Distoleon tetragrammicus (Fabricius, 1798)

Acevedo, Fernando, Monserrat, Víctor J. & Badano, Davide, 2013, Comparative description of larvae of the European species of Distoleon Banks: D. annulatus (Klug, 1834) and D. tetragrammicus (Fabricius, 1798) (Neuroptera, Myrmeleontidae), Zootaxa 3721 (5), pp. 488-494 : 491-493

Distoleon tetragrammicus (Fabricius, 1798)

Examined specimens. Italy: Veneto, Bovolone (Verona), V.2010, F. Sanna leg., 1 third instar larva; Liguria, Pompeiana (Imperia), VII.2010, D. Badano leg., 1 third instar larva; Sardinia, Berchidda (OT), VII.2010, M. Verdinelli & S. Cossu leg., pitfall, 1 third instar larva; Sardinia, Alghero (Sassari), Capocaccia, IX.2010, D. Badano leg., 1 third instar larva; Tuscany, Elba, Portoferraio (Livorno), IX.2010, L. Forbicioni leg., 1 third instar larva; Liguria, Perinaldo (Imperia), VII.2011, D. Badano leg., 3 third instar larvae; Val d’Aosta, Aymavilles (Aosta), Pont d’Ael, VIII.2011, D. Badano leg., 1 third instar larva; Lazio, Rocca Priora (Roma), X.2011, M. Gigli leg. 1 third instar larva; Liguria, Cipressa (Imperia), I.2012, D. Badano leg., 4 third instar larvae; France: Gard, Générac, VIII.2011, D. Badano leg., 10 third instar larvae; Greece: Corfù, Kato Pauliana, V.2012, D. Badano leg., 1 third instar larva.

Description. Size. Average body length 10.60 mm; head capsule length 3.00 mm (min–max 2.41–3.33), head capsule width 2.45 mm (2.22–2.72), mandible length 2.54 mm (2.24–2.76), ratio head capsule width/length 0.82, ratio mandible length/head capsule length 0.85.

General coloring. Brown with a darker pattern (Fig. 10), ventral side paler with a dark brown marking (Fig. 14), head dark brown, lateral and ventral sides with extensive dark markings (Figs. 10, 11); mandibles dark brown (Fig. 11).

Head. Longer than wide; external margin of the labrum with a small median incision (Figs. 10, 11); antennae longer than the eye tubercle (Fig. 11); eye tubercles prominent; mandibles relatively robust, shorter than the head capsule, equipped with 3 pairs of equidistant teeth of which the apical pair is the largest (Figs. 11, 15); 1 seta between each pair of teeth, few (3–4) setae between the basal tooth and the insertion of the mandible (Figs. 11, 15); labial palpi dark, apical segment longer than the others (Fig. 16).

Body. Elliptical in shape, covered by black setae and provided with thoracic scoli (Figs. 10, 14).

Thorax. Pronotum thickly covered by short setae; mesothoracic spiracle raised on tubercle, subcylindrical (Fig. 12); thoracic scoli prominent, especially the anterior pair (Fig. 12).

Legs. Pale in color, often yellow, covered by dark setae (Figs. 14, 17).

Abdomen. Dorsal side with a median series of dark circular markings with central pale area, creating an annulated pattern (Figs. 10, 13); abdominal spiracles brown, slightly raised; VIII sternite provided with odontoid processes (Fig. 18); IX sternite with a ventral-posterior pair of spiniform setae; rastra prominent, armed with 4 pairs of sub-equal digging setae (Fig. 18).

Biological and behavioral notes. The larvae were collected in Mediterranean woods and shrublands, at the base of trees and under stones. D. tetragrammicus larvae are relatively common on rock escarpments and under rock overhangs, even inhabiting artificial structures, as they have been observed on stone walls and in deposits of detritus in the corners of concrete buildings. During the inspection of an arenaceous escarpement in Générac, France, 15 1st instar larvae were detected buried in close contact in a very small recess, surely representing the oviposition site of a female specimen. These larvae did not show any sign of cannibalism or aggressiveness among them. A similar behavior is reported for the larvae of Neuroptera Ascalaphidae , which rest on the stem where the oviposition occurred for a short period, before dispersing.

