ANTLIONS IN THE SUFFOLK SANDLINGS

57 ANTLIONS IN THE SUFFOLK SANDLINGS

Trans. Suffolk Nat. Soc. 37 (2001)

ANTLIONS IN THE SUFFOLK SANDLINGS

E. J. M. KIRBY

Any rare or unusual animal attracts disproportionate attention and, in the context of Britain, the antlion (Euroleon nostras) is both. Over the past few years it has been realised that the insect is established in parts of the Sandlings (Plant, 1998; 1999). Having seen antlions in Australia, I was curious to find out more about the local species. Because of its former presumed absence and present rarity it was difficult to find much easily accessible information and what there was often did not answer questions which quickly sprang to mind. This note describes some observations about its structure and life history made around Westleton and Dunwich during 2000.

Distribution of pits With practice antlion pits were easy to recognise; they were circular in outline, ranging from about 20 to 55 mm in diameter measured from the crest of the pit (Figure 1). They were found where there was fine sand, generally overhung by a mat of roots and top soil or vegetation (Plate 1). Such conditions were found in the abandoned sand quarries which occur throughout the Heath, in Dunwich forest in places where a bulldozer had dug a shallow trench and on roadsides where a vehicle had gouged out a part of the verge (Plate 2). High concentrations of pits were found where rabbit holes had been dug into sandy banks but which were no longer used. The quantities of fine loose sand, free of encroaching vegetation, necessary for pit making appeared to be the result of natural erosion as well as the activity of mining bees or wasps or rabbits and other creatures. In places the concentration of pits was up to the equivalent 350/m2 , for example on the floor of a large unused rabbit or fox hole. Sometimes, where they occurred along the margin of an overhang the edge of one pit abutted on that of its neighbour.

Active pits were seen from May when observations started until early in October. The presence of an antlion was occasionally confirmed by gently scooping out the sand from the centre of the pit with a 35 mm film canister and trickling it out onto the palm of a hand or by sieving. Sometimes the insect made its presence known by flicking sand out of the pit, presumably stimulated by movement near the pit. Sometimes, repeat visits to a known site revealed no active pits; this usually occurred after heavy rain which had flattened out the topography but further examination showed that antlions were present in the damp sand but were unable to reconstruct their pits until the sand had dried out. After 16 September, when 34 mm of rain fell, no pits were seen in areas where larvae has been present and after this several heavy falls of rain and the lower evaporating power of the atmosphere prevented the sand from drying out.

The search for antlion pits was in no way a serious census. Many possible sites were not visited and, where antlions were known to be present, places difficult of access were not explored but even so more than 3000 pits were seen.


Suffolk Natural History, Vol. 37 58

Typically, pits were circular in plan, composed of fine sand only (no large sand grains, bits of vegetation or remains of a previous meal). A well formed, isolated pit often had a small embankment around it. Plaster casts were made of a few pits and a cross section profile of a model pit is shown in Figure 1. Larvae put on to a large, deep tray of sand soon made a pit. The method was similar to that described by Lucas (1982) and where the sand was flicked out of the tray onto a clean surface, it was seen that the larger grains were thrown furthest.

Structure of the larva Larvae varied in size from about 7.5 to 15 mm from the tip of the jaws to tail, presumably depending on nutrition and instar. No differences apart from size where seen which might be used to characterise the different instars.

Viewed from above the larva (excluding the head) was rhomboid, widest at the third thoracic segment. In profile the body was elliptic and, when out of its pit, the head was generally held low to the ground (Figure 2).

The head was flattened and covered in hairs (Figure 3). The long curved jaws had strong spines on the inner edge and long finer hairs elsewhere. The jaws, when viewed from below, showed the outline of the (maxillae) lacinea, closely adpressed to the mandibles. The only other visible mouth parts were a pair of small palps on the underside of the head near the base of the jaws. On the upper surface of head the small antennae (about a quarter the length of the jaws) were each made up of 16 segments, the first larger than the rest and the last terminating in a short fine bristle. They were placed on the ‘corners’ of the head, just behind the jaws and forward of the eyes, each with six or seven occelli.

Figure 1: A model of a cross section of an antlion pit. The model was made using a plaster cast of a pit. The image of the antlion was inserted electronically and the angle of repose of the sand is shown.



The first thoracic segment extended into a neck so that the head with its jaws was held well forward. The neck was very flexible and if the upper abdomen were prodded the insect could turn its head around over its body and grab the offending object. The pair of legs on this segment was inserted so that they were held parallel with the outer edge of the head (Figure 3). The legs, especially the front pair had long fine hairs on all segments and each was terminated in two small sickle shaped claws.

