Page:EB1911 - Volume 06.djvu/692

 The Paussidae are a very remarkable family of small beetles, mostly tropical, found only in ants’ nests, or flying by night, and apparently migrating from one nest to another. The number of antennal segments varies from eleven to two. It is supposed that these beetles secrete a sweet substance on which the ants feed, but they have been seen to devour the ants’ eggs and grubs. The Gyrinidae, or whirligig beetles (fig. 9), are a curious aquatic family with the feelers (fig. 9, b) short and reduced as in most Paussidae. They are flattened oval in form, circling with gliding motion over the surface film of the water, and occasionally diving, when they carry down with them a bubble of air. The fore-legs are elongate and adapted for clasping, while the short and flattened intermediate and hind legs form very perfect oar-like propellers. The larva of Gyrinus (fig. 9, c) is slender with elongate legs, and the abdominal segments carry paired tracheal gills.

—The members of this tribe may be easily recognized by their wing-nervuration. Close to a transverse fold near the base of the wing, the median nervure divides into branches which extend to the wing-margin; there is a second transverse fold near the tip of the wing, and cross nervures are altogether wanting. There are four malpighian tubes, and all five tarsal segments are usually recognizable. With very few exceptions, the larva in this group is active and campodeiform, with cerci and elongate legs as in the Adephaga, but the leg has only four segments and one claw.

The Silphidae, or carrion beetles, form one of the best-known families of this group. They are rotund or elongate insects with conical front haunches, the elytra generally covering (fig. 10) the whole dorsal region of the abdomen, but sometimes leaving as many as four terga exposed (fig. 11). Some of these beetles are brightly coloured, while others are dull black. They are usually found in carrion, and the species of Necrophorus (fig. 11) and Necrophaga are valuable scavengers from their habit of burying small vertebrate carcases which may serve as food for their larvae. At this work a number of individuals are associated together. The larvae that live underground have spiny dorsal plates, while those of the Silpha (fig. 10) and other genera that go openly about in search of food resemble wood-lice. About 1000 species of Silphidae are known. Allied to the Silphidae are a number of small and obscure families, for which reference must be made to monographs of the order. Of special interest among these are the Histeridae, compact beetles (fig. 12) with very hard cuticle and somewhat abbreviated elytra, with over 2000 species, most of which live on decaying matter, and the curious little Pselaphidae, with three-segmented tarsi, elongate palpi, and shortened abdomen; the latter are usually found in ants’ nests, where they are tended by the ants, which take a sweet fluid secreted among little tufts of hair on the beetles’ bodies; these beetles, which are carried about by the ants, sometimes devour their larvae. The Trichopterygidae, with their delicate narrow fringed wings, are the smallest of all beetles, while the Platypsyllidae consist of only a single species of curious form found on the beaver.

The Staphylinidae, or rove-beetles—a large family of nearly 10,000 species—may be known by their very short elytra, which cover only two of the abdominal segments, leaving the elongate hind-body with seven or eight exposed, firm terga (figs. 13, 14). These segments are very mobile, and as the rove-beetles run along they often curl the abdomen upwards and forwards like the tail of a scorpion. The Staphylinid larvae are typically campodeiform. Beetles and larvae are frequently carnivorous in habit, hunting for small insects under stones, or pursuing the soft-skinned grubs of beetles and flies that bore in woody stems or succulent roots. Many Staphylinidae are constant inmates of ants’ nests.

—In this tribe may be included a number of families distinguished by the softness of the cuticle, the presence of seven or eight abdominal sterna and of four malpighian tubes, and the firm, well-armoured larva (fig. 15, c) which is often predaceous in habit. The mesothoracic epimera bound the coxal cavities of the intermediate legs. The Lymexylonidae, a small family of this group, characterized by its slender, undifferentiated feelers and feet, is believed by Lameere to comprise the most primitive of all living beetles, and Sharp lays stress on the undeveloped structure of the tribe generally.

The Lampyridae are a large family, of which the glow-worm (Lampyris) and the “soldier beetles” (Telephorus) are familiar examples. The female “glow-worm” (fig. 15, b), emitting the well-known light (see above), is wingless and like a larva; the luminosity seems to be an attraction to the male, whose eyes are often exceptionally well developed. Some male members of the family have remarkably complex feelers. In many genera of Lampyridae the female can fly as well as the male; among these are the South European “fireflies.”

—Several families of rather soft-skinned beetles, such as the Melyridae, Cleridae (fig. 16), Corynetidae, Dermestidae (fig. 17), and Dascillidae, are included in this tribe. They may be distinguished from the Malacodermata by the presence of only five or six abdominal sterna, while six malpighian tubes are present in some of the families. The beetles are hairy and their larvae well-armoured and often predaceous. Several species of Dermestidae are commonly found in houses, feeding on cheeses, dried meat, skins and other such substances. The “bacon beetle” (Dermestes lardarius), and its hard hairy larva, are well known. According to Sharp, all Dermestid larvae probably feed on dried animal matters; he mentions one species that can find sufficient food in the horsehair of furniture, and another that eats the dried insect-skins hanging in old cobwebs.

—This is an important tribe of beetles, including families with four malpighian tubes and only five or six abdominal sterna, while in the thorax there is a backwardly directed process of the prosternum that fits into a mesosternal cavity. The larvae are elongate and worm-like, with short legs but often with hard strong cuticle.

The Elateridae or click beetles (fig. 18) have the prosternal process just mentioned, capable of movement in and out of the mesosternal cavity, the beetles being thus enabled to leap into the air, hence their popular name of “click-beetles” or “skip-jacks.” The prothorax is convex in front, and is usually drawn out behind into a prominent process on either side, while the elytra are elongate and tapering.