​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.

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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.