Figure 2: Side view of antlion larva. It was removed from its pit and photographed before it started to walk backwards and bury itself in the sand. The line of tubercles with dark hairs can be seen on its flank.

Figure 3: Head of an antlion larva, photographed from below. The outline of the lacinea can be seen on the lower side of the jaws. These together with the mandibles form a tube along which the insect injects toxins and enzymes and sucks out the digested contents of the prey. Two tubercles on the thorax are visible in the bottom RH corner of the photograph.



The abdomen was soft and was generally a sandy, fawn colour, but varied among individuals from a light to dark shade. The dorsal surface had three ranks of darker pigmented spots running from the first thoracic segment to the tail, one along the dorsal line and one at either side. On either side of the body there was a ‘lateral line’ of dark tubercles, each sprouting a tuft of long, finely pointed hairs (Plate 3). These were about 1 mm long, considerably longer than the hairs on the rest of the body which were stronger and most dense on the crest of each segment and curved towards the crest. The tubercles varied in size, largest on the third abdominal segment, the widest point on the body.

Pupa The larva constructed a spherical cocoon about 15 mm in diameter

(Plate 5). At the outside the silk threads enmeshed sand grains so that the cocoon appeared as a sand ball. The wall of the cocoon was about 1–2 mm thick and its inner surface was a smooth grey silken layer. In this the larva pupated, eventually to free itself by cutting a circular hole at the upper pole of the sphere (Plate 5). It dug through the sand until its head was just emergent from the sand, when the insect underwent its final metamorphosis into the imago. No pupae were observed either in the cocoon or cutting themselves free or digging through the sand, but some idea of their structure was gained from the cast left in the sand (Figure 4). In particular the head showed that sickle shaped jaws of the larva were exchanged for stout cutting mandibles with serrated edges in the pupa (Figure 4, inset). Also a pair of compound eyes

Figure 4: An excavated antlion cocoon and pupal cast soon after the imago had emerged. To save space the picture has been turned sideways. The sand surface is at the right hand side of the page. Inset: the cast head of the pupa showing the mandibles, compound eyes and antennae.



was evident and the antennae were relatively larger and club shaped; the head was generally hairy. The cast remains of the larva were found in the vacated cocoon. The mandibles and legs could be recognised in a small, compacted pellet together with the other parts of the larva.

Imago The often mentioned resemblance of the imago to a damsel fly was clear in the resting imago (Plate 4). At rest the two pairs of wings were held closely over the body; they were finely and intricately reticulated, generally clear but with occasional dark patches, not associated with a particular cell. The head had large compound eyes and long curving club shaped antennae. The mouth parts were small and insignificant.

The thorax was relatively small with long thin legs; these were generally hairless, but with a few pointed spines. The long cylindrical abdomen was about three quarters as long as the wings so that, at rest, the wings completely covered the thorax and abdomen (this differs from dragon and damsel flies in which the wings are considerably shorter than the body, so that in the resting damsel fly, the abdomen sticks out beyond the wings).

Only four imagos were seen and the observations of the structure were made on live resting specimens and no detailed examination of mouth parts or genitalia were made. An almost daily patrol of pit sites was made from mid-July to mid-August in the late afternoon (16.00–19.00). There were in the order of 200 pits along the usual patrol route which was traversed two or three times on each occasion but only one emerged imago was found (at 17.15 h). It was clinging to the sand cliff backing the pit site and was probably still drying its wings. Over a period of about 1h it made no movement and when picked up made no attempt to fly away. When forced to fly it had a weak flapping flight, with the wings appearing to move more or less independently.

Three cocoons were taken and reburied in sand, about 50 mm deep, in a plant pot enclosed in a mesh sleeve and kept in a unheated glasshouse. In the period 30 July to 8 August all ‘hatched’ to produce an imago, but none was inspected at the time when imago emergence began, and when seen (17.00–20.00 h), had climbed about 25–30 mm above the sand surface to expand and dry their wings. At first the wings were without colour and the dark patches appeared only later.

In all cases the imago was left outside, unconfined, and had flown away by next morning.

Discussion Most attention in antlion studies has been focused on the larvae. By far the

largest proportion of the total life cycle of the insect is spent as a larva and during this phase it accumulates most of its body mass and reserves for its activity as pupa and imago. Its success in its peculiar life style relates to behavioural and morphological adaptations which enable it to detect, trap, kill and consume prey.

The behaviour of Euroleon nostras when constructing a pit is similar to that of several other antlion spp. (e.g. Lucas, 1982) and the backward progression on a circular path, flicking sand as it goes, is easily observed.



