Snail shell beetle

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Snail shell beetle
Drilus flavescens

Drilus flavescens

Systematics
Class : Insects (Insecta)
Order : Beetle (Coleoptera)
Subordination : Polyphaga
Family : Click beetles (Elateridae)
Subfamily : Agrypninae
Tribe : Snail shell beetle
Scientific name
Drilini
Blanchard , 1845
Drilus concolor
Adult female of Malacogaster passerinii
Snail shell beetle larva
Larva of the genus Drilus on a snail

The snail-shell beetles ( Drilini) are a tribe in the subfamily Agrypninae of the click beetle family (Elateridae). Until it was reclassified by Kundrata & Bocak (2011), the group was in family rank, although the relationships were considered unclear until recently. Around 90 species in four genera are known worldwide . In Europe there are 16 species, in Central Europe there are only two with Drilus concolor and Drilus flavescens .

Characteristics of the beetle

male

The male beetles are 2.4 to 10.8 millimeters long. They have a moderately sclerotized , elongated, dorsoventrally flattened, parallel-edged body that is slightly widened at the back. They are either dark brown to black in color (genus Paradrilus , most species of the genus Drilus and some of the genus Selasia ), or light brown to yellow (most species of the genus Selasia ). They rarely have strongly red-colored body parts (genus Malacogaster ). The body of the animals is sparse, often long and moderately upright hairy. The pronotum and the elytra (elytra) shine at times and have a fine to coarse, dense sculpting. The head has downward (hypognathic) mouthparts. It is small and only slightly drawn into the prothorax. Including the compound eyes , it is almost as wide as the pronotum on the front edge. In some genera it is elongated at the front. The deflections of the feelers are widely separated from each other and are located on the front of the head. When viewed from above, they can hardly be seen. The eleven-segment antennae are strong and reach about a third of the length of the wing. They taper towards their tip. In the genus Paradrilus they are thread -shaped , in the genera Drilus and Malacogaster they are strongly toothed to fan-shaped, in the genus Selasia they are fan-shaped with very long lamellae. The first antenna segment ( scapus ) is pear-shaped, somewhat asymmetrical and strongly built. The length of the second to fourth sensor element is different, but the following elements become increasingly shorter towards the tip of the sensor. All limbs are moderately dense, upright with flowers. The hemispherical protruding compound eyes are located on the side of the head. The frontoclypeal suture is absent, the anterior edge of the frontoclypeus is concave. The front of the labrum is membranous and has long setae at the tip . The mandibles are long, slender, and clearly curved. In some species they have a central tooth on the inner edge. The maxilla has overgrown chewings ( Galea , Lacinia ). The maxillary palps are small and four-limbed. The labium is transverse and has no ligula. The labial palps are small and four-limbed. The pronotum is flat and in some species has a shiny surface on the disc. The edges are always finely to coarse structured. The dotting is either dense at the edge and increasingly looser towards the center, or it is roughly uniform everywhere. The pronotum is 1.22 to 1.45 times wider in the middle than it is long, with the widest point in the front third. It is seldom broadest at the base. The scutellum is small and simply rounded at the tip. The cover wings are flattened and almost parallel-edged or slightly broadened behind, with the widest part in the apical third. They are 2.20 to 2.90 times longer than they are wide at the shoulder. They shine and are sparse to dense, with fine upright flowers. Longitudinal grooves are absent, at most they are very weak, irregularly dotted in longitudinal lines. The hind wings are fully developed in the males of all species; there are never more brachyptic or aptere individuals. The apical field of the wings is very short and carries one or two sloping sclerites anteriorly and another scleritis posteriorly. The radial cell is large and has a somewhat acute posterobasal angle. The cross vein R 3 is very long and runs diagonally. The medial field has two or three exposed wing veins and no second cubitoanal cell (WC). The legs are slender and slightly flattened. The hips ( coxes ) are clearly separated, the thigh rings ( trochanters ) are strong. The splints ( tibia ) do not have any thorns apically. The tarsi consist of five slender tarsi segments. The claws are simple. The abdomen has seven ventrites (abdominal sclerites). The aedeagus is of the trilobal type and has a fine, sickle-shaped base and usually has basal processes . The phallus is long, usually longer than the paralere. Basally it is broadest and sometimes clearly curved.

female

The adult females are 13 to 35 millimeters long and have a larva-like appearance with an elongated (genera Drilus , Malacogaster ) or strongly built body ( Selasia ). The body is slightly flattened dorsoventrally, has parallel side edges ( Drilus ), or is clearly widened at the back ( Selasia ). The cuticle is soft and weak in the genus Drilus , and moderately sclerotized in Selasia . They are yellowish brown to dark brown in color. Their heads are small and have forward-facing (prognathic) mouthparts. It is slightly wider than it is long and about half as wide as the prothorax. The small, flat compound eyes are on the side. The sensor deflections are inconspicuous and widely separated from each other. The strong, thread-like, ten-link antennae are short and about as long as the head. The pear-shaped scapus is strong and somewhat asymmetrical. The second antennae ( pedicellus ) is shorter. At the tip of the antennae in the genus Drilus the limbs are very small and narrow. The mandibles are long, slender and clearly curved towards the tip. They have a tooth on the inner edge. Both maxillary and labial palps are small and have very slender basal limbs. The penultimate limb is wider than it is long, the last one is pointed. The segments of the thorax are larva-shaped and somewhat narrower than those of the abdomen. Both pairs of wings are completely missing. The legs are built similar to the males, but shorter. The hips are wide apart and the claws are paired. The abdomen has nine visible segments, the widest being in the rear third. The last, ninth segment is much smaller and narrower than the rest.

