Page:The American Cyclopædia (1879) Volume IX.djvu/323

 INSECTS 311 under lip is attached the tongue, which in some is abortive and in others long and changed into a suctorial organ. In the suck- ing insects the under lip is transformed into a tube, enclosing delicate lancet-like filaments or bristles, modifications of the mandibles and maxillae; in the hymenoptera (bees, &c.) the mouth is intermediate between the chewing and the suctorial, having parts belonging to both; in the lepidoptera (butterflies, &c.) the mandibles are very small, but the under jaws are changed each into a semi-canal which may be rolled up spirally. The eyes are either simple or compound, the first occurring chiefly in the larvso of the metamorphic orders, and the second in perfect insects ; some have both kinds in the perfect state, and some adults, larva, and pupee are blind. The compound organ is made up of many simple eyes, each having its cornea, conical vitreous body, pig- ment, and nervous filament ; the number of these facets is sometimes more than 25,000. The simple eyes (stemmata) consist of a cor- nea, a lens lodged in an expansion of the optic nerve, and a surrounding pigment layer ; they are placed either on the sides of the head, or in small groups on the vertex. The thorax supports the legs and wings, and consists al- ways of three rings, called respectively pro- thorax, mesothorax, and metathorax, each bear- ing on its ventral arch a pair of legs ; the wings arise from the dorsal aspect of the two poste- rior rings. The limbs consist each of a two- jointed hip, a thigh, a leg, and a kind of finger or tarsus of two to five joints terminated by the claws ; in the jumpers, like the grasshop- pers, the hind legs are very long and muscular ; in the swimmers, like the water beetles, the tarsi are flattened, ciliated, and arranged for oars ; in the flies, the feet are provided with pads and hooks by which they are enabled to hang suspended from smooth surfaces ; the an- terior limbs are often enlarged, as in the mole crickets, which dig in the ground, and armed with spines, as in the mantis, which uses them to seize its prey ; in some of the butterflies the anterior limbs are mere rudiments, useless as means of progression. The wings are mem- branous expansions, rendered firm by solid nervures; there are never more than two pairs, and one or the other may be wanting ; in the butterfly they are covered with a col- ored dust consisting of microscopic scales ; in the beetles the first pair becomes thick and hard, forming the elytra, which cover and pro- tect the second pair ; the wings are sometimes half membranous, half corneous, at others di- vided into barbed plumules, or wanting and replaced by the knob-like balancers. The legs and wings are moved by striated muscles, at- tached directly to the cutaneous skeleton; those of the wings of the diptera have their fibril! separable into series of disks, the as- tonishing rapidity of their movements being dependent on alternate contraction and relaxa- tion. The abdomen is composed of rings movable upon each other, sometimes to the number of nine ; they bear in the perfect in- sect neither legs nor wings, but are provided with various appendages useful in the econo- my of the animal, as the delicate bristles of the ephemera, the nippers of the earwig, the spring of the podurella, the sting of the bee and wasp, and the ovipositor of the grass- hopper and the ichneumons. Besides the an- tennss, the palpi about the mouth, the end of the suctorial tube, the ovipositor, and the feet in some instances, are delicate or- gans of touch ; the tongue, when present, as in bees and flies, is undoubtedly the seat of an acute sense of taste. Though insects apparently perceive by the sense of smell what food is proper for themselves or their young, the seat of this sense has not been satisfactorily determined; Dnme'ril and Cuvier, reasoning from analogy, concluded that it was placed at the openings of the respiratory trachero ; Hu- ber, from his experiments on bees, placed it in the mouth, Kirby in the anterior portion of the head or the nose, and others in the anten- na and palpi. Hearing is acute in many in- sects ; the shrilling of the locust, the tick of the deathwatch, the song of the cricket, &c., would be useless unless they could be heard by their companions ; in the orthoptera especially an auditory apparatus is connected with the stigmata of the thorax and the anterior legs ; the sense has also been placed inward at the base of the antennce. The sounds of insects are produced by the friction of one part of the external skeleton on another, by the vibration of special organs, or by a particular soniferous apparatus, always due to the action of volun- tary muscles and unconnected with the respira- tory system ; the buzzing of flies seems to de- pend on the rapid vibrations of the thorax du- ring flight and on the passage of air through the thoracic stigmata, perhaps intensified by the motions of the wings themselves ; some beetles produce a sharp sound by rubbing the last abdominal segments against the curved points of the wing covers, or the thoracic rings against each other ; the sounds of butter- flies and of the death's-head moth are referred to friction of the hips together, and to various causes not at all satisfactory. The nervous system consists of a brain and spinal cord ; the former is constituted by the ganglia which em- brace the oesophagus, and is situated in the first segment ; the spinal cord is made up gen- erally of a double series of ganglia united by longitudinal cords, in number corresponding to that of the segments of the body ; the three thoracic ganglia are much the largest, and from them are given off the nerves to the legs and wings. The alimentary canal is generally com- plicated and more or less convoluted ; it con- sists of a pharynx, oesophagus, first stomach or crop, second or gizzard with muscular walls for tritnration, third or chylific ventricle of soft and delicate texture, a small intestine, cajcum, and rectum ; as in the higher animals,