FIGURES 10–18. 3rd instar larva of Distoleon tetragrammicus (Fabricius, 1798) ( Italy, Liguria: Pompeiana). 10. Habitus, dorsal view; 11. Head, dorsal view; 12. Detail of thorax, dorsal view; 13. Abdomen, dorsal view; 14. Habitus, ventral view; 15. Head, ventral view; 16. Detail of palpomeres; 17. Thorax and abdomen, ventral view; 18. Detail of VIII and IX sternites, ventral view.

Discussion

The third instar larvae of the genus Distoleon are distinguished by the following characters: mandible equipped with 3 pairs of teeth, pronotum with a thick covering of large setae interspersed with spiniform ones, mesothoracic spiracle protruding on tubercle, VIII sternite armed with odontoid processes, IX sternite provided with prominent rastra bearing 4 pairs of digging setae comparable in size. Both the examined species present in the mandible 1 seta between each pair of teeth, few (4) setae between the basal tooth and the insertion of the mandible, although 2 setae between the basal and the second tooth are reported by Satar et al. (2006) for D. tetragrammicus .

The larvae of the two European species of Distoleon are noticeably similar in morphology, however they are easily set apart by their distinctive body coloration and pattern. D. tetragrammicus is overall dark brown and the annulated pattern of the dorsal side of the abdomen is a diagnostic character of all larval stages while D. annulatus is yellowish, sand-like, and the median stripe on the dorsal side of the abdomen is simply composed by a series of dark spots. The head pigmentation is distinctive in both species: D. tetragrammicus is dorsally unmarked except a pair of small spots in the occipital area while the ventral side is covered by extensive dark markings, on the contrary D. annulatus is characterized by the presence of a pair of large dark triangular markings on dorsal side and the ventral side is very pale and unmarked, except a dark area at the insertion of the mouthparts. The coloration patterns of the ventral side of the abdomen is noticeably different in the two species, in particular D. annulatus shows a pair of dark spots covering the odontoid processes that are absent in the other species. Finally, the larva of D. annulatus is slightly smaller than the congener.

D. tetragrammicus is a common species in Mediterranean Europe. It shows a considerable ecological plasticity and its larvae are potentially found wherever a dry and fine substratum, suitable to dig, is present. The larva of D. tetragrammicus probably represents the first non pit-building antlion discovered, as the observations of Bonnet are assignable to this taxon (Bonnet 1780; Réaumur 1742). Nevertheless the first accurate description was realized much later by Brauer (1854) who also treated the internal anatomy (Brauer 1855). This species was also included in the works of Hagen (1873) and Redtenbacher (1884). Steffan (1975) was the first to investigate ecological requirements, reporting the presence of this species in sandy soils with a rich component of humus and in alluvial deposits; the present study confirms the observation of the above mentioned author. Finally, Satar et al. (2006) described the third instar larvae and the eggs.

On the contrary, D. annulatus remains poorly known. This species is rarer and with a significantly narrower distribution in Europe than the previous species, being exclusively reported for Spain, Sicilian islands (Pantelleria, Vulcano, Lampedusa), Crete and Dodecanese islands (Aspöck et al. 1980; [Bernardi] Iori et al., 1995; Monserrat & Triviño, 2013). The ecology of this species is inadequately known, however it is probably associated with rocky habitats, as suggested by the actual findings.

The larvae of the species of the genus Distoleon are appreciably similar to the other genera included in tribe Nemoleontini by Stange & Miller (1990) such as Creoleon Tillyard, 1918 and especially Neuroleon Navás, 1909 (Steffan 1975; Devetak et al. 2010; Gepp 2010; Badano 2013), thus a detailed study of the larvae of the members of this tribe appears necessary to investigate the internal relationships in this complex group of Myrmeleontidae .