Characteristics of the larvae

The larvae have an elongated, slightly flattened and sometimes slightly C-shaped body. Except for the last larval stage, all larvae have conspicuous hairs made up of long setae, especially on the back of the abdomen. In addition, the larvae of the genera Malacogaster and Drilus have heavily sclerotized and pigmented tergites and slightly less sclerotized ventrites. The mouthparts are directed forward on the head. The head is sclerotized, short and partially retracted into the prothorax. There is a pinhole ( ocellus ) on each side . Your labrum has grown together with the head capsule. The antennae are tripartite, with the area with the sensory receptors on the penultimate segment, which is shorter than the last. The mandibles are narrow, sickle-shaped and widely separated at the base. Their tips are slightly curved upwards and they have channels for sucking on the prey. The maxillary palps are four-limbed, the labial palps two-limbed. The larvae have a dense, hairy pre-oral filter. The prothorax has almost parallel side edges, but widens slightly towards the rear. It is almost as long as the meso- and metathorax combined. The legs are five-limbed, with the praetarsi carrying two spatula-shaped setae.

On the abdominal segments there are laterally well-developed processes on the tergites and pleura , which carry long setae. On the first through eighth abdominal segments, spiracles are formed which have a double opening. On the ninth tergite there is a pair of strong, immobile urogomphi , which have numerous well-developed setae and are pointed. The tenth abdomen segment forms a suction cup-shaped pygopodium . Light organs are not developed. In the last larval stage, the body of the larvae is strongly C-shaped and the body tapers sharply from back to front. Setae in the genera Malacogaster and Drilus are then restricted to the back of the second to tenth abdominal segment, the rest of the body surface is smooth, unsclerotized and brightly colored. Only on the rear parts of the back of the abdomen are the larvae moderately sclerotized at this stage. The lateral processes on the tergites and pleura are severely receded and have only short setae. The mandibles are also strongly regressed, as are the legs, which are also less sclerotized than in the previous stages and which have no claws. On the ninth tergite there are two pairs of immobile, well sclerotized urogomphi, the tenth segment of the abdomen is somewhat regressed.

distribution and habitat

Most genera and species occur in the Mediterranean area, with the occurrence of the genus Paradrilus being limited to the glacial retreats on the Iberian Peninsula. The genera Drilus and Malacogaster occur widely in the Mediterranean area, only two Drilus species, Drilus concolor and Drilus flavescens , are also found north of the Alps. The genus Selasia is afrotropic , distributed on the Arabian Peninsula and on Socotra .

Snail-shell beetles colonize different habitats in which terrestrial molluscs also live. In Central Europe they are most common in the flatlands or in low mountain forests, in the Mediterranean region they are from coastal swamp areas to high altitudes. The afrotropically living species of the genus Selasia are found mainly in forests and bushy habitats.

Way of life

The beetle larvae predatory feed on molluscs living on land. Food intake is not documented in adult males. They are diurnal, but have their maximum activity in the evening before sunset, when the humidity is high and the temperatures are lower. They occur in the lowlands of southern Europe from the end of March; in the mountain areas they can be found until the end of July and can be found in the herbaceous layer and in the low shrub layer. Although the males are able to fly, their flight is slow and hesitant, which means that their ability to spread and mobility is very poor. All previously known females of the genera Drilus , Malacogaster and Selasia have no wings and have a larva-like physique on the thorax and abdomen. Like the larvae, they can be found in empty snail shells or when hunting on the ground.

The larvae of the genus Drilus and Malacogaster presumably feed on a single snail individual at each larval stage. There are beetle species with a wide range of prey, but some seem to specialize in individual snail species or genera. The larvae sometimes enter through the opening in the shell, but more often they eat a round hole through the wall of the snail shell. At each stage, the snail is first eaten and the empty shell is used as protection. The new larval stage usually leaves the housing through a second self-made exit hole. The biological reason for the behavior is that snails often attach themselves to solid surfaces with their shells, whereby the larva would be trapped. In addition, the housing opening is often restricted by teeth and burrs. Ultimately, pupation also takes place in a snail shell. In the early larval stages it can happen that several larvae live together in a snail shell. The way of life of the larvae of the genus Paradrilus is unknown, but presumably similar. The snail-shell beetles are mostly only known from the adult males and due to their rarity they are rarely represented even in the largest European-related beetle collections. The last record of Paradrilus opacus was made at the end of the 19th century, so it is conceivable that the species has become extinct. The males of the genus Selasia are often attracted by artificial light sources.

