Coleoptera / by George Dimmock [book]

George Dimmock
1884 unpublished
The Beetles, or Culeoptera as they are termed by naturalists, briefly defined, are six-legged insects, which have thick and horny fore wings and chewing niuuth-parts, and which undei'go complete metamor])hosis. The most striking of these characters is the peculiar horn-like, opaque, usually quite rigid fore wings, which, in the beetles, are termed elytra (singular elytron), from the Greek elutron, meaning a sheath, a word used by Aristotle to designate the forewings of beetles. These elytra
more » ... s. These elytra give a general aspect to beetles which make them easily recognizable as such, however much they may vary in other respects. As a rule the elytra close together, meeting in a straight line along the ]iosterior portion of the back or dorsum of the insect, and shielding beneath them the delicate hind wings, unless hind wings arc absent, as is the case with a small number of beetles. The elytra take no active part in the flight of Colcoptera, but generally are opened outward at right angles to the body of the insect, remaining at rest in that i)osition, while the membranous hind wings perform the necessary strokes for locomotion. In beetles that have rudimentary iiiml wings, and in those of which the hind wings are absent, the elytron of one side usually is united linnly along the back to that of the other side, to form a single shield, which protects the abdominal portions of tlie insect beneath it. In one family, the Staphylinida', or rove-beetles, and in some less commonly known beetles belonging to other families, the elytra arc much too short or too small to cov>rr the whole abdomen, although the ytaphylinid;e nuuiage to bring the entire wings beneath the elytra by a complex system of folding. The name Coleo].)tera (from koleos, a sheath, and pteroii, a whig) was first employed for beetles by John Ray, an early English naturalist, in 170.5, and has been generally adopted by subsequent naturalists, although Fabricilis, in 177J>, termed beetles Eleutherata, on account of their free maxilkp, anil Schluga, in 1707, used for them the term Vaginata, from r(.'araiu-e, which has given rise to popular names, such as 'meal-worm' for the larva of Tuiithrio nioUtor, and ' wire-worms ' for the larva-of many Elaterida\ The thicker and more fleshy larva-of Colcoptera, such as are those often dug up .aliout roots, or split from their mines in wood, are in pojmlar parlance 'grubs.' The larval of beetles mostly haxe six legs, or feet, near the anterior end of their body, that is a pair of legs for each of the first three segments behind the headthe thoracic segments. In the Curculionidas and in some other beetles of which the larvae live within their food, the latter are legless. Certain larviv have more or less developed traces of anal legs, sometimes a product of the evaginated lateral portions of the Fic. :H4.-Larva of IiUq}s jtruilurta. Fungus, liark, decayed wood, dead twi food in its position between masticatory organs which operate laterally. All the mouth-]iarts of adult Coleoptera arcv subject to slight modifications and reductions in certain families and genera, and in the larv.a? the oral organs are not rarely consiilerably modified. The larva' of Dytiscida' and Hydropliilida' present remarkable modifications of form and use, which, as in the case of other striking variations from the normal form of larval nn)utli-]iarts, will lie described under their res]iective families. The prothorax, the middle of the three ]iortions into which all beetles are divided with considerable distinctness, is hollowed out in front to receive tlie neck or liead, and is articulated behind with the mesothorax. The ])rothorax bears the forward legs. Its dorsal surface is often termed the |ii'onotum. The mesothorax, metathorax, and abdomen form the last of tlie three portions of a beetle, which portion is often called the trunk. The mesothorax and metathorax are very intiinatelj' united ; upon the former are borne the elytra and middle pair of legs, and upon the latter the wings and hind pair of legs. Both mesothorax and metathorax are hidden from view, on the dorsal side of the insect, by tlie elytra, except that in most beetles a small triangular jiortion of the mesothorax is visible between the elytra at their bases; this triangle is the scutellum. The median ventral ])ieces of the three different ])arts of the thorax arc termed, resjiectively, prosternum, mesosternum, and metasternum, an, as is the case with manv pupa' of Lncanidw and Scaralneidre, than in images. While the alxlomen of adult beetles have no appendages except the genitalia, a few beetle larva?, among which are those of Gyrinidro and Psephenida^, liave gill-a])pendasres borne bv several abdominal semnents. Adult beetles enjoy locomotion of several sorts, flying, springing, c!iml)ing, crawling, or running ; a few add swimming : their larvte sometimes swim, but more commonly only crawl or run. The legs of ailult beetles, as well as those of their Larvre, are appendages of the three thoracic segments, and are modified according to the purposes for which they are used. In some larvse one or more pairs of legs are reduced to the merest rudiments or entirely disappear, but in the mature beetles no such reduction takes place. The typical constitution of the legs of adult Coleojitera is as follows : the basal joint, short but of various forms, is termeil a coxa, and the cavities into which the coxfe are set on the under side of the thoracic segments, on each side of the median line, are ' called coxal cavities. The form of the coxa' is of importance in distinguishing the families of Coleoptera. Jointed to the coxa is a jjiece named the trochanter; the connection of the trochanter with the next following piecethe femuris very intimate in some beetles, in others the hind trochanters are prominent lobes upon the inner side of the femora. The femur, or thigh, is the first long piece of each leg ; the tibia, another long piece, follows it, and to the latter piece is jointed a foot or tarsus, consisting of from three to five short joints, the terminal joint generally having two claws, and between the claws a small appendage, the onychium. The tarsal joints are hairy beneath, and those of the anterior (and sometimes also the middle) pair of legs of many male beetles are modified to clasp more firmly the female during copulation. The most peculiar of these modified tarsi are those of the males of the water-beetles belonging to the family Dytiscida> ; their tarsi will be further described when treating of that family. The form and proportion of beetles' legs are quite varied. Legs fitted for walking and running, ambulatorial and cursorial legs as they are called, are slender, and rather long in ])roportion to the size of the insect. Fossorial legs, or those intended for digging, are short, stout, and flattened. Springing legs, or saltatorial legs, have generally much thickened femora. Natatorial legs, those used for swimming, are oar-shaped or ]iaddle-sha]ied, and are bordered with hairs. The elytra, which are really horny forward wings, and in which the renmants of veins often can be seen, are generally opened out nearly or quite at right angles to the body in flight, and serve to balartce and to steer the beetles, while the two membranons wings, which are ordinai'ily hidden beneath the elytra, unfold, and, by their rapid vibrations, furnish the impulse in flight. Jousset de Bellesme thinks the elytra govern direction in flight by changing their position, and consequently altering the position of the centre of gravity of the beetle. In the Cetonians, a portion of the Scarab»id;e, the elytra are said to remain closed in flight, the wings projecting out from beneath their bases. The inflexed outer margins of the elj'tra are designated as epiplcurae.
doi:10.5962/bhl.title.9556 fatcat:5vaqbufafndv3cj2gl3ncvj2h4