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LIFE HISTORIES OF TWO NORTH AMERICAN SCORPIONS
CENTRUROIDES VITTATUS (SAY) (BUTHIDAE) AND
VAEJOVIS BILINEATUS POCOCK (VAEJOVIDAE)
by
WILLIAM DAVID SISSOM, B.S.
A THESIS
IN
ZOOLOGY
Submitted to the Graduate Facultyof Texas Tech University in
Partial Fulfillment ofthe Requirements for
the Degree of
MASTER OF SCIENCE
Approved
<r
August 1980
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C^fi * ^ ^ ACKNOWLED GMENTS
I w ish , in particular, to express my thanks and appreciation
to Dr. Oscar F. Franc k e, the chairman of my committee, who gave
so freely of his time throughout the course of this study. His
encouragement, willingness to share knowledge and ideas in discu<;sion,
and friendship made all of this work possible. I also wish to
thank the other members of my committee, Drs . Robert W. Mitchell and
Willi am P. Morri son for serving in that capacity. Their reviews
and comments on the manuscript are sincerely appreciated.
Several individuals helped in collection and care of specimens,
and I am grateful to them: James C. Cokendolpher, Dr. 0. F. Franck e,
Jane Gro en, Steve Jones, Dale K ing, Dr. Robert W. Mitche ll, James V.
Moo dy, and Fred W. Wagner. Mr . Cokendolpher also brought to light
many interesting ideas concerning life histor ies.
I also wish to extend a very special thanks to Lisa Nelms for
her help and encouragement throughout the latter portions of this
study. Her time and consideration were given unselfishly during
that time and are truly appreciated.
Finally, my parents provided m e w ith support, both moral and
financial, and understanding while I was a student at Texas Tech
University. To them I am extremely grateful and appreciative, for
without them all of this would have been impossible.
1 1
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1 1
TABLE OF CONTENTS
ACKNOWLEDGMENTS
LIST OF TABLES iv
LIST OF FIGURES vi
INTRODUCTION 1
Purpose and Scope of the Thesis 1
Review of Previous Research 3
The Hypotheses 13
MATERIALS A ID PROCEDURES , 17
LIFE HISTORY OF CENTRUROIDES VITTATUS (SAY) 23
Introduction , 23
Results 23
Discussion 40
LIFE- HISTORY OF VAEJOVIS BILINEATUS POCOCK 50
Introduction 50
Results 50
Discussion 58
GENERAL DISCUSSION 69
Tests of Hypotheses 69
Field vs . Laboratory Observations 72
SUMMARY , 30
LIST OF REFERENCES 83
iii
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LIST OF TABLES
TABLE Page
1. Measurements of adult females and second instar young, and
growth factors from previous life history studies . . . . 10
2. Estimation of the number of molts (N) from second instar
to adult in three litters of Centruroides vittatus (Say)
based on the prediction formula 14
3. Estimation of the number of molts (N) from second instar
to adult in five litters of Vaejovis bilineatus Pocock
based on the prediction formula 15
4. Duration of instars and cumulative age at time of molting
in days in laboratory-reared Centruroides vittatus (Say) . 31
5. Measurements in mm and growth factors of observed instars
(mean + standard deviation); predicted 95% confidence
intervals for instars not observed in the laboratory; and
selected measurements in mm of field-collected specimens
of Centruroides vittatus (Say) 32
6. Duration of instars and cumulative age at time of molting
in days in laboratory-reared Vaejovis bilineatus Pocock • • 56
7. Measurements in mm and growth factors of observed instars
(mean + standard deviation) ; predicted 95% confidence
intervals for instars not observed in the laboratory; and
selected measurements in mm of field-collected specimens
of Vaejovis bilineatus Pocock 5^
iv
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TABLE Page
8. Results of Duncan's multiple range test to determine
significance of size differences between litters of
Vaeiovis bilineatus 68
9. Measurements and growth factors of five structures
from an immature Paruroctonus mesaensis collected
near Indio (Riverside Co.), California 76
10. Measurements of adult females of Paruroctonus mesaensis
from the 0. F. Francke collection 78
V
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LIST OF FIGURES
FIGURE Page
1. Diagrammatic representation of carapace length used in
life history studies 22
2. Diagrammatic representation of chela length used in
life history studies 22
3. Diagrammatic representation of metasomal segment V length
used in life history studies 22
4. Dorsal view of late embryo of Centruroides vittatus . . . 26
5. Ventral view of late embryo of Centruroides vittatus . . . 26
6. Bar graph showing the presumed correlation between occurrence
of deaths in second instar Centruroides vittatus to times
of exuviation 30
7. Internal face of pedipalp femur of second instar Centruroides
vittatus 36
8. Internal face of pedipalp femur of third instar Centruroides
vittatus 36
9. Movable finger of pedipalp chela in third instar specimen of
Centruroides vittatus 39
10. Movable finger of pedipalp chela in fourth instar specimen of
Centruroides vittatus 39
11. Movable finger of pedipalp chela in fifth instar specimen of
Centruroides vittatus 39
12. Plot of carapace length vs , chela length in Centruroides
vi
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FIGURE ^aee
vittatus 43
13. Plot of carapace length vs . metasoma V length in Centruroides
vittatus 46
14. Bar graph showing the presumed correlation between the
occurrence of deaths in second instar Vaejovis bilineatus
to times of exuviation 5^
15. Plot of carapace length vs . chela length in Vaejovis
bilineatus 64
16. Plot of carapace length vs . metasoma V length in Vaejovis
bilineatus 66
V I 1
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INTRODUCTION
Purpose and Scope of the Thesis
There are two basic methods for determining the number of instars
required by scorpions to reach maturity. The better method is the
direct method, where scorpions are reared from birth to maturity and
the number of instars counted. Because this is usually difficult,
indirect methods w ere developed to estimate the number of instars,
and these methods are used both in the laboratory and in the field.
In the laboratory the number of instars can be indirectly
determined if the rearing of young fails to yield a complete life
history. Measurements are taken from all the instars observed, and
by extrapolation confidence intervals for dimensions of the structures
measured are obtained for succeeding instars. These results are
then compared w ith specimens know n to be adults and the instar
representing the adult stage is ascertained.
In the field morphometric methods are used. Large samples are
measured and grouped into modal size groups which presumably
represent the number of instars in the life cycle. However, actual
ecdyses are not observed, and growth is based on estimations of
population averages.
The primary purpose of this research is to present a third
indirect method and to test its accuracy by comparisons with data
from the literature and with observations obtained from laboratory
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rearings of two North American scorpion species.
It has long been k nown that arthropod growth is limited by the
presence of an exoskeleton and that dimensions of body structures grow
at a near-constant increment at each ecdysis. This latter characteristic
was first reported by Dyar (1890), and the growth increment has since
been know n as Dyar's constant. In scorpions there is generally an
increase of 25-35% in dimensions between consecutive instars (Francke
1976) , although some structures on a given molt can increase in a
linear dimension by as much as 60% . Thus, each structure has its
own growth increment and this can vary with each molt. For these
reasons I prefer to use the term "growth factor" rather than "Dyar's
constant".
If this growth factor is known or estimated (and expressed as
the dimension of a structure divided by its dimension in the previous
instar), and measurements of adults and young of known instar are
available (which is often the case in scorpions), then an equation
can be derived to determine the number of molts between the known
immature stage and adult. If A is the dimension of the adult
structure, Y is the dimension of the same structure in a young
specimen of k nown age , G is the growth factor of the measured
structure, and N is the number of molts required by the young
specimen to reach adulthood; then the equation may be expressed as
follows:
NA = Y ( f / ) ,
that can be transformed into logarithms, giving:
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log A = log Y + N log G.
