Neuroleon microstenus

Devetak, Dušan, Lipovšek, Saška & Pabst, Maria-Anna, 2010, Larval morphology of the antlion Neuroleon microstenus (McLachlan, 1898) (Neuroptera, Myrmeleontidae), with notes on larval biology, Zootaxa 2428, pp. 55-63 : 56-62

First instar larva of Neuroleon microstenus

Body colouration: head black with shiny black jaws, ventral side of the thorax and abdomen whitish straw, dorsal side light brown.

Size: body length (including mandibles) 4–4.5 mm, body width (widest part) 1.5 mm, head length (without mandibles) 0.9–1 mm, head width 0.9–1.2 mm, length of the mandibles 1–1.1 mm.

Head. Anterior margin of the head is weakly sinuate ( Figs. 1, 2 ). The head surface is covered with numerous dolichasters and scarce plumose hairs ( Figs. 3, 5 ). All dolichasters occurring in N. microstenus are grooved longitudinally. On the frontal margin of the head, between the mandibles, there are six longer dolichasters arranged in two groups and close to them there is a pair of campaniform sensilla ( Fig. 3 ). Antenna ( Fig. 6 ) about 2.5 times longer than basal width of mandible. The number of antennal segments varies from 15 to 17. On the distal part of the last flagellomere sensilla basiconica are found ( Fig. 8 ). Shiny black mandibles are equipped with three teeth ( Fig. 4 ). Distance between tooth 1 and 3 is longer than greatest mandibular width. Tooth 3 is longer than 2 which is longer than 1. Mandible typically with 2 black longitudinally grooved bristles (sensilla chaetica) between mandibular base and tooth 1, 1 bristle between tooth 1 and 2, and 1 bristle between tooth 2 and 3. No bristles between the third tooth and pointed end of mandible. Bristles on outer margin of mandible extend from its basal region to the level of tooth 3. Mandibular bristles significantly shorter than tooth 3. Mandibles and maxillae bear a great number of contact chemoreceptor organs—sensilla coeloconica ( Figs. 9–11 ). The sensilla are set on the floor of relatively shallow depressions in the cuticle and are scattered in the whole length of the jaws including mandibular teeth. The labial palps are four-segmented. On the tip of the labial palps sensilla basiconica occur ( Fig. 12 ). Eyes consisting of seven stemmata are borne on prominent eye tubercles ( Fig. 6 ). Six stemmata of each eye are grouped on the dorsal anterior side of the tubercle, the seventh, rudimentary stemma is positioned on its ventral side ( Fig. 7 ). On eye tubercle, several dolichasters are present.

Thorax. Prothorax has fine granular sculpture with longitudinally grooved dolichasters and plumose hairs scattered on its dorsal side ( Fig. 13 ). Between pro- and mesothorax a pair of ungrooved sensilla trichodea occurs. The spiracles of mesothorax are equipped with marginal dent-like structures and are situated laterally on prominent tubercles ( Fig. 14 ). Paired segmental extensions—scoli—equipped with tufts of long, longitudinally grooved bristles (sensilla chaetica), laterally fringe the meso- and metathorax and abdominal segments ( Figs. 15–17 ). The bristles are classified as dolichasters. Wrinkled cuticular surface of thorax and abdomen is rich in bristles and plumose hairs ( Fig. 18 ). Legs are covered with longitudinally grooved bristles ( Figs. 19–21 ). The tip of the tarsus of all legs has two claws. On the tibia of fore leg and on the distal part of the tarsus of hind leg campaniform sensilla are found ( Fig. 22 ). Between the claws two short sensilla trichodea occur ( Fig. 23 ).

Abdomen. Sternite 8 bears a pair of small submedian teeth ( Fig. 24 ). Sternite 9 is equipped with stout and pointed fossorial or digging bristles ( Fig. 25 ). On the distal part of the sternite there are two bulges, each of them bearing four digging bristles. On each bulge, the most lateral digging bristle is the longest one and the median is the shortest. All digging bristles are longitudinally grooved. In the proximal part of sternite 9 there is a pair of campaniform sensilla ( Fig. 24 ).

Behaviour: In laboratory conditions, larvae were fed on aphids. When larvae are put on sand surface, they move only forwards, whilst in sandy substrate they move backwards.

Second and third instar larvae

Morphology of second instar is very similar to third instar, but differ in body size.

Body colouration: head brown to reddish brown with black or dark brown pigmentation on dorsal side of the head forming “V” mark ( Fig. 27, 28); jaws shiny black; ventral side of the thorax and abdomen light brown, dorsal side brown. Size of second instar larvae: body length (including mandibles) 8–10 mm, body width (widest part) 2.5–3 mm, head length (without mandibles) 1.5–1.6 mm, head width 1.4–1.5 mm, length of the mandibles 1.5–1.7 mm. Size of third instar larvae: body length (including mandibles) 10–13 mm, body width (widest part) 3–3.5 mm, head length (without mandibles) 2–2.2 mm, head width 1.8–2 mm, length of the mandibles 2–2.1 mm. Anterior margin of the head and head surface are covered with numerous bristles (sensilla chaetica); many of them have broken tip or are missing in the whole length. All bristles are grooved longitudinally. Shiny black mandibles are equipped with three teeth. Distance between tooth 1 and 3 is twotimes longer than greatest mandibular width. Tooth 3 is longer than 2 which is longer than 1. Mandible is typically equipped with 3–5 short bristles (sensilla chaetica) between mandibular base and tooth 1, 1 bristle between tooth 1 and 2, and 1 bristle between tooth 2 and 3. No bristles are found between the third tooth and pointed end of mandible. The intact bristles occur only in freshly hatched larvae; in older animals most of them are broken and often only a base of the bristle is seen. Bristles on outer margin of mandible are scarce and extend from its basal region to level of tooth 3. Inner jaw bristles are shorter than tooth 1, outer jaw bristles are shorter than tooth 3. Thoracic and abdominal morphology is like in first instar larvae. Sternite 9 is equipped with stout and pointed bristles. On the distal part of sternite 9 there are two bulges, each with four fossorial or digging bristles which are longitudinally grooved (Fig. 26). These digging bristles are longitudinally grooved less clearly than other bristles of the same larva.

