MASS. of Dimor2ism. 380-43 4o-4e r@or 4o-4...

A NEW HYPOTHESIS OF SEASONAL-DIMORPHISM INLEPIDOPTERA.- II.

BY ALFRED GOLDSBOROUGH MAYER CAMBRIDGE MASS.

(2) 2Vew Hyfloesis of Seasonal-

Dimor2ism.I know of only one experiment upon

the effect of excessive heat upon Lepi-doptera, and that was performed uponthe pupae of Vanessa anli@a, byFischer (’95) who, it will be remem-bered, subjected them, when fl’esh, for

3 hours and then daily for 2-3 hours toa temperature of 4o-42 C. keepingthem at all other times at 350-380 C.The butterflies which issued resembledthose which would have resulted fl’omexposure to cold of o- C.

It has occurred to me that in thisremarkable fact we may have a clue toat least a partial explanation of theaction of cold upon seasonally-dimor-phic Lepidoptera. It is well knownfrom the researches of Dutrochet, Ross-bach, and Plateau that if organisms besubjected to gradually increasing heatthe metabolic processes as evinced byincreased excretion in protoplasm, andmore rapid rate of development, becomemore and more active; until suddenlyall movements cease and heat rigor setsin. This is not death hoveverfor if theorganism be now cooled down, recoverytakes place, and the life processes return

with normal vigor. According to Pla-teau* the temperature of heat rigor invarious insects varies between 380-43C. It is highly probable then that thehigh temperature of 4o-4e C. pro-duced eat r@or in the pupae, andtherefore the metabolic processes werechecked, exactly as they would havebeen by the benumbing influence ofcold. If this be true it becomes prob-able that the peculiar color-aberrationscaused by cold are only an expressionof the decreased mela3ol/sm in thepupae. It will be remembered thatheat of 35 C. produces an aberration inV. antzbpa which is just the reverse ofthat caused by cold. In this case thepeculiar coloration could be explained as

one of the results of the increased meta-bolism in the pupae.Now it may well be that it is an advan-

tage to a pupa which is destined to with-stand the winter’s cold to inherit a

tendency toward a low metabolism, forresistance to the cold would naturallyrequire the possession of low metabolicprocesses hence those pupae whichalready possessed low metabolism would

*Plateau, ’7; Bulletin d l’Academie royale des science,

de Belgique, xxxiv, pp. 35-37.

60 PS2CttE. [May i897.

be in better condition to withstand theeffects of the cold. Natural selectionwould then operate to weed out allpupae having high metabolic processes,for they would be more likely to fl’eezewhereas those individuals in which themetabolism was low would be pre-served. Also this inherited tendencyin the overwintering pupae .to possesslow metabolic activity might become so

strongly fixed that it would be founddifficult to alter it by the mere subjectionof the pupae to a high temperature,such as 3z-35 C. Moreover it woulddoubtless be of advantage to the insectsif they had the power to resist the in-fluence of such warmth, for there areoften hot periods of weather in theautumn through which the over-win-tering pupae must pass; but theirdevelopment must not be hastenedthereby, for if the butterflies emergedthey or their progeny would probablyperish of the cold.

Professor Weismann’s former (875’Sz) idea that in the seasonally-dimor-phic butterflies of the temperate zonethe phylogenetically older form issuesfrom the wintered chrysalids, and repre-sents the form which existed during theglacial epoch, is to my mind improb-able. His hypothesis may be true asfitr as the European Vanessa levana-prorsa is concerned, because, as is wellknown, the butterflies of Europe aremore closely related to those of Siberia,than to those of Africa or India. (SeeBath (’95)).* The cause of this lies

Bath, W. H. I895; Entomologist Vol. 8, p. z47.

in the well known fact that there hasbeen, according to Geikie, no land con-nection between Europe and Africasince the close of the glacial epoch.Moreover the deserts of Sahara, andArabia, and the snow clad peaks of theHimalayas form an insuperable barrierbeyond which tropical forms could notpass to enter the northern regions. InAmerica, however, the case is very dif-ferent, for almost 50% of all the knownspecies ofLepidoptera of the world comefl’om South America,* and there can bebut little doubt that the ancestors of mostof the North American butterflies camefl’om South America. The ancestorsof the North American forms havegradually crept in from South Americaafter the glacial epoch, and as theirrange extended further and furthernorth, they were obliged to becomeadapted to the cold, or perish. Thisadaptation would mean the acquisitionof a low metabolism in the over-win-tering pupae. In this connection it isinteresting to notice that Merrifield (’93)has shown that in England those pupaeof Pyrameis atalanla which form inthe autumn all perish of the cold. Thisinsect nsually hibernates as an imago,and is not seasonally-dimorphic.deed, the seasonally-dimorphic butter-flies, of the temperate zone, accordingto Scud(let (.Butterflies of New Eng-land, t889, p. 1384) probably all winter

* Schatz (’92) finds that there in South America 7genera of butterflies comprising 4560 species, and thatof these genera proper to South America alone. InNorth America, the other hand, there according toEdwards only 66 genera and 612 species.

May $97.] PSICttE. 61

over as ibupae. Dr. Scudder has alsoobserved, that in a few cases of seasonal-dimorphism we fitad differences betweenthe earlier and later appearing membersof the spring brood, the later membersshowing an approach toward the sum-me" lowell.