By rearrangement the work ing formula can be obtained:
^ log A - log Ylog G
This equation is a modification of original growth equations
by Huxley (1932) and Teissier (1960), and w as put into this usable
form for scorpions by Francke (personal communication).
A secondary purpose of this thesis is to report observations on
the life histories of two common North American scorpions:
Centruroides vittatus (Say) and Vaejovis bilineatus Pocock. The
number of instars to maturity will be predicted and tested by rearing
specimens. Various instars w ill be compared morphologically and
actual growth factors of three measured structures will be calculated.
Review of Previous Research
Studies on the post-birth development of the order Scorpiones
are relatively few in number, but the last decade has produced a
surge of interest in this area. At present species of five families
have been investigated: Buthidae, Chactidae, Diplocentridae,
Scorpionidae, and Vaejovidae. These investigations will be considered
herein according to the methods used in determining the number of
instars to maturity for the particular species.
In Table 1 measurements are given for second instar young and
adult females. These measurements were obtained from all published
sources in which adequate measurements (for calculation of growth
factors) were presented.
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Direct Methods
The first account of a scorpion reared from birth to maturity
was that of Schultze (1927), who reared Heterometrus longimanus (Herbst),
a scorpionid. He reported that the first two molts occurred in the
first 18 days after birth and made careful observations on instar
duration and behavior. He recognized eight instars to adult, but
reported no measurements . Consequently, the rate of growth of each
stage is unknown.
A rather detailed study of the biology of the chactid Euscorpius
italicus (Herbst) included a report on its life history (Angermann
1957). He was the first to observe maturation molts to occur at two
instars — the sixth and seventh. Only one female required the
additional molt to attain maturity in his study. Measurements of
carapace length were included in this paper.
Another scorpionid, Pandinus gambiensis Pocock, was reared to
maturity by Vachon ejt al . (1970), and males of that species were found
to mature at the seventh or eighth instar. They divided morphological
characters into two groups based on life history information: (1)
those stabilized at birth (such as the number of pectinal teeth,
trichobothria, and tarsal spines on the legs) and (2) those changing
with age (such as size and setation). Characters stabilized at birth
are considered very important in taxonomy. Measurements of the
metasoma, chela, and body (prosoma and mesosoma) were given in this
study for several instars . Because these measurements are inadequate
for calculation of growth factors, they are omitted from Table 1.
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The remaining direct-method studies were done on buthids. Shulov
and Amitai (1960) raised Orthochirus innesi Simon to adulthood at the
sixth instar. The number of hairs on the pectinal fulcra was observed
to increase during the immature stages in that species. Total body
length of all stages was measured and reported.
Buthus occitanus Amoreux w as reared in the laboratory by Auber
(1963). She reported first instars ("larvae") to lack claws on the
tarsi and to possess well-formed pectines with the full complement of
teeth. She also reported sexual maturity to be attained with the
seventh instar. Later (Auber-Thomay 1974), she reared Androctonus
australis (L.) and again observed the constancy in pectinal tooth
counts throughout development. A.. australis was found to mature at
the eighth instar. In both contributions measurements of chela
length of all stages in the life cycle were presented.
Several studies have been done with members of the genus Tityus.
Both Matthiesen (1962) and San Martin and Gambardella (1966) found
T. serrulatus Lutz and Mello to be parthenogenetic, females giving
birth at the sixth instar. Matthiesen presented no measurements, but
San Martin and Gambardella provided measurements of some of the
instars. The measurements given do not allow calculation of growth
factors and will be omitted from Table 1. Matthiesen (1970) was
able to raise several specimens of T_. bahiensis (Perty) and found
females to attain maturity at the sixth instar. In this contribution
he presents measurements of the movable finger of the pedipalp chela
for the instars.
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Lourengo (1978) reared six individuals of Tityus trivittatus
fasciolatus Pessoa to the sixth instar and found males to mature at
either the fifth or sixth instar. Some field-collected males were
noticeably larger and were estimated to be seventh and eighth instars.
Females were considered to be mature at the sixth instar. Measurements
of carapace length and-metasomal segment V length were reported
in :this contribution.
A very detailed account of the life history of Isometrus maculatus
(De Geer) was presented by Probst (1972). ^. maculatus achieves sexual
maturity at the seventh instar, but 5-10% of the females were found
to mature at the sixth instar. Not only did Probst present measurements
(carapace length, chela length, and metasoma V length), but he
calculated growth factors for each of the structures and compared
growth of males and females. He found that allometric sexual
dimorphism, which is strong in this and many other buthids, is not
distinct until the fifth or sixth molt.
A detailed morphological analysis of the different instars in the
life cycle w as done on Buthotus minax occidentalis (Vachon and Stockmann)
by Stockmann (1979). He observed increases in the number of hairs on
the fulcra and middle lamellae of the pectines and in the number of hairs
around the basitarsal spurs. In addition he noted increases in the
number of sensillae on the pectines and in the number of lyrifissures
on the articulations of the appendages. Males of B . minax occidentalis
reach maturity at the sixth or seventh instar and females do so at the
seventh or eighth. No measurements were reported in this study.
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Francke (unpublished data) was able to rear two litters of
Centruroides gracilis (Latreille) to maturity w ith very low mortality
permitting detailed analysis of the life history of this species. C_.
gracilis was found to mature at the seventh instar (some males mature
at the sixth) with an average growth rate of 27.5%. Measurements
of carapace length, pedipalp femur, tibia, and chela lengths, metasoma
V length, and telson length were used in that study.
Indirect Methods
The number of instars in the life history of eight scorpion species
has been estimated by the use of indirect methods. Five of these
species were reared through part of their life history in the laboratory
and three species have been observed in the field (two studies have
been done on the same species). By necessity the number of instars
was determined morphometrically, and measurements w ere presented in
all studies except that of Smith (1966) . In that paper was presented
a graph plotting the logarithms of measurements obtained from two
structures, and the measurements can be estimated from this graph.
Laboratory Method
A subadult specimen of Diplocentrus spitzeri Stahnke
(Diplocentridae) molted to maturity in the laboratory (Francke, in
press) and by using the growth factors obtained from that molt, it
was hypothesized that D. spitzeri attains maturity at the sixth
instar. The observations presented in this paper represent the
only observations made on this family of scorpions to date.
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size class for first instar young, which do not appear on the surface
with the rest of the population) , and stated that P_. mesaensis has
eight instars. Dissection revealed that P. mesaensis matures at the
presumed seventh instar, and this was interpreted to mean that P_.
mesaensis has a postmaturation molt , something that has never been
observed in scorpions.
Polls and Farley (1979) in a more detailed study found seven
distinct size classes (including first instars) and concluded that
P. mesaensis has seven instars in the life cycle. They reported
that there was definitely no postmaturation molt and calculated an
average growth factor of 1.38, which is rather high in comparison
with other scorpions. This large growth factor was attributed to
differences between field and laboratory data.
Fox (1975) also analyzed populations of Paruroctonus baergi
(Williams and Hadley). There were presumably seven distinct size
classes in its popula tion, or a total of eight instars (including
first instars). Again a postmaturation molt was hypothesized.
Finally Smith (1966) published information from field studies
on Urodacus abruptus Pocock (Scorpionidae) and recognized six instars
in that scorpions's life cycle. He included no measurements but
plotted logarithms of measurements of two structures on a graph.
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13
The Hypotheses
Earlier, I presented a derivative of the well-known growth equations
of Huxley (1932) and Teissier (1960), which will allow prediction of the
number of molts to maturity. This formula wi ll be used for the first
time in hypothesizing the number of molts to maturity in two scorpions:
Centruroides vittatus and Vaejovis bilineatus. The hypotheses wil l be
tested by rearing litters of these species in the laboratory.
For Centruroides vittatus the values for dimensions of the
structures of second instars and adults are presented in Table 2.
