DRILINI Blanchard, 1845 

False Firefly Beetles

This unusual species is very local on chalk grassland and woodland in the southeast. Males are occasionally found in numbers in late spring and summer.

POLYPHAGA Emery, 1886

ELATEROIDEA Leach, 1815

ELATERIDAE Leach, 1815

Drilus Olivier, 1790

D. flavescens (Fourcroy, 1785)

7-11mm (♂)

25-35mm (♀)

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Introduction

This group was formerly much larger and classified as a distinct family along with other soft-bodied beetles such as Lampyridae Latreille, 1817 and Omalisidae Lacordaire, 1857 within the ‘Cantharoid’ series; in 1910 Olivier listed 20 genera and in 1944 Wittmer listed 35 as belonging to the Drilidae. Since then many eastern Palaearctic genera have been transferred to other families such as Lampyridae, Lycidae Laporte, 1836, Omethidae LeConte, 1861 and Rhagophthalmidae (a group of uncertain limits and placement) and the tribe as presently understood includes only 4 genera of Western Palaearctic and African species. Recent molecular data demonstrates a close relationship between Omalisidae, ‘Drilidae’ and Elateridae Leach, 1815 and only a distant affinity to the ‘Cantharoid’ series of families, and the drilids are placed as a terminal lineage of the Agrypninae Candéze, 1877. A defining feature of the tribe, soft-bodied males and only partially developed females, is also seen in other groups such as Lampyridae and Phengonidae, and is considered as an ancestral trait of elaterid morphology caused by interrupted metamorphosis. The tribe now includes the following genera: Drilus Olivier, 1790 with about 35 species is known from North Africa, Asia Minor to central Asia, Sri Lanka and Europe. Selasia Laporte, 1836 includes about 50 species and is restricted to mainly tropical regions of Africa. Pseudeuanoma Pic, 1901 occurs in Asia Minor and Eastern Europe while Melacogaster Baudi, 1833 is more widespread, occurring in North Africa, Asia Minor, Southern Europe and the Canary Islands; these last two genera contain only a few species but they remain dynamic as species are sometimes transferred between them. The monotypic genus Paradrilus Kiesenwetter, 1866, which includes a single European species not recorded since the nineteenth century and now probably extinct, is now transferred to Omalisidae. All four genera occur around the Mediterranean, which is the region of greatest diversity, but this quickly decreases further north; of the 20 or so European species only 2 species of Drilus occur in central Europe and only one of these (D. flavescens) extends to the U.K. So far as is known all species have a specialized lifestyle and are predators of snails.

Ecology

Adults are generally rare in collections, more especially so the females and some are known only from males. The species occur in a wide range of habitats that host populations of terrestrial molluscs, the U.K. species Drilus flavescens (Fourcroy, 1785) occurring especially on calcareous grassland and woodland, but wider afield they are most diverse from lowland to low altitude mountain woodland in central Europe, and from coastal marshland to high Mediterranean altitudes further south. In general only the males are encountered, they inhabit grassland and lower shrub layers in woodland and scrub areas and may be seen walking on exposed areas or on pathways, and during warmer parts of the day on flowers. Many species are capable of flight but this tends to be weak and is very rarely seen. Most species have a short season, in southern latitudes between March and August and further north a little later but they are usually finished by August in temperate regions. Males of all European species are diurnal and active mostly towards the evening when the temperature is decreasing and the humidity is increasing, those of many African species are nocturnal and attracted to light. Despite the males occurring on flowers and foliage they are thought not to feed.  Adult females are larviform and those of Drilus, Melacogaster and Selasia are apterous with larviform thoracic and abdominal morphology, and they often occur alongside early instar larvae foraging on the soil or inside empty snail shells. They are very fecund, producing between 300 and 650 eggs which they deposit in the soil over a short period, sometimes within a single day or placing all the eggs in a shallow pit in a single session. Early instar larvae are agile and run quickly in search of prey, and they may occur in numbers feeding in a single snail, larger larvae look like Eggar or Tiger moth larvae and each instar develops within a single snail, moving on after they have moulted, generally consuming one or two snails each year over their development which takes between one and four years. Late instar larvae of at least some species have been observed boring exit holes through the shells to leave before searching for a new host. The final instar changes form after consuming its snail and becomes soft-bodied, white and almost glabrous before pupating in the shell during the spring.

Description

Adult males are small, 2.5-11mm, soft-bodied, elongate and parallel-sided or weakly dilated towards the apex and rather flat beetles, most are dark brown to black, some are light brown or yellow (most Selasia) or rarely black with bright red markings (Melacogaster and some Euanoma). The foreparts are usually shiny and most are sparsely but distinctly pubescent. Head hypognathous and transverse with prominent convex eyes and curved slender mandibles. The antennae are 11-segmented, long and filiform to serrate or, in Selasia, flabellate and with all segments densely pubescent. Maxillary palps 4-segmented, labrum transverse, the palps 3-segmented, all palps truncate at the apex. Pronotum transverse and widest about the middle with sinuate or rounded lateral margins, posterior angles generally sharp and the basal margin sinuate, anterior margin straight or slightly concave. Prosternum long in front of the coxae (a typical elaterid feature) and without a distinct process which, considering the family placement, is unusual. Scutellum small and usually rounded. The anterior margin of the mesosternum varies; straight in Euanoma, shallowly, in Drilus, or deeply emarginate in Pseudeuanoma. Elytra flattened and generally widest behind the middle, often nearly parallel-sided, shiny and with fine erect pubescence, the surface smooth; without longitudinal costae, finely punctured and without striae, at most with irregular rows of larger punctures. Hind wings fully developed in all males, often visible through the somewhat translucent elytra. The legs are moderately long, slender and flattened, the coxae well-separated, the femora attached to the apex of robust trocanters, the tibiae smooth and usually curved, without apical spurs. Tarsi 5-segmented and slender, the segments not, or only weakly lobed.