Behaviour of second and third instar larvae: On sand surface larvae move backwards or forwards. They pursue prey by digging slowly backwards in sand and then whipping the head backwards to grasp prey when it is beneath the larvae. Larvae never ran forwards after prey.

Remarks on development

In the laboratory rearing, second instar larvae moulted into third instar larvae at the end of June, rarely in July or August. Third instar larvae form ball-like cocoons made of silk and covered with sand grains glued to silk. The diameter of cocoon is 7–9 mm. Pupation lasts 28–30 days. Adults emerged in July and August, rarely in June. The pupa only partly emerged from the cocoon before the adult appeared. Shortly after completion of the adult form, a single pellet of larval excrement (meconium) was deposited. This is elongate, measuring 3.5–4 mm in length and 1 mm in diameter, hard and shining.

Discussion

Although a number of species of myrmeleontid larvae are mentioned in the literature, only a small proportion is adequately described (reviews: Gepp 1984; Stange & Miller 1990). More papers describing larval morphology and biology deal with pit-building antlion species than non-pit-builders (Gepp 1984). Larvae of N. microstenus have prominent eye tubercles and sparse short bristles on the jaws. Prominent eye tubercles and sparse mandibular bristles are both characteristic for non-pit-builders, like Distoleon (Satar et al. 2006) and other Neuroleon species (Steffan 1975). On the abdomen apex of N. microstenus the two groups of digging bristles are arranged in the same way as in Neuroleon ochreatus (Navás, 1904) (Steffan 1975) . Four digging bristles on a bulge are also found in Distoleon (Satar et al. 2006) but their length differs from bristle length in Neuroleon where they are not of equal size. Black pigmentation on the dorsal side of the head forming “V” mark in N. microstenus is similar to the pigmentation pattern in a closely related species, Neuroleon nemausiensis (Borkhausen, 1791) (Steffan 1975) . Three types of sensory receptors—campaniform sensilla, sensilla coeloconica and sensilla basiconica—are recognized for the first time in antlion larvae. Campaniform sensilla are recorded on the head, legs and abdomen. It is proposed that two campaniform sensilla on the 9th abdominal sternite play a role in digging into sandy substrate because these mechanoreceptors are known to detect stretch forces in cuticle (McIver 1985; Chapman 1998; Römer 2003). Sensilla coeloconica are present on the mandibles and maxillae of Neuroelon microstenus , and sensilla basiconica were found on the tips of the antennae and labial palps. These receptors play role in detection of chemical substances important in feeding and sensing animal’s environment (Zacharuk 1985; Galizia 2008; Glendinning 2008). Koch (1983) recorded chemoreceptor sensilla on the mandibles of Euroleon and Satar et al. (2006) mentioned antennal sensilla as “three finger shaped lobes” in Distoleon . Chemoreceptors of Neuroleon are present on maxillae and both surfaces of mandible, even on mandibular teeth. These receptors play a role during catching behaviour when prey is grasped and fixed with mandibles and tip of mandibles and teeth penetrate a prey. It is suggested that antlions estimate taste of prey with these chemoreceptors.

Figures

FIGURE 25. Tip of the abdomen with digging bristles of first instar larva. FIGURE 26. Bulge with four digging bristles of third instar larva.

FIGURES 1 – 8. SEM micrographs of first instar larva of N. microstenus. 1. First instar larva, dorsal view. 2. Head, dorsal view. 3. Frontal margin of the head with six dolichasters, dorsal view. 4. Head with jaws, ventral view. 5. Frontal margin of the head, ventral view. 6. Eye tubercle with six stemmata, dorsal view. 7. Ventral surface of the eye tubercle with seventh stemma (marked with an asterisk). 8. Right antenna with sensilla basiconica on the last flagellomere.

FIGURES 9 – 16. SEM micrographs of first instar larva of N. microstenus (continued). 9. and 10. Sensilla coeloconica (see asterisks) on the first and second mandibular tooth. 11. Sensilla coeloconica on the tip of the right mandible. 12. Labial palp with sensilla basiconica. 13. Prothorax, dorsal view. 14. Mesothoracic spiracle on a prominent tubercle. 15. Ventral view of two mesothoracic scoli. 16. Scoli on the abdominal segments 6 – 8.

FIGURES 17 – 24. SEM micrographs of first instar larva of N. microstenus (continued). 17. Scolus on the fourth abdominal segment. 18. Sensilla chaetica and plumose hairs on the right side of prothorax. 19. Fore leg covered with bristles. 20. Tarsus of fore leg. 21. Hind leg with paired claws. 22. Two campaniform sensilla and paired claws of hind leg. 23. Two sensilla trichodea between the claws of hind leg. 24. Digging bristles and campaniform sensilla (see arrows) on the sternite 9.