The growth factor used in the formula is the average value of all
observed growth factors from published laboratory studies (Table 1)
wh ere growth and ecdyses are measurab le. This average growth factor
is approximately 1.29.
On the basis of the obtained values of N (rounding to the nearest
molt) , the prediction is that C _. vittatus should molt five times between
the second instar and adult. Including the first molt (from first to
second instar) this gives a total of six molts in the life cycle.
The ref ore , I hypoth esize that C_. vittat us reaches maturity at the
seventh instar.
For Vaejovis bilineatus the data are presented similarly in
Table 3. From the obtained v alues of N the prediction is that V;.
bilineatus should molt four times between the second instar and adult.
Including the first molt a total of five molts should occur in the life
cycle. Ther efor e, I hypothesize that V. bilineatus matures at the
sixth instar.
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14
Table 2. Estimation of the number of molts (N) from second instarto adult in three litters of Centruroides vittatus (Say) based on theprediction formula.
Specimen orParameter
LITTER #1Female
log A2nd instar
log Y
mean
Growth Factorlog G
CalculatedLITTER #2
Femalelog A
2nd instarlog Y
N
mean
Growth Factorlog G
CalculatedLITTER #3
Femalelog A
2nd instarlog Y
N
mean
Growth Factorlog G
Calculated
Average_Alog AAverage Y
log YGrowth Factor
log GCalculated N
N
All measurements are
FormulaValue
A
Y
G
N
A
Y
G
N
A
Y
G
N
A
Y
G
N
AverageAverage
Average
NN
N
CarapaceLength
4.810.681.69
0.231.290.114.09
5.330.731.620.211.290.114.73
4.680.671.670.221.290.114.09
4.940.691.660.221.290.114.27
for LITTERfor LITTER
for LITTER
in mm.
ChelaLength
8.130.912.66
0.421.290.114.46
9.150.962.410.381.290.115.27
7.980.902.560.411.290.114.46
8.420.932.540.401.290.114.82
#1: 4.46#2: 5.33
/ 3: 4,55GRAND AVERAGE N (ALL LITTERS): 4.78
Metasoma VLength
5.640.751.66
0.221.290.114.82
6.800.831.480.171.290.116.00
5.600.751.540.191.290.115.09
6.010.781.550.191.290.115.36
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15
Table 3. Estimation of the number of molts (N) from second instarto adult in five litters of Vaejovis bilineatus Pocock based on theprediction formula.
Specimen orParameter
LITTER #1Female
log A2nd instar mean
log YGrowth Factor
log GCalculated N
LITTER #2Female
log A2nd instar mean
log YGrowth Factor
log GCalculated N
LITTER #3Female
log A2nd instar mean
log YGrowth Factor
log G-*• ** o
Calculated NLITTER #4
Femalelog A
2nd instar meanlog Y
Growth Factorlog G
Calculated NLITTER //5
Femalelog A
2nd instar mean
log YGrowth Factor
log GCalculated N
All measurements are
FormulaValue
A
Y
G
N
A
Y
G
N
A
Y
G
N
A
Y
G
N
A
Y
G
N
CarapaceLength
4.150.621.52
0.181.290.114.00
3.550.551.370.141.290.113.72
4.150.621.410.151.290.114.27
3.750.571.420.151.290.113.82
3.530.551.410.151.290.113.64
in mm.
ChelaLength
4.900.691.70
0.231.290.114.18
4.200.621.500.181.290.114.00
5.050.701.570.181.290.114.73
4.630.671.610.211.290.114.18
4.270.631.590.201.290.113.91
Metasoma VLength
4.360.641.46
0.161.290.114.36
3.800.581.260.101.290.114.36
4.500.651.260.101.290.115.00
4.230.631.350.131.290.114.54
3.810.581.320.121.290.114.18
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Table 3. (Continued)
16
Specimen orParameter
FormulaValue
A
Y
G
N
CarapaceLength
3.830.581.430.161.290.113.89
ChelaLength
Metasoma VLength
Average Alog A
Average Ylog Y
Growth Factorlog G
Calculated N
Average N for LITTER HAverage N for LITTER #2Average N for LITTER #3Average N for LITTER /MAverage N for LITTER #5
4 . 6 10 . 6 61 . 6 10 . 2 11 . 2 90 . 1 1
4 . 2 0
4 . 1 84 . 0 34 . 6 74 . 1 83 . 9 1
4 . 1 40 . 6 21 . 3 40 . 1 31 . 2 90 . 1 1
4 . 4 9
GPxAND AVERAGE N (ALL L IT T E R S ): 4 , 1 9
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MATERIALS AND PROCEDURES
Approximately 250 specimens of Centruroides vittatus and Vaejovis
bilineatus were available for life history studies. This number not
only includes females and their lit ters, but also approximately 50
specimens from the 0. F. Francke collection used for comparative
purposes. Only specimens taken near or at the collecting localities
of the pregnant females were used for comparison.
Specimens of C_. vittatus came from several sources: (1) four
pregnant females from Rio Grande Village, Big Bend National Park,
Brewster Co., Texas on 29 July 1978 by 0. F. Francke and J. V. Moody
(only two of these females gave birth, n = 24 on 3 August, and n = 21
on 21 August 1978); (2) one pregnant female from Glenn Springs, Big
Bend National Park , Brewster Co. , Texas on 23 March 1979 by R. W.
Mitchell (aborted 28 young in June 1979); (3) one pregnant female
from White River Lak e, Crosby Co. , Texas on 3 September 1979 by S.
Prien (gave birth 20 September 1979, n = 32); (4) one immature female
from two miles north of Wink , Winkler Co. , Texas on 12 November 1977
by J. Groen (molted to maturity in the laboratory); and (5) a freshly
molted adult male from 20 miles east of Bracketville, Kinney Co. ,
Texas on 4 June 1980 by J. C. Cokendolpher (found underneath a rock
with its exuvia).
Five pregnant females of V. bilineatus were collected at Villa
Hidalgo, San Luis Potosi, Mexico by R. W. Mitchell et al. in March
17
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18
1977. All females gave birth in the laboratory during that summer.
In addition an immature specimen collected with the pregnant females
molted to maturity in the laboratory.
All specimens were maintained in an environmental chamber as
described by Francke (1976, 1979, in press). Temperature was constant
( 2 6 . 6 + 1 C) except for a one month period (June 1979) when the
heating system failed. During this time scorpions were moved to room
temperature (approximately 21° C ) .
Young scorpions born in the laboratory were separated and
placed in individual containers (3 oz. jars with lids) as soon as
the litter dispersed from the mother's back. A bit of paper towel
or a layer of soil was placed on the floor of the jar to provide
a substrate, and a moistened sponge was used for watering. A variety
of food was offered to the young scorpions, including newly hatched
crickets, newly emerged wingless Drosophila sp., young cockroaches
of two species [ Nauphoeta cinerea Saussure and Supella supellectilium
(Serville)], and termites. All of these prey items were introduced
into the containers alive, and all were accepted quite readily.
In addition chopped "steaks" of Nauphoeta were fed to the earlier
instars when living prey of small size were not readily available.
That young scorpions accept such food matter has been documented
by Francke (1979). All litters of scorpions were fed at least
twice week ly (more often depending upon the availability of food),
and adults were generally fed three to four times per month.
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1<
Data were collected with each molt or death of a laboratory
specimen . Exuviae from the molts were stored in two and three dram
via ls , and dead specimens w ere preserved in 80% ethanol. With each
a label was placed bearing identification information, instar number,
and date of the event.