Once the pit is complete and the larva is concealed at the base of the inverted cone it is clearly aware of what is going on around it. Small disturbances of the sand outside the raised edge of the pit, provoke the larva to flick sand about and Mencinger (1998) has shown that it can locate the direction of the movement of prey species and throw sand towards the potential prey with considerable accuracy, raising the question of how it senses the proximity of other small creatures. It is unlikely to be able to see them because, first, the eyes may be under sand and, secondly, the source of movement may be over the horizon of the pit. By analogy with other sand dwelling animals it is likely that it can detect vibrations in sand. Vibrations caused by small animals can be propagated through sand (Brownell, 1977) and, for example, in sand dwelling scorpions such vibrations are detected by hairs on the legs and other specialist organs (Brownell & Farley, 1979).

Therefore, the hairs on the antlion’s body and legs may have sensory functions. There are, however, three or more types of hairs on the larva: long fine hairs on the legs, short stiff curved hairs on the body generally and long, finely pointed hairs associated with tubercles on each segment. The long hairs on the legs may have a sensory function. The appearance and distribution of the hairs in the tubercles may also suggest a sensory function. The tubercles form a lateral line along the body (Figure 2), a common arrangement for mechanoreceptors in many organisms such fish and amphibians which can detect water movement, serving to perceive and locate prey, predators, or animals of the same species A similar function might be performed by the antlion’s lateral line of hairs, enabling the larva to detect and track prey as it enters the pit and slides swiftly towards it.

Some hairs may have other functions; Plant (1999) considered that the abdomen “adorned with forward pointing bristles” prevented the larva from moving forwards and Lucas (1982) has suggested that the fore legs, which are held close to the head (Figure 3), may be used to sift sand around the head, prior to throwing it towards the margin of the pit and the hairs on the forelegs may assist in this function. Physiological or anatomical studies would elucidate the way in which the antlion is aware of the world around it.

Other morphological adaptations are more obvious; the spines on the inner edge of the jaws enable the insect firmly to hold its prey and sharp curved points puncture the prey’s body and inject digestive fluids. The speed at which, for example, woodlice were subdued suggested that the injected fluid probably contained toxins as well as digestive enzymes.

The ecology of the antlion lava is completely dependent on the mechanics of sand. Pit construction depends on the presence of fine, loose, dry sand with uniform particles. When fully prepared the pit walls are at the angle of repose characteristic of the sand (Figure 1). This is a just-stable condition where any disturbance provokes an avalanche (Jaeger & Nagel, 1992). Thus potential prey, straying over the lip of the pit, disturbs the equilibrium of the sand. This sets off an avalanche which activates a layer of sand only a few grains thick so that the prey slides uncontrollably into the bottom of the pit where the antlion awaits.



The change in the mechanical properties of sand when it becomes wet (the sand castle effect) may limit the location of suitable sides for pit building. In general, pits were found where there was some shelter from rain either from an overhang or in a disused burrow. Exposure to heavy rain both battered down the pit and immobilised the larva.

In this study few observations were made on the pupal stage and those only on the cast skin of the pupa. The pupa undergoes a remarkable transformation in the structure of the mandibles compared with those of the larva. The long curved jaws are replaced by two serrated blade shaped mandibles whose only function appeared to be to cut a hole in the cocoon to release the pupa before its short journey to the sand surface where a further metamorphosis to the imago takes place.

Little of the remainder of the life cycle of the insect was observed. After expanding and drying its wings the imago remained stationary, well camouflaged by the wings folded over its body. If provoked, it had a weak fluttering flight quite different from that of a damsel fly. It remained motionless until darkness and in all cases had vanished by next morning, consistent with the report that mating takes place at night possibly in the upper branches of pine trees. An absolute requirement for pine trees (Yasseri & Parzefall, 1996), however, seems questionable. At only one site where pits were found was there pine trees nearby and the majority of sites were on the heathland with only birch or oak trees in the vicinity.

Conservation and unanswered questions The brief study described in this paper provided an insight into the structure and autecology of this fascinating insect apparently unique, in Britain, to a small part of East Suffolk. It and related species are common in Europe, the USA, Australia and other parts of the world where they are a subject for biological research, a source of amusement for children, an inspiration for artists and other aspects (many of which are described in the website www.antlionpit.com). Its presence in Suffolk poses interesting questions about how long it has been here and how stable the population is and provides opportunities for finding out more about its structure and life cycle.

