Many of the tropical American Elateridae emit light from the spots on the prothorax and an area beneath the base of the abdomen; these are “fireflies” (see above). The larvae of Elateridae are elongate, worm-like grubs, with narrow bodies, very firm cuticle, short legs, and a distinct anal proleg. They are admirably adapted for moving through the soil, where some of them live on decaying organic matter, while others are predaceous. Several of the elaterid larvae, however, gnaw roots and are highly destructive to farm crops. These are the well-known “wire-worms” (q.v.).
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| Fig. 19.—Catoxantha bicolor. Java. |
The Buprestidae are distinguished from the Elateridae by the immobility of the prosternal process in the mesosternal cavity and by the absence of the lateral processes at the hind corners of the prothorax. Many tropical Buprestidae are of large size (fig. 19), and exhibit magnificent metallic colours; their elytra are used as ornaments in human dress. The larvae are remarkable for their small head, very broad thorax, with reduced legs, and narrow elongate abdomen. They feed by burrowing in the roots and stems of plants.
Bostrychoidea.—This tribe is distinguished from the Malacoderma and allied groups by the mesothoracic epimera not bounding the coxal cavities of the intermediate legs. The downwardly directed head is covered by the pronotum, and the three terminal antennal segments form a distinct club. To this group belong the Bostrychidae and Ptinidae, well known (especially the latter family) for their ravages in old timber. The larvae are stout and soft-skinned, with short legs in correlation with their burrowing habit. The noises made by some Ptinidae (Anobium) tapping on the walls of their burrows with their mandibles give rise to the “death tick” that has for long alarmed the superstitious.
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| Fig. 20.—Hydrophilus piceus (Black Water Beetle). Europe. |
Clavicornia.—This is a somewhat heterogeneous group, most of whose members are characterized by clubbed feelers and simple, unbroadened tarsal segments—usually five on each foot—but in some families and genera the males have less than the normal number on the feet of one pair. There are either four or six malpighian tubes. A large number of families, distinguished from each other by more or less trivial characters, are included here, and there is considerable diversity in the form of the larvae. The best-known family is the Hydrophilidae, in which the feelers are short with less than eleven segments and the maxillary palpi very long. Some members of this family—the large black Hydrophilus piceus (fig. 20), for example—are specialized for an aquatic life, the body being convex and smooth as in the Dyticidae, and the intermediate and hind-legs fringed for swimming. When Hydrophilus dives it carries a supply of air between the elytra and the dorsal surface of the abdomen, while air is also entangled in the pubescence which extends beneath the abdomen on either side, being scooped in bubbles by the terminal segments of the feelers when the insect rises to the surface. Many of the Hydrophilidae construct, for the protection of their eggs, a cocoon formed of a silky material derived from glands opening at the tip of the abdomen. That of Hydrophilus is attached to a floating leaf, and is provided with a hollow, tapering process, which projects above the surface and presumably conveys air to the enclosed eggs. Other Hydrophilidae carry their egg-cocoons about with them beneath the abdomen. Many Hydrophilidae, unmodified for aquatic life, inhabit marshes. The larvae in this family are well-armoured, active and predaceous. Of the numerous other families of the Clavicornia may be mentioned the Cucujidae and Cryptophagidae, small beetles, examples of which may be found feeding on stored seeds or vegetable refuse, and the Mycetophagidae, which devour fungi. The Nitidulidae are a large family with 1600 species, among which members of the genus Meligethes are often found in numbers feeding on blossoms, while others live under the bark of trees and prey on the grubs of boring beetles.
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| Fig. 21.—(a) Tenebrio molitor (Flour Beetle). Europe. (b) Larva, or mealworm. |
Fig. 22.—Blaps mortisaga (Churchyard Beetle). Europe. |
Heteromera.—This tribe is distinguished by the presence of the normal five segments in the feet of the fore and intermediate legs, while only four segments are visible in the hind-foot. Considerable diversity is to be noticed in details of structure within this group, and for an enumeration of all the various families which have been proposed and their distinguishing characters the reader is referred to one of the monographs mentioned below. Some of the best-known members of the group belong to the Tenebrionidae, a large family containing over 10,000 species and distributed all over the world. The tenebrionid larva is elongate, with well-chitinized cuticle, short legs and two stumpy tail processes, the common mealworm (fig. 21) being a familiar example. Several species of this family are found habitually in stores of flour or grain. The beetles have feelers with eleven segments, whereof the terminal few are thickened so as to form a club. The true “black-beetles” or “churchyard beetles” (Blaps) (fig. 22) belong to this family; like members of several allied genera they are sooty in colour, and somewhat resemble ground beetles (Carabi) in general appearance.
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| Fig. 23.—Meloe proscarabaeus (Oil Beetle). Europe. |
Fig. 24.—Lytta vesicatoria (Blister Beetle). Europe. |
The most interesting of the Heteromera, and perhaps of all the Coleoptera, are some beetles which pass through two or more larval forms in the course of the life-history (hypermetamorphosis). These belong to the families Rhipidophoridae and Meloidae. The latter are the oil beetles (fig. 23) or blister beetles (fig. 24), insects with rather soft cuticle, the elytra (often abbreviated) not fitting closely to the sides of the abdomen, the head constricted behind the eyes to form a neck, and the claws of the feet divided to the base. Several of the Meloidae (such as the “Spanish fly,” fig. 24) are of economic importance, as they contain a vesicant substance used for raising medicinal blisters on the human skin. The wonderful transformations of these insects were first investigated by G. Newport in 1851, and have recently been more fully studied by C. V. Riley (1878) and J. H. Fabre. The first larval stage is the “triungulin,” a tiny, active, armoured larva with long legs (each foot with three claws) and cercopods. In the European species of Sitaris and Meloe these little larvae have the instinct of clinging to any hairy object. All that do not happen to attach themselves to a bee of the genus Anthophora perish, but those that succeed in reaching the right host are carried to the nest, and as the bee lays an egg in the cell the triungulin slips off her body on to the egg, which floats on the surface of the honey. After eating the contents of the egg, the larva moults and becomes a fleshy grub with short legs and with paired spiracles close to the dorsal region, so that, as it floats in and devours the