Tw o methods w ere used in predicting the number of instars to
matur ity. At the onset of the investigation, second instar young and
adult females were measured, and the values obtained were substituted
into the proper place in the formula N = °^ ^ ~ ^ ° " — . The growthlog G
factor used was the average of all reported values (1.29) from
laboratory studies including measurements. The value of N obtained
represented the predicted n umber of molts between the second instar
and adu lt, and the total number of molts w as obtained by adding one
(the mo lt from first to second instar).
The failure of specimens from the litters to reach maturity
required the use of the indirect method for estimating the number of
instars to maturi ty. From the size of the last instar observed 95%
confidence intervals we re used to predict the sizes of the next instars,
as explained in detail by Francke (1979). Known adults are compared
with the ranges of predicted measurements so that the instar at which
maturity is reached can be determined. Structures used are the same
as by Francke (carapace len gth, chela lengt h, and metasoma V length),
and the dimensions measured are illustrated in Figures 1-3. All
measurements were taken on an American Optical Model 569 binocular
dissecting microscope equipped with an ocular micrometer calibrated
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20
at 20X. Illustrations were done with a Wild M7A dissecting scope
equipped with a drawing tube (TYP 256575).
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1
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LIFE HISTORY OF CENTRUROIDES VITTATUS (SAY)
Introduction
Of three litters of Centruroides vittatus (Say) born in the
laboratory during 1978 and 1979, two litters (n = 21 and n = 24)
were the pale yellow form formerly referred to as £. pantheriensis
Stahnke, and one litter (n = 32) was the typical striped form of
C _. vittatus. Five specimens from these litters were kept alive well
into the fifth instar (three specimens are still alive), one immature
specimen molted to adult after its capture and return to the
laboratory, and one adult specimen was captured with its last exuvia.
These and additional specimens from the 0. F. Francke collection were
used to elucidate the details of the life history of this common
North American scorpion.
Results
The young of Centruroides and other buthids are born surrounded
by a "birth membrane", and they escape from this structure before
ascent to the mother's back (Baerg 1961; Shulov and Amitai 1960;
Stahnke 1966; Williams 1969) where they stay until shortly after
their first mol t. The abortion of 28 young in the laboratory by
a female C_. vittatus captured in August 1979 allowed the opportunity
to observe young scorpions within their "birth membranes". Although
these specimens are of little value in studying the postbirth
development of C . vit tatus, there have been few descriptions or
illustrations of the development or position of scorpions in their
23
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26
H
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27
At birth the young scorpions are creamy white , and the appendages
possess a yellowish tinge. Portions of the carapace (particularly the
interocular area), pedipa lps, metasoma, and legs have considerable
dusky mark ings . The tergites bear rudiments of the two submedian
stripes which are so characteristic of typical C_. vittatus. The
ventral surface of the cauda has median, and sometimes two lateral,
dark bands; and metasomal segment V is completely darkened. Color
of the young gradually darkens until the next molt .
The lack of a number of typical adult characters at this stage
is rather distinctive (for a discussion relative to this, see Francke
1976:255-256). For this reason the stage has often been referred to
as a larval stage by French observers. A number of species at this
stage have been described (Auber 1963; Francke 1976; Lourengo 1978;
Polls and Farley 1979; Shulov and Amitai 1960; Williams 1969), and
the appearance of first instar C_. vittatus agrees very w ell with
other scorpions. Pectines are fully developed and contain the
full complement of pectinal teeth; the tarsi are blunt and lack claws;
the aculeus is soft and b lunt; trichobothria are absent; and except
for two or three fine setae on the tarsi of the legs, these cuticular
structures are absent as well. In addition granulation and carinal
development are lacking, pedipalp chelae lack dentate margins, and
the cheliceral teeth appear as barely discernible protuberances on the
inner margins.
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Second Instar
The second instar (first nymphal stage of French observers) is
the stage at w hich young scorpions become independent from their
mothers. It is this stage which has caused problems in many life
history studies, because mortality due to starvation, molting
difficulties, or other causes is very high. This is true of the
present study, as well as several others (Francke 1976, 1979;
Hjelle 1974; Lourengo 1979). Mortality of second instar £ . vittatus
in this study was approximately 80%. Upon examination of the
specimens from two of the litters (n = 21 and 24) , it became probable
that many of the deaths were caused by molting difficulties, as
evidenced by separation of the old cuticle from the prosoma and by
the correlation of dates of death and dates of molts (Figure 6 ) .
Mortality in the third litter (n = 32) was largely due to starvation,
as specimens apparently refused the food offered them.
Specimens surviving the second instar (n = 15) molted to the
third instar at an age of 152.7 + 3 7 . 3 days. The second instar was
found to last 143.1 + 37.5 days (range 104-256 days). For actual
chronologies of specimens molting in the laboratory see Table 4. For
measurements of second instar structures consult Table 5.
At the second instar, young scorpions possess most of the
characters found in adults. The body is now covered with setae, and
trichobothria are found in normal numbers on the pedipalp chela and
tibia; but the internal face of the femur in C. vittatus lacks one —
trichobothrium i «, according to Vachon's (1974) terminology, is missing
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30
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3A
(Fig. 7 ) . This same trichobothrium w as found to be missing in second
instar Buthotus minax occidentalis by Stockmann (1979), in Tityus
bahiensis and T . serrulatus by Stewein and Delgado (1968), and in
Centruroides gracilis by Francke (unpublished data). According to
Vachon (1974), this trichobothrium is missing in all second instar
buthids.
Carinae are present and moderately developed, though crenulations
and serrations are quite feeble. Development of carinae is rather
gradual throughout the instars, and it is very difficult to age
specimens using this character.
Other structures are also well developed at the second instar.
The aculeus is now hardened, sharp, and useful in subduing prey. The
teeth of the chelicerae are hardened, and the nodules are present on
the ventral surface of the cheliceral fingers. The dentate margins of
the pedipalp chela fingers are well developed, and internal and
external lateral granules are present. Supernumerary granules are
absent in this stage and do not appear until the fourth instar (Figs..
9-11). Color of this and succeeding instars will be discussed later,
as comparisons w ill be made between the pale and striped forms of this
species.
Third Instar
Of 15 specimens reaching the third instar, six died during this
stage (mortality = 4 0% of second instar survivors, or 7.8% of the
original number). One of these specimens died shortly after its
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35
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36
7
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37
second molt before it was able to expand its exoskeleton fully, and it
was omitted from third instar measurements for this reason.
Specimens surviving the third instar (n = 8, and one specimen
remains alive) molted to the fourth instar at an age of 223 + 45.3 days.
The third instar w as found to last 87 + 29.3 days (range 38-118 days).
For individual chronologies consult Table 4. For measurements of
third instar structures consult Table 5.
Other than s ize, there were few differences between the second and
third instar scorpions. The previously missing trichobothrium on the
pedipalp femur internal face is now present (Fig. 8 ) . Supernumerary
granules on the fingers of the pedipalp chela are still absent (Fig. 9 ) .
Fourth Instar
Five of the eight specimens reaching the fourth instar completed
this stage and one remains alive at this time. The fourth molt was
accomplished at an age of 342 + 127.7 days, and the duration of the
fourth instar w as found to be 128 + 74 days (range 59-210 days). For
chronologies of the individuals surviving this instar consult Table 4.
For measurements of fourth instar structures consult Table 5.
The only noticeable structural difference between third and
fourth-instar scorpions, other than size increase, is the presence of
supernumerary granules on the pedipalp chela fingers (Fig. 10). At the
fourth instar there is usually only one (but sometimes tw o) of these
granules betw een consecutive lateral granules, both on the internal
and external sides of the dentate margin (Fig. 10 ). The number of
these granules increases at the fifth instar (Fig.11).
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39
. sg -
4 4 .
9 11
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40
Fifth and Succeeding Instars
Four of the five remaining specimens died during the fifth instar,
and the fifth specimen remains alive. Therefore, the duration of this
stage is unknown at present, but three specimens in different stages
are still alive and could possibly survive this instar, or even reach
maturity.
