I N S E C T S 145 Respiratory System, Inspiration by tracheae is one of the main characteristics of an insect. Trachea) are tubes ramifying in the interior of the body, the walls of which are composed of two membranes with a spiral thread between, and extending intj the wings and other append ages ; but in the perfect insect the main trachea) are subject to modification, and are more or less expanded into vesicles to suit the requirements of creatures with great powers of flight, or of strong movement in other ways. The manner in which air is communicated to these trachea?, in order that the necessary oxygen may be obtained from it, is twofold in its nature. In insects that live in free air the latter is received through lateral openings termed spiracles or stigmata, which vary in number in different insects, but there is usually one on each side of most of the segments. A spiracle usually consists of a longitudinal slit in a membrane, protected by delicate mechanism, and also by special muscles, which can close it hermetically if necessary. Many aquatic insects also breathe through spiracles, and in these cases a quantity of air is collected (or entangled) in delicate pubescence on the surface of the body, the insect coming to the surface to obtain a fresh supply at intervals. But in the majority of aquatic insects, and especially of their larvae or pupa}, air is obtained by means of external threads or plates, expansions of the integument, the function of which is to absorb air from water and convey it to the trachea) by means of delicate ramifications of the traclieal system in their substance. The number and position of these external appendages (or branchiae) is as varied as are the conditions under which the insects live ; in some only a single elastic tube is present, which can be protruded to the surface of the water, and its length adapted to the varying depth of that element; in some (as in many dragon-flies) the plates are in the rectum, and the air is obtained by the forcible taking in and expulsion of water by means of powerful anal valves (which serve also for locomotion). It is obvious that those larvae that exist parasitically in the substance of the body of other larva), &c., must still obtain air, and it is presumed that this is sometimes effected at the expense of the respira tory system of their hosts. It has long been known that rudimentary branchiae exist in aerial insects, and, though this was at one time supposed to be an attribute of one or two forms only, it is now known to occur frequently. According to the researches of Gegenbaur and Palme:), those branchiae exist side by side with the ordinary spiracles; hence they conclude that there is no direct connexion be tween the branchial system of the larva and the spiracles of the imago. It is still perhaps an open question whether these branchiae in the imago serve any functional purpose. Alimentary and Digestive Systems. The food of insects is either solid or liquid, and the parts of the mouth arc modified, according to requirements, into two main condi tions, termed mandibulatc and haustellate ; but the latter term is somewhat vague, inasmuch as the modifications are by no means homologous in all haustellate insects, although the structure is subservient to the same function. Again, in both divisions the food may be either vegetable or animal in its nature, and according as this may be the parts of the digestive system are modified. The most simple digestive system consists merely of a tube extending from mouth to anus, with no very distinct division into parts. But in insects the arrangement is considerably more complex, yet varying enormously. The most complete system consists of (esophagus, with the salivary glands (modified into silk-producing glands in Lepidoptera, etc.), crop or proventriculus, gizzard, stomach, small and large intestines, and an arrangement of small canals termed the Malpighian tubes. Some authors distinguish also other divisions of the intestines answering to those of higher animals. By some the term proventriculus is applied to the crop, by others to the gizzard. The gizzard is usually
- absent in haustellate insects; but, as most of these are truly
| mandibulate in their larval stage, much modification is undergone during metamorphosis. The digestive secretion of all parts of the system appears to be essentially alkaline, and assimilation goes on from all (excepting perhaps the lower intestine) pari passu with digestion, the latter being commenced in the crop. The Malpighian tubes are a set of long slender vessels (varying much in number) situated in the lower portion of the system at the junction of the small and large intestines. Their function has been warmly contested amongst physiologists, many considering them biliary organs, while probably an equal number maintain they are solely urinary, and a few hold that both these functions may be attributed to them. The recent researches of Plateau and others are in favour of their being solely urinary. Von Siebold has asserted that the biliary system consists of certain cells in the walls of the stomach. It is possible the whole intestinal canal is at times called upon to play a role quite independent of digestion and assimilation ; it may be made subservient to metamorphosis through being distended with air, thus assisting the rupture of the integuments for the escape of the imago ; but this can probably only obtain in insects with incomplete metamorphoses. Circulatory System. Almost as much uncertainty exists, or has existed, as to the true nature of this system as in other points of internal structure and physiology. Originally it was believed that no circulatory system existed, an idea that was speedily dissipated. If we examine a larva of which the integuments are tolerably transparent, we perceive, even without dissection, a large vessel running along the dorsal portion of the creature just beneath the integument, and we perceive also that it dis tinctly pulsates. This is the " dorsal vessel " or " heart," and it terminates anteriorly in a cephalic aorta. Examined i more minutely by dissection, it is seen to consist of a | number of chambers and constrictions, each chamber having a lateral valvular opening on either side, through which the blood is received into the vessel by regular currents and conveyed to the cephalic aorta, whence it escapes into the body in currents which have no vascular walls, and is again received into the dorsal vessel from lateral currents, such, at least, is the most generally received opinion. Certain it is that the blood (which is ordinarily a colourless liquid) circulates through all parts of the body, even to the antennae, legs, and wings, and the circulation can be well observed in the wings of some insects in which these organs are unusually transparent, in that case distinctly fol lowing the course of the nervures. But many physiologists 1 have believed that the blood is conveyed over the body by j means of the tracheae, some distinguishing certain tracheae to which this function alone, and not that of respiration, is proper. The majority of these, however, state that the blood simply flows between the two integuments of which the walls of a trachea are composed, and to this system the term " peritracheal " has been given. According to the results obtained from the experiments of the most recent observers, we prefer to doubt the existence of this peritra cheal circulation. The relative frequency of pulsations varies much according to the insect and its state of activity or excitement. They disappear almost entirely in insects in a state of hibernation, and are much reduced in the pupae of those that undergo complete metamorphoses. Muscular System. The muscles are attached to the inner side of the chitinous integument, and lie just beneath it. They are composed of numerous parallel fibres without any tendinous sheaths, but the fibres are apparently sometimes united at their extremity into a kind of tendon, which has XHI. 19