It is highly probable that the vastmajority of Lepidoptera which existedin North America before the glacialepoch, simply retreated southward uponthe approach of the ice. For to havethem remain we are obliged to assumethat not only they, but their food plants,also, became acclimated to the cold.Many of those which did remain, andsucceeded in defying the cold wouldprobably for the most part become sothoroughly acclimated to it that theywould finally refer a cold climate,and when the ice retreated they wouldprobably follow it northward leavinga few representatives stranded, as itwere, upon the tops of the highest:mountains as has been shown by Groteand, also, Scudder (’89 p. 588) tohave been the case with Oeneis semideaof Mount Washington.

In the case of t?aibilio ajax, whichwas experimented upon by Edwards,I have reason to believe that it couldnot have existed in North America dur-ing the ice period. For its nearest alliesare all in Mexico, and South America,none of them being found in the colderparts of North 2kmerica. Moreover itsfood plant, the papaw (Asimina trilo3a,Dunal.) belongs to the characteristicallytropical family Anonaceae, and cannotlive in a cold climate. This leads me

to suggest that walshii or telemonidesare not primitive forms as would be thecase were Weismann’s i875 hypothesistrue. I also predict that if the over-wintering pupae of tapilio ajax besubjected to a constant heat of from

30-35 C. they will be transformed intothe summer form marcellus.

Conclusions: In lepidoptera of thetemperate regions it is an advantage forthe summer pupae to possess high meta-bolic’processes, for nd,er these circum-stances their development is rapid. Onthe other hand it is an advantage for theoverwintering pupae to posses low meta-bolic process, for under these circum-stances they would be the better able towithstand the influence of warm periodsof weather in the autumn; for if thebutterflies emerged at this time they ortheir progeny would probably perish ofthe cold. Moreover ,in order that thepupae .may withstand the influence ofthe winter’s cold it is essential that theypossess a low metabolism. Natural selec-tion would then operate to cause allsummer pupae to inherit a high meta-bolism, while all overwintering pupaewould be forced to inherit a low meta-bolism. Pupae which possess a constitu-tional tendency toward high metabolismgive rise to the summer form of imago,while those pupae which possess a con-stitutional tendency toward low meta-bolism give rise to the overwinteredform of butterfly. The summer andwinter forms of imago are only expres-sions of this difference in constitution ofthe summer and winter pupae.

It is well known through the re-

62 /:’S [May 897.

searches of Barker,* Brandes," Butler, Dorfmeister, G. ’79.--Mittheil. des naturwis.and others that tropical butterflies sen Vereins flit Steiermark, Jahrgangexliibit seasonal dimorphism. It has 879’ p" 3. Plate.

occurred to me that this maypossibly be Edwards, W.H. ’68.-" The Butterflies ofNorth America," Philadelphia, and Bos-due to the direct influence of the vary- ton, I868.

ing hmidity upon the pupae. Pupae Edwards, W. H. ’75. raThe Canadian ento-reared in a dry atmosphere may give mologist, Vol. vii, p. 228.rise to the dry season form, while thosereared in a humid atmosphere may giverise to the wet season form. I amassured by Mr. E. A. C. Olive of Cook-town, Q3eensland, Australia that thedevelopment of pupae in that region ismore rapid in the wet season than inthe dry.

I freely admit that Weismann’s"adaptive" seasonal-dimorphism mayexist, but I believe that the explanationgiven in this paper is more probable.I hope that some of the many ableentomologists who are carrying out re-searches upon seasonal-dimorphism willtest its truth or falsity by experiment.

Literature t?ecordt’ng Temperature Exjberi-ments ujou Lebidoplera.

Dorfmeister, G. ’64. Mittheil. des naturwis-sen. Vereins ftir Steiermark, Heft II. p.99, Plate.

* Barker, C. W. Notes Seasonal.I)imorphism ofRhopalocera in Natal. Trans. Ent. Soc. Lond. x895, pp.413-428.

Brandes, G.; I)er Saison-Dimorphismus bei einheimi-schen und exotischen Schmetterllngen. Zeit. Naturw.Leipzig, 66. Bd., pp. 277-3oo, Fig. T. x894.

Butler, A. G.; Notes Seasonal-Dimorphism in Cer-tain African Butterflies. Trans. Ent. Soc. Lond. x895, pp.519-522.

Edwards, W. H. ’77. The Canadianentomologist, Vol. viii, p. 203.

Edwards, W. H. ’8o. Psyche, Vol. 3, No.69, p. 3.

Edwards, W. H. ’8. Psyche, Vol. 3, No.82, p. i74.

Edwards, W. H. ’84. The Canadian Ento-mologist, Vol. xvi, p. 3.

Fischer, E. ’95. Transmutation der Schmet-terlinge infolge Temperatirunderungen,Berlin, I895.

Merrifield, F. ’9o, ’9, ’92, ’93, ’94. Trans.ent. soc. London, 89o p. x3 89r p.55 892, P. 33 893, P. 55 x894, P.425.

Merrifield, F. ’96. Proe. ent. soe. Lond. ii,896.

Reiehenau, W. yon, ’82. Kosmos, Vol. 2,x882, p. 46.

Standfuss, M. ’96.-- Handbueh der palark-tischer gross-Schmetterlinge, Jena, 896,p. 236.

Stange, G. 86.Stettinerentomolog. Zeit-ung, 886, p. 279.

Weismann, A. ’75. Studien zur Desendenz-Theorie, I. Leipzig, x875.

Weismann, A. 82. Studies in the Theory ofdescent. Translated by Raphael Meldola,London, 882.

Weismann, A. ’95.Zoologische Jahr-bticher, Syst. Abth. Bd. 8, p. 6. Seealso translation of the same by W. E.Nicholson, The Entomologist, Vol. xxix,896, PP. 29, 74, o3, 53, x73, 2o2,240.

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