Structurally, fifth instars resemble fourth instars rather closely.
At the fifth instar there are usually more supernumerary granules on
the fingers of the pedipalp chela, but due to the small sample sizes
of known fourth and fifth instars, I do not know how reliable this
character will be in separating these two age groups. Probably the
best external criterion to differentiate fifth instars from subsequent
stages is size, as specimens of these age groups are difficult to
identify by other means. Information pertinent to the late instars,
including predictions of the number of stages in the life history of
£. vitt atus , will follow immediately in the discussion.
Discussion
Growth
Even though laboratory-reared litters of C. vittatus failed to
yield a complete life his tory, the data obtained from them, from one
specimen which molted to maturity after its capture, and from an
adult found with its last exuvia are sufficient to observe growth in
this species, and predict 95% confidence intervals for instars not
observed in the laboratory. For each structure (carapace length, chela
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41
length , and metasoma V length) a growth factor was calculated for the
second through fifth instars (see Dyar 1890; Francke 1976), and this
growth factor was used in predicting 95% confidence intervals for
sixth, seventh, and eighth instars.
The carapace growth factor of C_. vittatus is fairly steady through
the third molt but tapers at the fourth. Francke has found that the
carapace growth factor of C_. gracilis continues to taper through the
sixth, and final , mol t. The same cannot be said for the chela growth
factor, as it remains relatively constant throughout all observed
instars of £. vittatus. In C_. gracilis, however, Francke found that
chela growth factor shows rather definite allometry: Males experience
a significant increase in chela growth factor at the maturation mol t,
whereas in females it remains rather constant. Proper comparison
cannot be made here due to the small sample size of fifth instars in
this study, and the confidence limits obtained for succeeding instars
have been based on male and female chelae lengths considered together.
The values for these confidence limits (Table 5, Fig. 12) may be
slightly distorted for this reacon. Indeed, field-collected adult
males show a slight trend toward a higher ratio of chela length to
carapace length than females of the same age (Fig. 12) . Therefore,
it is probable that males of C . vittatus show considerable allometry
at the maturation molt.
It has long been know n that Centruroides scorpions show drastic
allometry in the metasoma, and this character is used to sex adults of
these scorpions on sight. Only metasoma V was analyzed in this
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— TI
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44
contribution , but it should be pointed out that allometry involves all
five metasomal segments. Growth of m.etasoma V in C . vittatus was
found to be relatively constant until the fifth instar, when it
becomes possible to sex specimens due to a slight increase in male
growth factors. Of two specimens molting to maturity (one male from
Kinney Co ., and one female from Winkler Co., Texas), more drastic
allometry occurred at the maturation molt. The metasoma V growth factor
of this molt w as 1.61 for the male , and 1.33 for the female. Even
though this information came from only two specimens, Francke
(unpublished data) observed the same in £. gracilis: Male metasoma
V growth factors averaged 1.58, while females averaged 1.36. Because
such drastic allometry occurs in metasoma V, separate confidence
intervals were predicted at sixth, seventh, and eighth instars for
males and females using 1.58 for the male growth factor and 1.29
(average value for the observed growth factor in this study) for
the female growth factor. The confidence limits thus obtained for
the males are based on observations of C . gracilis and a maturation
molt for a male £. vittatus. These confidence intervals are subject
to error , but the available field observations of C . vittatus indicate
that growth of this structure in the two species is quite similar. It
should be noted at this point that field-collected specimens agree
closely with the predicted confidence limits (Fig. 13 ) , and there is no
reason to doubt that these limits are reasonably accurate.
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45
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46
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47
Number of Molts to Maturity
In predicting the number of molts to maturity, it was necessary to
use the indirect approach developed for scorpions by Francke (1976, 1979)
and used later by Lourengo (1978, 1979). Confidence limits for the size
of the three measured structures were obtained for late instars using
the method described in detail by Francke (1979) and are presented in
Table 5. Measurements from field-collected specimens were compared
with these confidence limits (obtained from the laboratory-reared
specimens) , and the number of molts to maturity is based on these
comparisons.
Maturity of field-collected males was determined by dissection of
hemispermatophores. This was done by making a longitudinal incision
in the pleural membrane of the mesosoma and looking inside for the
hemispermatophore. By using this technique the specimen remains
undamaged. Field-collected males fit predicted confidence limits for
both sixth and seventh instars. This is demonstrated in Figures 12
Cwhere chela length is plotted against carapace length) and 13
Cwhere metasoma V length is plotted against carapace length). Table 5
provides direct comparisons between measured specimens and the
confidence intervals.
The adult male from Kinney C o., Texas, which had just molted to
maturity prior to its collection, falls within seventh instar confidence
intervals for all three structures. The exuvia collected with it fits
sixth instar confidence intervals for all three structures.
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48
Maturity of females was determined by means other than dissection
to prevent damage to specimens. Six females that gave birth were
found to be seventh instars, and females of comparable size were judged
also to be mature. One specimen is slightly larger than predicted size
for seventh instars (Figs. 12 and 13), but is far too small to be an
eighth instar, and I consider it to be a large seventh instar.
The female from Winkler Co. , Texas, which molted to maturity in the
laboratory, fell within seventh instar confidence intervals for all
three adult structures. Carapace measurements from the two exuviae of
this specimen were slightly lower than sixth (-0.09 mm) and fifth
C-0.14 mm ) instar confidence intervals; chela measurements w ere also
lower than predicted sixth (-0.07 mm) and fifth (-0.18 mm) instars; but
metasoma V fell within those levels. Therefore, I consider this
specimen to be a small adult at the seventh instar.
It should also be considered whether or not £. vittatus attains
maturity at the eighth instar. Because no field-collected adults
Cn = 26) fit near the predicted eighth-instar confidence intervals
(Table 5 ) , it becomes unlikely that this species matures at the eighth
instar.
In light of the above evidence, I conclude that males of C . vittatus
mature at the sixth or seventh instar, and that females do so at the
seventh instar. The possibilty exists that females may also mature at
the sixth instar, but no evidence here suggests this to be true.
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LIFE HISTORY OF VAEJOVIS BILINEATUS POCOCK
Introduction
Five pregnant females of Vaejovis bilineatus Pocock from Villa
Hida lgo, San Luis Pot osi , Mexico gave birth in the laboratory (n = 17 ,
20, 22 , 25 , and 2 6 ) , and their litters we re reared. The litter of
one female (n = 17) died as second instars, and specimens were used
only for litter size inform ation, duration of first instar, and
meas ure ment s of second instar structures. Fourteen specimens from the
other litters entered the fourth instar and one of those entered the
fifth. An immature female collected w ith the adults at Villa Hidalgo
molted twice and reached maturity in the laboratory and remains alive
at the time of w ritin g. Theref ore, by combining these two sources
a complete life history was obtained. As in the study of C _. vittatus
the 0. F. Franck e collection provided useful comparative material
for life history predictio ns. These are the first observations on life
history for the genus Vae jov is, which is the most diverse genus of
scorpions in North America . Only three other vaejovids have been
studied: Uroctonus mordax by Francke (1976); Paruroctonus baergi
by Fox (1975); and Paruroctonus mesaensis by Polls and Farley (1979).
Comparisons will be made between all four vaejovids where relevant.
Results
First Instar
The young of Vaejovis bilineatus are born enclosed by the "birth
mem bra ne" as are many other scorpions. Upon freeing themselves from
50
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51
this membrane, they climb onto the mother's back , where they stay until
after the first mol t. Once on the mother's back, the neonates orient
themselves in an organized fashion with the prosoma directed forward
and the metasoma back ward. This neat layering effect was reported
first by Williams (1969) for several Vaejovis and is apparently
characteristic for the genus. In this laboratory the same was
observed for Vaejovis coahuilae Williams and Vaejovis waueri Gertsch
and Soleglad.
