Page:The American Cyclopædia (1879) Volume IX.djvu/324

 312 INSECTS it is shortest in the carnivorous families, and very long in the vegetable feeders ; it is kept in place by numerous tine trachea) which en- velop its whole extent ; in the sucking insects there is only a sucking stomach opening from the oesophagus, into which the fluid food is first taken, as in the first stomach of ruminants. The anus opens on the last segment, except in some non-feeding pupae, in which both it and the mouth are wanting; the salivary glands are well developed, opening into the pharynx ; the villosities of the third stomach seem to secrete a gastric juice, the biliary secretion being poured into this cavity ; the office of a liver is performed by crecal appendages lying upon the ventricle ; similar organs on the small intestine sometimes perform the office of a pancreas. An adipose tissue is found in all insects, especially toward the end of the larva state, gradually disappearing in the perfect condition, freely traversed by trachean branches ; the fatty con- tents are intimately connected with the func- tions of nutrition. The circulatory system consists of a contractile chambered dorsal ves- sel which serves as a heart, and a cephalic aorta which conducts the blood into the body ; the blood moves from behind forward, and passes from the aorta all over the system, forming regular currents without vascular walls, and returning as venous blood to the lateral vessels ; the blood is usually a colorless liquid, containing a few small oval corpuscles. Eespiration is carried on by a system of trachea spread through the entire body, which open externally by stigmata, and admit air either directly or by means of lamelliform or tubular prolongations which have been com- pared to branchiae ; they divide into branches, gradually becoming smaller, ending csecally, so that the air passes out by the same way that it enters. The branchial tracheae are found in certain aquatic larvae and pupee, and never in the perfect insect ; they do not commu- nicate externally, but the air is received by en- dosmosis and exosmosis. The stigmata of the pulmonary tracheso are usually bordered with a fringe of hairs, and can be opened and shut by internal muscles, whose action gives to the ab- domen of many insects well marked movements of respiration ; there is generally a pair on the upper portion of the interstices between the rings, being wanting between the head and prothorax and the last two abdominal seg- ments; the tracheae are often dilated into large reservoirs of air. Respiration is very active in insects, and performed by the movements of the abdominal segments ; they require a great deal of air, and are very quickly asphyxi- ated by deprivation of oxygen ; though not producing much animal heat ordinarily, some- times, as in the bees when hived, the respiration is accelerated and their temperature percep- tibly elevated. The Malpighian vessels, which were formerly supposed to be biliary, are now ascertained to be urinary organs, secreting uric acid products ; they are small convoluted tubes, yellowish or brownish, and open into the pos- terior extremity of the stomach. Many insects have secretory follicles just under the skin, whose ducts open between the segments or be- tween the joints of the limbs, or by the side of the anus; the fluid secreted is generally of a disagreeable odor, and sometimes, as in the bugs, very fetid. The females in many of the hymenoptera, as the bees and wasps, have a glandular apparatus in the anal region, which secretes an irritating poison introduced into the tissues of their enemies by their hollpw stings. Most insects undergoing a complete metamorphosis have in their larva state silk organs, whose secretion they use in the forma- tion of their cocoons and webs; they consist of two long, flexuous tubes on the side of the body, continuous in front with two small ex- cretory ducts opening on the under lip ; in a few the silk is spun from a spinneret project- ing from the anus ; the wax-secreting appara- tus has been described under BEE. The sexes are distinct, and the females often differ great- ly from the males, as in the glow-worm ; among the bees and ants the females are much less numerous than the males, and certain indi- viduals of neither sex, or neuters, do the work and protect the colony. Most insects lay eggs, though a few, like the aphides, are viviparous ; by means of an ovipositor many introduce their eggs into a deep-seated nidus, in or near which the young can find the food suited for them, almost always different from that required by the parents. There are generally two symmet- rical ovaries and testes, situated in the ab- dominal cavity, and two oviducts uniting into a single one at the posterior end of the body. In their progress to maturity insects change their skins many times, and many of them un- dergo transformations as singular as those al- ready mentioned in the frogs ; on coming from the egg they are very different from their pa- rents and from their pupa forms. Before ar- riving at their perfect state they usually pass through the larva and pupa form, which may be entirely different, or vary chiefly in the development of wings, according as the meta- morphosis is complete or not. Insects with complete metamorphosis when they leave the egg or are in the larva state are more or less worm-like, with an elongated soft body di- vided into movable rings, normally 13 in number, sometimes with and sometimes with- out feet ; in no respect do they resemble the parents; the eyes are generally simple, and occasionally absent; the mouth is al- most always armed with jaws for chewing, even in insects which are sucking in the per- fect state ; these larva are called caterpillars or maggots, according to their size, form, and habitat. After remaining in this state, either in the water, in the air, or under ground, a cer- tain length of time, varying according to the species, and undergoing several moults, rudi- mentary wings form under the skin, and they change into nymphs, chrysalids, or pupae ; the