For many species, two or three larval stages are assumed, mostly on the basis of limited data. At least one Drilus larva kept in the laboratory ate eight snail individuals one after the other, each followed by a molt (in the empty snail shell). The length of the life cycle and the number of larval stages are therefore possibly greater than previously thought.

Taxonomy and systematics

The first known species of snail shell beetle were first described from Western and Southern Europe by Olivier in 1790 and by Bassi in 1834. Laporte described the afrotropic genus Selasia in 1836 and other species from the eastern Mediterranean, North Africa and the Caucasus were first described. Initially, only the three genera Drilus , Malacogaster and Selasia were assigned to the group as a family . The other three types known today were only added in the early 20th century. Historically, the group was a family kinship near the fireflies (Lampyridae) Omalisidae and other beetles with weakly trained sclerotization within the superfamily Cantharoidea seen (sensu Crowson, 1955). Crowson restricted the family in 1972 to the genera Drilus , Malacogaster and Selasia and also placed the genus Pseudeuanoma provisionally instead of the Omalisidae in this family. However, he mentioned that other, less species-rich genera, such as Euanoma and Paradrilus, could also be assigned to the snail-shell beetles , but that he had no research material. The oriental and East Palearctic genera, which were originally assigned to the family, were, however, assigned to the fireflies , Rhagophthalmidae , Omethidae and red cap beetles (Lycidae). The classification used by Leschen, Beutel & Lawrence (2010) is based on Lawrence & Newton (1995) and distinguishes the following six genera: Drilus , Euanoma , Malacogaster , Paradrilus , Pseudeuanoma and Selasia . The monophyly of the shell beetle has long been insufficiently safe justified as there is no single synapomorphy gave that occurred in all six classes. The relationship of the family to the other families of the superfamily was also unclear. Beutel (1995) assumed a close relationship to the families Omalisidae, Lycidae and Lampyridae, whereas Branham & Wenzel (2003) saw the family as a sister group to all other families in the group. Molecular genetic studies by Bocakova et al. 2007 suggested a close relationship between Omalisidae, Drilidae and click beetles (Elateridae) and consider the families Lycidae, Lampyridae and Cantharidae to be more distantly related.

After an investigation of rDNA and mtDNA of the weakly sclerotized groups of the Elateroidea by Kundrata & Bocak (2011) it was shown that the previously represented or assumed classification of the Drilidae had to be abandoned and both the composition of the group and the family rank no longer was to be kept upright. Instead, the family was assigned as a tribe in the subfamily Agrypninae of the family of click beetles (Elateridae), whereby the genera Pseudeuanoma and Euanoma were placed in the family Omalisidae. According to current opinion, the tribe of the snail shell beetle includes the following genera:

The following types occur in Europe:

supporting documents

  1. ↑ a b c d e f g h i Richard AB Leschen, Rolf G. Beutel, John F. Lawrence: Handbuch der Zoologie - Coleoptera, Beetles, Volume 2: Morphology and Systematics (Elateroidea, Bostrichiformia, Cucujiformia partim) . de Gruyter, 2010, ISBN 978-3-11-019075-5 , p. 104 ff . (English).
  2. ↑ a b Robin Kundrata & Ladislav Bocak: The phylogeny and limits of Elateridae (Insecta, Coleoptera): is there a common tendency of click beetles to soft-bodiedness and neoteny? . Zoologica Scripta 40, No. 4, April 2011, pp. 364-378 doi : 10.1111 / j.1463-6409.2011.00476.x .
  3. ↑ a b Drilidae. Fauna Europaea, accessed November 3, 2014 .
  4. ^ Karl Wilhelm Harde , František Severa : Der Kosmos-Käferführer. The Central European beetle . Franckh-Kosmos, Stuttgart 2000, ISBN 3-440-06959-1 , p. 184 .
  5. ^ A b Els Baalbergen, Renate Helwerda, Rense Schelfhorst, Ruth F. Castillo Cajas, Coline HM van Moorsel, Robin Kundrata, Francisco W. Welter-Schultes, Sinos Giokas, Menno Schilthuize (2014): Predator-Prey Interactions between Shell-Boring Beetle Larvae and Rock-Dwelling Land Snails. PLoS ONE 9 (6): e100366. doi : 10.1371 / journal.pone.0100366
  6. ↑ a b c José Luis Zapata de la Vega & Antonio Sánchez-Ruiz (2012): Catálogo actualizado de los Elatéridos de la Península Ibérica e Islas Baleares (Coleoptera: Elateridae). Arquivos entomolóxicos 6: 115-271.

literature

  • Richard AB Leschen, Rolf G. Beutel, John F. Lawrence: Handbuch der Zoologie - Coleoptera, Beetles, Volume 2: Morphology and Systematics (Elateroidea, Bostrichiformia, Cucujiformia partim) . de Gruyter, 2010, ISBN 978-3-11-019075-5 (English).

Web links

Commons : Schneckenhauskäfer (Drilini)  - Collection of images, videos and audio files