The first molt occurred at an age of 9.8+0.45 days. Molting
of all the young took place in a single day. Dispersion of the young
took two to four days to complete, and this process began at an age of
16.75 + 1.71 days (7 + 1.63 days after the first molt). The young
were sorted after all were dow n, and this occurred when they were20.4 + 3.78 days of age.
First instars of V_. bilineatus are creamy white at birth but
gradually darken until time of exuviation. As in all other scorpions,
they lack many adult features. Tarsi are blunt and lack claws, the
aculeus is soft and b lunt , trichobothria and setae are absent,
granulation and carinae are absent, and the denticles on the margins
of the pedipalp chela fingers are absent. Pectines, however, are
well developed and contain the complete number of pectinal teeth.
Second Instar
As in £ . vittatus, mortality was very high in the second instar.
Only 40 of 110 specimens (36.4%) survived this stage, and deaths were
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52
probably due to a number of causes. Several individuals died early in
this stage, probably due to starvation; a number of others died during
or shortly after peak molting periods; and the remainder simply never
molted (Fig. 14 ) . The average age at the second molt was 106 days, but
11 of the specimens which died failed to molt after 160 days. These
specimens probably died because of problems in initiating molting, but
the exact nature of these problems cannot be discerned because
environmental conditions w ere constant and equal for all specimens.
The young Vaejovis were fed chopped roach "steaks" because they
shied away from available living prey. Although this food is accepted,
there is one disadvantage to its use. VThen left in the container for
very long (sometimes even overnight), it molds easily and the hyphae
may not only trap the young scorpion, but may also damage the specimen
to where measurements are not possible. Fortunately, this was the case
primarily in the second instars where sample size was rather large.
Another problem resulted in the loss of specimens. The litters
of all _V.. bilineatus were reared on a soil substrate, and a number
of specimens were lost as a result. These "missing" specimens probably
dug into the soil and died, where they dessicated and crumbled so that
recognition was difficult and measurements impossible. This was
especially true of second and third instars, in which the specimens are
very small.
Specimens surviving the second instar (n = 40) molted to the third
instar at an age of 106 4 15.62 days. The duration of the second instar
was 96.2 + 15.69 days, and the range of observations was 60-140 days
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54
00
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18
16
14
12
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8
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2
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60 80 100 120 140 160 180 200 220 240 260
AGE IN DAYS
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56
Q J
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CM r^ CN r^ ON <3^ CMC CN O^ CN CTi CTi COrH i-l O rH O O rH
r H O O v O C J ^ l n v O l n c o a ^ O O C O O O O O O O r H C O C O c O C O O O c oC O O O O O O O O O O C T i r ^ O O O O C T i O O O v O C r i O C T i r H C O r H c y i C T i C Nr H r H r H O O r H O O O O O O O r H O O r H O r H O r H O O r H
o o o o o o o o o o o o o o o o o c r > c T i c T > c T i c y ( < T i o >r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H
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E3CJ
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i n e N < r c N v o c N - s f c o o c r > o o o v o o i n c r iO O ^ o ^ r - c 3 ^ o a ^ O s 1 • r ^ O r ^ o o O r H o or H O r H O O r H O r H r H O r H O O r H r H O
o o o o o o o o o o o o o o o or H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H
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l n v o ^ ^ o o o ^ O r ^ c M c o < ^ l n v o ^ ^ o o c ^ oC M C N C N C N e N C O C O O 0 C O C O C O C O C O r O C 0 > ; r
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58
instar structures are presented in Table 7.
The single specimen that molted to the fifth instar did so at
an age of 323 days and the duration of its fourth instar was 147 days.
To ensure better accuracy for instar predictions, 95% confidence
intervals for measurements were calculated from fourth instar specimens
(n = 9 ) . Therefore, confidence intervals on measurements for the
fifth instar were calculated as well as for sixth, seventh, and eighth
(Table 7 ) . The use of these confidence intervals in prediction of
the number of instars to maturity is discussed in the following section.
Discussion
Growth
Although the laboratory-reared litters of v;. bilineatus failed to
reach maturity , it w as possible to observe relative growth of measured
structures (carapace length, chela length, and metasoma V length) at
the early instars. Information on growth at later instars was primarily
taken from a single specimen in one litter which molted to the fifth
instar and from a field-collected female that molted twice (and
reached maturity) in the laboratory.
The carapace growth factor of V . bilineatus was steady throughout
life averaging 1.26 _+ 0.05. This observation is based not only on the
litters but also on the specimen molting to maturity in the laboratory
The magnitude of the growth factor and its constancy is consistent with
reports on Uroctonus mordax (Francke 1976). The observed average growth
factor of that specimen, reared to the fifth instar, was 1.31.
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59
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60
C O
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61
Fox (1975) reported carapace measurements for his recognized
population size classes in Paruroctonus baergi and P. mesaensis.
Whereas the measurements were not considered to represent the limits
of carapace size in instars, they do represent approximations of
"instar" size and growth factors can be calculated from them. In
both species carapace length was reported to decrease steadily throughout
life. The calculated growth factor for P_. baergi decreases from an
average of 1.41 (which is rather high) between second and third
"instars" to 1.11 (which is very low) between seventh and eighth
"instars". In P_. mesaensis the growth factor decreases from 1.37
(from second to third "instars") to 1.17 (from seventh to eighth
"instars") . Not only does this differ from reports on Vaejovis
and Uroctonus, but it is in conflict with reports of Polls and
Farley (1979) for _P. mesaensis.
Polls and Farley (1979) did not report carapace length but
measured two other prosomal characteristics instead. They reported
seven size classes (or instars) for P . mesaensis and calculated growth
factors between these size classes. The growth factors fluctuated
considerably and ranged from 1.30 to 1.58. The mean overall growth
factor reported w as 1.38, which is rather high in comparison to other
scorpions. The differences observed between these two field studies
will be discussed fully in the general discussion, and comparisons
between field and laboratory methods w ill be made.
Chela growth is also relatively constant throughout life in V .
bilineatus, and averages 1.28 + 0.05. The average growth factor
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62
in U. mordax w as observed to be 1.26 (Francke 1976) but was rather
low in the molt from second to third instar (1.20). It is quite common
for a single specimen to show considerable fluctuation in the growth
factor betw een instars, and this is not reflected in the averages.
Neither Fox (1975) nor Polls and Farley (1979) reported on growth in
the pedipalp chela, so comparisons cannot be made with Paruroctonus.
Metasomal segment V in Vaejovis does not show the allometry that
members of Centruroides exhibit, and growth in this structure in V_.
bilineatus is rather constant between instars. The average growth
factor was 1.32 +. 0.07. Several specimens showed high growth factors,
but these occurred betw een the second and third instars, and no
inferences could be made from those observations. The growth rate of
metasoma V was slightly higher than that in the other structures, and
this was found to be true in IJ. mordax as well (Francke 1976).
Again, metasoma V w as not analyzed in either field study of Paruroctonus
and comparisons cannot be made.
Number of Instars to Maturity
As stated previously, methods for predicting the number of instars
to maturity are the same as used by Francke (1976, 1979) and Lourengo
(1978, 1979). Measurements of observed instars and 95% confidence
intervals for instars not observed are presented in Table 7.