Despite occasional reports during the last 300 years it is not clear that Euroleon nostras has been present throughout the period (Plant, 1999). It is feasible that it could have remained undetected, for the larval pits can easily pass for rabbit paw marks to the casual passer-by and even experienced naturalists might miss them, for example, after rain and the imago is only transient and needs some persistence or good luck to see it. It is clear, however, that Euroleon nostras has been firmly established in the area around Dunwich and Westleton for the last five years or so. Indeed the present study seemed to indicate that it could be found almost anywhere the habitat was suitable.

Of the necessary factors, East Anglia provides low rainfall and sunny conditions which maximise the time when the condition of sand is suitable for pit making and the Sandlings has many areas with deposits of free draining, fine sand. Another element which seems necessary for successful colonisation



is the presence of shelter, which is often provided by the overhanging top soil and plant roots at the margin of an excavated area or disused burrows into a sandy bank. Therefore to some extent the provision of suitable habitats depends on man’s intervention as, for example, a well populated site in Dunwich forest which was the result of a recent bulldozer scrape. Other suitable habitats may be provided on the root plate of an uprooted tree (Plant 1999). None of these sites may be permanent; they may be overgrown, the sand may be washed off (root plates) or quarry edges may be broken down by red deer which like to prance about on precipitous slopes.

Of the habitat factors necessary for the imago Yasseri and Parzefall (1996) emphasise the importance of pine trees which are thought to be essential assembly points for mating. As many sites in the Westleton area were distant from any pine trees it would be interesting to find out about the mating and feeding behaviour of the imago and if the presence of pine trees are essential or if suitable conditions can be provided by, for example, birch and oak trees.

At the end of summer some larvae pupate and produce an imago, while others continue for another season (Plant, 1999); the imagos mate and lay eggs all within the space of a month (Yasseri & Parzefall, 1996). Little seems to be known about the development of the eggs, whether they hatch immediately or go into some form of diapause over winter. The non-pupated larvae will not be able to feed over the winter; as the season progresses the sand will take longer to dry out and apart from the surface layer, will remain at field capacity and larvae, either newly hatched or non-pupated will be immobilised and unable to construct pits until the sand dries out in the following spring. How the larvae survive over the winter poses an interesting question.

References Brownell, P. & Farley, R. D. (1979). Detection of vibration in the sand by

tarsal sense organs of the nocturnal scorpion, Parurotonus mesaensis. Journal of Comparative Physiology 131: 23–30.

Brownell, P. (1977). Compressional and surface waves in sand: used by desert scorpions to locate prey. Science 197: 479–481.

Jaeger, H. M. & Nagel, S. R. (1992). Physics of the granular state. Science 255: 1523–1531.

Lucas, J. R. (1982). The biophysics of pit construction by antlion larvae (Myrmeleon, Neuroptera). Animal Behaviour 30: 651–664.

Mencinger, B. (1998). Prey recognition in larvae of the ant-lion Euroleon nostras (Neuroptera, Myrmeleontidae). Acta Zoologica Fennica 209: 157–161.

Plant, C. W. (1998). Investigations into the distribution, status and ecology of the ant-lion Euroleon nostras (Geoffroy in Fourcroy, 1785) (Neuroptera: Myrmeleontidae) in England during 1997. Trans. Suffolk Nat. Soc. 34: 69–79.

Plant, C. W. (1999). The Suffolk ant-lion Euroleon nostras. British Wildlife 10: 303–309.



Yasseri, A. M. & Parzefall, J. (1996). Life cycle and reproductive behaviour of the antlion Euroleon nostras (Geoffroy in Fourcroy, 1785) in northern Germany (Insecta: Neuroptera: Myrmeleontidae), in Canard, M, Aspöck, H, & Mansell, M W (eds.) Pure and Applied Research in Neuropterology. (Proceedings of the fifth international symposium on Neuropterology, Cairo, Egypt 1994) pp. 269–288.

E. J. M. Kirby The Studio Blythburgh Road Westleton Saxmundham Suffolk IP17 3AS

Plate 1: Antlion pits under an overhang of compacted gravel. The gravel overlays a stratum of soft sand which has eroded faster than the gravel (p. 57).

Plate 2: A roadside verge with traffic damage and overhanging vegetation. Many antlion pits were found in this area (p. 57).

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Plate 3: The tubercle on the third thoracic segment of an antlion larva. Compare the length of the tubercle hairs with other body hairs, e.g. in the lower right-hand part of the photograph (p. 60).

Plate 4: Antlion imago which emerged from a cocoon in a glasshouse. After emergence it was put on the birch twig where it quickly settled down and remained until after nightfall, when it flew away (p.61).

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Plate 5: Antlion cocoons. The two lower cocoons contained larvae or pupae. The pupa had cut an exit hole and emerged from the upper cocoon (p. 60).

ANTLIONS IN THE SUFFOLK SANDLINGS

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