Adult females are larger than the males; 13-35mm and larviform, elongate and rather parallel-sided to distinctly widened posteriorly (Selasia), soft-bodied and densely pubescent. The head is reduced and prognathous with small, flat eyes and 10-segmented short and robust antennae. The mandibles are long and slender, usually incurved apically and with internal teeth, the palps tiny with a slender basal segment, transverse subapical segment and pointed terminal segment. Thoracic segments larviform, often narrower than the abdomen. Elytra and wings absent. Legs very short, with widely separated coxae and tiny smooth claws. Abdomen with nine visible segments, often with lateral lobes and modified setae the ninth segment narrower and smaller than the others.

The larvae are elongate and, in all but the final form of the last instar, densely pubescent with well-sclerotized head and tergites and softer ventrites. Head small and partly retracted into the prothorax, with tiny 3-segmented antennae and narrow elongate mandibles which are channelled for sucking up liquids. The prothorax is widened posteriorly and about as long as the meso- and meta-thorax combined. Abdomen with well-developed lateral processes bearing groups of long and robust setae. The final form of the last instar is C-shaped, twisted and strongly narrowed anteriorly and posteriorly, adaptations to accommodate the snail shell, it is lightly sclerotized and without the dense pubescence seen in earlier instars, the ninth segment has two pairs of strong urogomphi bearing stiff setae.

Drilus flavescens (Fourcroy, 1785)

This is a generally scarce though often locally common species throughout central Europe from France to Italy and north to the Netherlands and U.K. where it is locally common on chalk grassland and woodland in the southeast; most records are from Hampshire, including the Isle of Wight, Sussex and Kent. Adults exhibit an extreme sexual dimorphism with larviform females and soft-bodied cantharid-like males. Females lack elytra and hind wings and so are terrestrial, living mostly in the shells of snails which they have killed with a poisonous bite and which they consume by sucking the body fluids with the help of injected digested enzymes, moving between snails as necessary. Adults appear from late spring until July or early August; males generally occur on foliage, flowers or high up on grass stems but a little later in the season may be seen wandering on pathways or on open areas of soil following the scent-trails of females, a function to which the expanded antennae are thought to be adapted. They mate end to end, often with other males in competition, and this may be a protracted process before the female moves on, sometimes with the male still attached or with other males in company, to resume feeding before oviposition begins. Males are short-lived and have not been recorded feeding. The females are very fecund, producing in excess of 300 eggs, they are deposited in shallow burrows or depressions in the ground, often among compact leaf-litter, and several sites may be chosen or all the eggs may be laid at one site over the course of a single day. The larvae are hypermetamorphic; first instars look like ‘normal’ beetle larvae and are very agile, dispersing rapidly in search of snails, when the prey is located the larva enters the shell and feeds on the snail’s tissues, at this early stage feeding there do not appear to be any toxins or digestive fluids injected into the snail. The fully-grown first instar moults inside the shell into a highly specialized second instar which has a suction apparatus on the terminal abdominal segment, it is heavily-sclerotized and pubescent, resembling superficially an eggar or tiger-moth larva. The later stage larva actively hunts for snails, upon finding prey it attaches itself to the shell and bites the snail, injecting a paralyzing toxin which will slowly liquefy the snail’s tissues, and then burrows through the tissue into the shell where it will feed and eventually moult before the next instar leaves to find another snail, larval development takes two or three years and 2-4 snails are consumed each year. The fully-grown larva, about 20mm long, enters a snail and feeds as usual but then develops into a soft, almost hairless form with reduced head and appendages and a twisted body to accommodate the shell, this secondary larva will overwinter in the shell and pupate there during the following spring, and the adult will eclose shortly after. On the continent there may be a specific association with the snail Fruticicola fruiticum (Muller, O.F., 1774) (Bradybaenidae), a species now extinct in the U.K.

Adult males are about 10mm long and entirely dark but for the orange or red elytra and are readily identified among the U.K. fauna by the strongly pectinate antennal segments. (The prosternum is long in front of the coxae, a typical elaterid feature, but the prosternal process is not developed posteriorly.) The fourth tarsomere is only weakly lobed ventrally, a feature that will distinguish Drilus from similar sized cantharids. Larvae are typical of the family, elongate and depressed with heavily sclerotized plates and lateral abdominal lobes bearing tufts of stiff spines. Females lack the larval vestiture and may be recognized from the small head, prothoracic segment and tiny legs, and the transverse and laterally-lobed abdominal segments. They are dark sandy-yellow in colour with dark and heavily sclerotized plates.

Similar Species
CANTHARIDAE spp.
     -Some species resemble D. flavescens
      males.
  • Habitus and size distinctive
  • Antennae more narrowly filiform; segments more regularly elongate.
  • Tarsi with at least third segment bilobed.
  • Claws distinct; bifid, split longitudinally, distinctly lobed at base or assymetrical.
Lampyris noctiluca
     -Resembles D. flavescens females.
  • Pronotum proportionally much larger.
  • Legs longer; femora distinctly visible from above.
  • Bioluminescent.
  • Adults and larvae generally darker.
  • Lampyris larvae lack multi-segmented antennae.

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