When measurements of field-collected adults are compared to the
confidence intervals obtained from the laboratory-reared litters
(Table 7, Figs 15 and 16) , almost all of them fit easily within the
confidence intervals predicted for sixth instars. However, three adult
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67
females (two giving birth to laboratory-reared litters, and the immature
specimen molting twice in the laboratory) fall withi n the lower limits
of the seventh instar. Although it is possible that these three females
are indeed seventh i nsta rs, I consider that they are actually large
sixth instar adults based on the following observations. Young of
diffe rent litter s of V_. bilineat us bo rn in the laboratory w ere
significantly different in size (Analysis of Variance gives an F value
of 6.03, d = 0.05). Young of one litter were shox»7n to be significantly
larger than the others (by Duncan's mutliple range test; see Table 8)
and extrapolation from the average size of second instars to adult
(using the growth factor obtained from this litter) shows that a
specimen of this litter could easily fall into the predicted seventh
instar confidence intervals after only four mol ts. I hypothesize that
the specimens in question came from litters of large individuals and
represent sixth instar adults.
Only two adult males were available. These w ere determined to
be mature by dissecting for hemispermatophores, and their size fell
w ithin predicted confidence intervals for the sixth instar.
In concl usion, maturity in V. bilineatus is reached at the
sixth instar for both males and females based on available laboratory
observation.
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Table 8. Results of Duncan's multiple range test to determinesignificance of size differences between litters of Vaejovis bilineatusAny two means not underscored by the line are significantly different.The number of the litter corresponds with the females in Table 3; nis the number of individuals of each litter measured; x is the meancarapace length of individuals w ithin the litter; s is the standarddeviation of that mean .
n= 16 n = 18 n = 15 n = 11 n = 26
X = 1.37 X = 1.41 X = 1.41 X = 1.42 x = 1.52
s = 0.034 s = 0.025 s = 0.042 s = 0.037 s = 0.029
litter #2 litter #3 litter #5 litter #4 litter H
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GENERAL DISCUSSION
Tests of Hypotheses
Laboratory Test
In the introduction I presented an equation which enables prediction
of the number of molts to maturity of any scorpion, provided adults
and immatures (of kn ow n instar) are available. Based on litters of
two s peci es, Centruroides vittatus and Vaejovis bilineatus , born in
the labora tory , I hypothesized the number of molts required by those
species to reach maturity.
For Centruroides vittatus I hypothesized that there should be six
molt s (and seven insta rs) in the life cycle. The best test of that
hypothesis would have been the rearing of litters to maturity where
the numbe r of instars could have been counted. Unfortunately, no
specimens from the three litters have yet reached the adult stage, and
indirect met hod s w ere used. Analysi s by the indirect method indicates
that females of C_. vitta tus m atur e at the seventh instar and that
males do so at the sixth or seventh. That males mature at two instars
cannot be predicted at the onset of investigation since size ranges of
instars are unkno w n at that time. If sixth instar males are used in
the formula in the place of the adult female measurement, then the
resulting number of molts from the formula would reflect six instars
to adu lt. The ref ore , the formula is strongly supported by the evidence
obtained from the indirect meth od. Because three specimens of £.
vittatus remain a liv e, evidence from the direct method may soon be
available.
69
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For Vaejovi s bilineat us I hypothesized five molts (six instars)
in the life cycle . Aga in, direct methodology was not possible and
the number of instars w as calculated by the use of the indirect method.
The hypothesis w as again supported rather strongly by the results
obtained. Both males and females fit 95% confidence intervals for
sixth in star s, and this w as the same value as predicted.
Test b^; Comparison w ith Literature Data
Measure ments from published life histori es, where available,
have been tabulated (Table 1 ) . For each structure the measurements
of those studies were substituted into the formula and a value of N
is predi cted. These calculated values can be directly compared to
the observed or inferred values of N as reported by the studies.
Only measure ments of adult females were used in the prediction formula,
and the number of mol ts calculated is relevant primarily to females.
Wher e a number of different measurements were given in publications
the standard mea sure ment s as used in this study (carapace length , chela
length, and metasoma V leng th) w ere chosen for the table. If the
standard measure ments w ere not w ere not available other structures
w ere used. For each structure a value of N was obtained, and from
these values for a single species an average N was calculated. These
values of N w ere compared w ith the observed values using a paired
T-test (Sokal and Rohlf 1969). The observed values did not differ
significantly from the predicted ones (t = 0.739; df = 17; a = 0.05
level) . Therefore, the predictive power of the formula is
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substantiated by previous observations and can be useful in future
studies, both in the laboratory and in the field.
Use of the Formula
In the use of the formula there are several considerations.
First , structures w hich show allometric sexual dimorphism, such as
metasomal segment V in Centruroi des, should be used very carefully
in predic tions . The predicted number of molts w ill be accurate for
one sex but w ill show considerable error in the other depending upon
the degre e of allometry involved. Second, structures that show
ontogenetic allometry (i.e., grow noticeably faster or slower than
others, or show sharp declines or increases in growth at the late
•instars) should also be carefully analyzed. An example of this is
carapace length in Centruroides. As observed in C_. gracilis (and
partially in C _, vitta tus) the average grow th factor decreases at the
late instars and drops to about 1.25 (and sometimes less). In my
predictions of instar number based on carapace length for C_. vittatus
(Table 2) , all calculated values of N w ere lower than for chela and
meta soma V leng th (N = 4.23 as compared to N = 4.77 and 5.32,
respectively). In Vaejovis bilin eatus , where growth of the various
structures w as si milar, the predictions were very good. In light of
this, it is good to obtain an average value for N from several structures
in the final formulation of the hypothe sis.
It is important to note that variation in the values of N (in
the special cases outlined above, as well as in general prediction)
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w ere favored for the following reasons: (1) difficulty in duplicating
field environme nts in the laboratory, including food intake and
temperat ure; (2) presumed differences in growth between field and
laboratory speci mens; (3) presumed differences in duration of intermolt
perio ds; and (4) presumed inhibition of ecdysis under laboratory
conditions. At the same time, they realized disadvantages of the
field method: Grow th, matur ity, and ecdyses of individuals is not
observed and can only be estimated based on population averages.
I must concur that many parameters of the life history occurring
in natural popu lati ons, particularly those of daily, seasonal, or
generation to generation occurrence cannot be accurately determined
in the laboratory. At the same time, certain parameters of life
history are more accurately determined in the laboratory, and should
be determined thus. It is these param ete rs x rith w hic h this contribut ion
is primarily concerned.
The estimations of these parameters in the field differ somewhat
from laborat ory r esu lts. The growth factor obtained in the field is
based on presumed population averages and is subject to error,
depending on the accuracy of the size classes considered to represent
instars. Due to large varia bilit y in size of scorpions (even between
litters at b i r th ) , grouping of scorpions from field data into size
classes is risky because of inevitable overlap between groups. The
error can be compounded in the calculation of the growth factor, which
has been done in the two field studies of ?_ , mesaensis.
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Fox (1975) recognized eight instars for P. mesaensis (eight size
classes) but did not calculate growth factors. As I have already
shown, the estimated growth factor is relatively high between the
second and third instars , and declines steadily until it reaches
1.17 betw een the seventh and eighth instars. The latter value is
obviously low'w hen compared to other scorpions, particularly when it
is realized that this value is the presumed population average at
that molt . Polls and Farley (1979) recognized only seven distinct
size classes, or seven instars, and calculated growth factors for
them. As may be expected, these growth factors were higher than
those obtained from Fox's (1975) data, ranging from 1.30-1.58
(average 1.38). Therefore, from two field studies on the same
species, there are conflicting interpretations. Because the two
studies did not use the same structural measurements, the relative
sizes of adults between the two studies cannot be determined. It
also seems unlikely that geographical variation causes this disparity.
Fox studied populat ions from Palo Verde (Riverside Co.) , California
and Mohawk (Yuma C o . ) , Arizona, whereas Polls and Farley studied
populations not far away (approximately 110 miles from Palo Verde),
near Palm Springs (Riverside C o . ) , California. All three localities
are located centrally within the known distribution of _P. mesaensis
(Francke, personal communica tion). I find it doubtful that populations
so close together could produce conflicting data.
An immature specimen of P. mesaensis from near Indio (Riverside
Co.), California was collected as a second instar and has molted three
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times to date in this laboratory. The exuviae and specimen have been
measured and growth factors calculated for five structures (including
those of field studies). These measurements and growth factors are
presented in Table 9. The overall average growth factor was 1.30
(range 1.23-1.38) w hich is intermediate betw een the two published
studi es. Even thoug h this observat ion is based on only one specimen
(and fifteen growth factors) , it is obvious that the growth factors
calculated here and those of the two field studies are different.
Structures measured by Polls and Farley (1979), other than total
length, are not the standard ones used in systematics, and therefore,
are hard to compare w ith other measurem ents. Problems are inherent
in two of the measure ments selected. The first of these, distance from
the edge of the prosoma to the medial ocelli , produced an extremely
large grow th factor of 1.58 betw een the first and second instar. It
has been previously noted (Pavlovsky 1924 ; Laurie 1896) that the
medial ocelli shift posteriorly in other species and that this growth
is not indicative of grow th in later instars and other structures.
Secondly, total length (as measured with a millimeter ruler) is subject
to error due to stretching of the intersegmental membranes of the
mesosoma in well- fed specimens. For these reasons these measurements
will be excluded from remaining calculations in this study.
Because a laboratory growth factor was obtained and adult females
of sizes com parable to those of the field studies are available, the
number of instars w as calculated using the prediction formula. The
average value of N using the standard measurements (carapace length.
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Table 9. Measurements and growth factors of five structures
from an immature Paruroctonus mesaensis collected near Indio(Riverside Co.) , California. "X" denotes measurement of theinterlateral ocellar distance, and "Y" denotes that from the edgeof the prosoma to the medial ocelli.
2nd instar exuvia
Growth factor3rd instar exuvia
Growth factor4th instar exuvia
Growth factor5th instar specimenAverage growth factor
Carapacelength
2.11
1.262.651.253.301.364.481.29
Chelalength
3.50
1.274.431.255.551.337.381.28
MetasomaV length
2.13
1.262.681.323.551.344.751.31
X
1.30
1.231.601.282.041.292.631.27
Y
0.81
1.351.091.201.401.381.931.34
Overall average growth factor = 1.30
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chela l engt h, and metasoma V length) w as 5.74 (range 5.40-6.00). These
resul ts ar e tabul ated (Table 10) and values of N for structures reported
by Polls and Farley (1979) are presented there. It appears that P_.
mesa ensi s re aches maturity at the eighth instar, which agrees with the
results of Fox (1975). The disparity of the growth factors calculated
from Fox (1975) and from this contribution are probably attributable
to error in estimating the boundaries of the size classes of the former
study.
To the obvious argument that growth rates may differ in the
laboratory and the field, one bit of evidence is available for
discussion. The male Centruroides vittatus from Kinney Co. , Texas
that wa s collected w ith its exuvia gave the following growth factors:«
carapace growth factor, 1.22; chela growth factor, 1.27; metasoma
V growth factor, 1.61. The first two values were the same as observed
in the litters of C_. vitt atus reared in the labo ratory, and the latter
value is the allometric growth in metasoma V that has been observed in
the labora tory (in C_. gracil is by Fra nck e, unpublished data). The
question still rem ain s, but this bit of evidence suggests that growth
rates in the laboratory and in the field do not differ, and that more
evidence is at least obtainable.
In conclusion the majo r differences between laboratory and field
studies is in interpretation. Both methods have distinct advantages,
and excellent results have been and can be obtained from both.
Depending upon the questions being ask ed, one method may be favored
over anot her: but it w as and is my purpose to show that determination
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Table 10. Measurements of adult females of Paruroctonusmesaensis from the 0. F. Francke collection. For each structureof each female, a value of N (obtained from the predictionformula) is given. The other variables in the formula are from
the second instar exuvia and growth factors from succeeding moltsof the immature specimen from Indio, California (Table 9 ) . Agrand average value (N) for the number of molts is given for thefirst three structures.
1.
2.
3.
4.
5.
Carapace lengthN value
Chela lengthN value
Metasoma V lengthN value
Interlateral ocellarN value
Edge of prosoma toN value
distance
medial ocelli
Fem. 1
8.505.54
15.406.00
10.405.875.055.683.755.23
Fem. 2
8.215.40
14.355.72
10.005.735.055.683.555.05
Fem. 3
9.055.79
14.855.85
10.205.805.305.883.905.37
Avg.
8.595.58
14.875.85
10.205.805.135.753.735.22
Grand average value of N = 5.74
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of the nimber of instars to maturity in scorpions, and of growth
rates betw een those instars , is more accurately done in the laboratory
because individuals are actually observed. With this in mind, studies
should be attempted in which both methods are used in elucidating
the details of the life history of scorpions
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SUMMARY
Tw o basic met hod s of determining the number of instars in the
post-bir th development of scorpions were recognized: the direct method
and indirect method. The direct method requires that scorpions be
reared to mat uri ty, the better metho d. Indirect methodology is of
two types : laboratory and field. The laboratory indirect method
w as developed for instances in which scorpions w ere reared but failed
to yield complete life histories. The number of instars to maturity
is obtained by extrapolation and subsequent comparison with adults
collected in the field. The field method requires estimation of size
classes in natural populations but is not as effective as the previous
methods.
A n ew indirect method for determining the number of instars to
maturity was presente d, A formula, derived from previous growth
equations (Huxley 193 2; Teissier 1960) allows the calculation of the
number of instars and may be expressed as follow s:
_ log A - log Y^ " log G
where A is the dimension of a structure in a known adult specimen, Y
is the dimension of the same structure in an immature specimen of
known age , G is the growth factor, and N is the number of molts
required for the immatu re specimen to reach the size of the adult.
Iti scorpio ns females are sometime s collected p regn ant, or with first
or second instars on their bac k s, mak ing the formula a practical one.
80
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SI
The formula w as tested by rearing two species, Centruroides
vittatus (Say) and Vaejovis bilineatus Pocock in the laboratory and
by comparisons w ith data from the literature. In each test the formula
was found to be highly accurate.
In addition to the number of instars typical life history
information for the two reared species is given. Growth in C ,
vittatus w as found to be quite similar to that of £. gracilis.
Carapace grow th declines w ith age and growth in metasomal segment
V is found to show drastic allometric sexual dimorphism at the
maturation molt . Sexual dimorphism wa s noticeable in specimens at
the fifth instar, but the growth factor showed a drastic increase
only at the matura tion molt . Chela growth also showed slight
indications of allometric sexual dimorphism based on field-collected
adults. Because no specimens as yet have reached sexual maturity
in the laboratory, the indirect method was used to predict that
male s of C . vit tatu s mat ure at the sixth or seventh instar and that
females do so at the seventh.
Grow th in V, bilin eatus v/as found to be rather constant in all
three structure s. No distinct sexual dimorphism was noticed in any
of the struc tures , and Indirect methods indicated that both males and
females matu re at the sixth instar.
The validity and predictive pow er of the formula is discussed,
and comment is made concerning choices for values of G. From
measure ments and grow th factors presented in laboratory studies an
average grow th factor (1.29) for all scorpions wa s calculated. This
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growth factor was used for all predictions and was considered more
effective than Przibram's rule.
Observed discrepancies in field reports for one species,
Paruroctonus mes aen sis , are discussed and compared w ith laboratory
life history data on the same species. The field method was determined
to be less relia ble in estimation of growth factors and the number of
instars to matu rity . Because laboratory methods enable direct
observation of ecdyses and growth of individuals, this method gives
better accuracy than field methods and should be used when possible
in determining these paramete rs.
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