Fig. 71.—Portion of the abdominal part of tracheal system of a Locust (Oedipoda): a, spiracular orifices; b, tracheal tubes; c, vesicular dilatations; d, tracheal twigs or capillaries. (After Dufour.)
We must repeat that such a system as we have just sketched forms a striking contrast to the imperfect blood-vascular system, and that Insects differ profoundly in these respects from Vertebrate animals. In the latter the blood-vessels penetrate to all the tissues and form capillaries, while the aerating apparatus is confined to one part of the body; in Insects the blood-circulating system is very limited, and air is carried directly by complex vessels to all parts; thus the tracheal system is universally recognised as one of the most remarkable of the characters of Insects. Many Insects have a very active respiratory system, as is shown by the rapidity with which they are affected by agents like chloroform; but the exact manner in which the breathing is carried on is unknown. In living Insects rapid movements of contraction and expansion of parts of the body, chiefly the abdomen, may be observed, and these body contractions are sometimes accompanied by opening and shutting the spiracular orifices: it has been inferred that these phenomena are respiratory. Although such movements are not always present, it is possible that when they occur they may force the air onwards to the tissues, though this is by no means certain. It is clear that the tracheal system is the usual means of supplying the organisation with oxygen, but it appears to be improbable that it can also act as the agent for removing the carbonaceous products of tissue-changes. It has been thought possible that carbonic acid might reach the spiracles from the remote capillaries by a process of diffusion,[[48]] but it should be recollected that as some Insects have no tracheal system, there must exist some other mode of eliminating carbonic acid, and it is possible that this mode may continue to operate as an important agent of purification, even when the tracheal system is, as a bearer of air to the tissues, highly developed. Eisig[[49]] has suggested that the formation of chitin is an act of excretion; if so this is capable of relieving the system of carbonic acid to some extent. Others have maintained that transpiration takes place through the delicate portions of the integument. Lubbock[[50]] has shown that Melolontha larvae breathe "partly by means of their skin." The mode in which the carbon of tissue-change, and the nitrogen of inspiration are removed, is still obscure; but it appears probable that the views expressed by Réaumur, Lyonnet, and Lowne[[51]] as to inspiration and expiration may prove to be nearer the truth than those which are more widely current. In connexion with this it should be recollected that the outer integument consists of chitin, and is cast and renewed several times during the life of the individual. Now as chitin consists largely of carbon and nitrogen, it is evident that the moulting must itself serve as a carbonaceous and nitrogenous excretion. If, as is suggested by Bataillon's researches,[[52]] the condition accompanying metamorphosis be that of asphyxia, it is probable that the secretion of the new coat of chitin may figure as an act of excretion of considerable importance. If there be any truth in this suggestion it may prove the means of enabling us to comprehend some points in the development of Insects that have hitherto proved very perplexing.
Peyrou has shown[[53]] that the atmosphere extracted from the bodies of Insects (Melolontha) is much less rich in oxygen than the surrounding atmosphere is, and at ordinary temperatures always contains a much larger proportion of carbonic acid: he finds, too, that as in the leaves with which he makes a comparison, the proportion of oxygen augments as the protoplasmic activity diminishes. Were such an observation carried out so as to distinguish between the air in the tracheal system and the gas in other parts of the body the result would be still more interesting.
We know very little as to the animal heat produced by insects, but it is clear from various observations[[54]] that the amount evolved in repose is very small. In different conditions of activity the temperature of the insect may rise to be several degrees above that of the surrounding medium, but there seems to be at present no information as to the physiological mode of its production, and as to the channel by which the products—whether carbonic acid or other matters—may be disposed of.
In the order Aptera (Thysanura and Collembola) the tracheal system is highly peculiar. In some Collembola it apparently does not exist, and in this case we may presume with greater certainty that transpiration of gases occurs through the integument: in other members of this Order tracheae are present in a more or less imperfect state of development, but the tracheae of different segments do not communicate with one another, thus forming a remarkable contrast to the amalgamated tracheal system of the other Orders of Insects, where, even when the tracheal system is much reduced in extent (as in Coccidae), it is nevertheless completely unified. Gryllotalpa is, however, said by Dohrn[[55]] to be exceptional in this respect; the tracheae connected with each spiracle remaining unconnected.
Water Insects have usually peculiarities in their respiratory systems, though these are not so great as might à priori have been anticipated. Some breathe by coming to the surface and taking in a supply of air in various manners, but some apparently obtain from the water itself the air necessary for their physiological processes. Aquatic Insects are frequently provided with gills, which may be either wing-like expansions of the integument containing some tracheae (Ephemeridae larvae), or bunches of tubes, or single tubes (Trichoptera larvae). Such Insects may either possess stigmata in addition to the gills, or be destitute of them. In other cases air is obtained by taking water into the posterior part of the alimentary canal (many dragon-flies), which part is then provided with special tracheae. Some water-larvae appear to possess neither stigmata nor gills (certain Perlidae and Diptera), and it is supposed that these obtain air through the integument; in such Insects tracheal twigs may frequently be seen on the interior of the skin. In the imago state it is the rule that Water Insects breathe by means of stigmata, and that they carry about with them a supply of air sufficient for a longer or shorter period. A great many Insects that live in water in their earlier stages and breathe there by peculiar means, in their perfect imago state live in the air and breathe in the usual manner. There are, in both terrestrial and aquatic Insects, a few cases of exsertile sacs without tracheae, but filled with blood (Pelobius larva, Machilis, etc.); and such organs are supposed to be of a respiratory nature, though there does not appear to be any positive evidence to that effect.
Blood and Blood-Circulation.
Owing to the great complexity of the tracheal system, and to its general diffusion in the body, the blood and its circulation are very different in Insects from what they are in Vertebrates, so that it is scarcely conducive to the progress of physiological knowledge to call two fluids with such different functions by one name. The blood of Insects varies according to the species, and in all probability even in conformity with the stage of the life of the individual. Its primary office is that of feeding the tissues it bathes, and it cannot be considered as having any aerating function. It is frequently crowded with fatty substances. Graber says: "The richness of Insect blood in unsaponified or unelaborated fat shows in the plainest manner that it is more properly a mixture of blood and chyle; or indeed we might say with greater accuracy, leaving out of consideration certain matters to be eliminated from it, that it is a refined or distilled chyle." Connected in the most intimate manner with the blood there is a large quantity of material called vaguely the fat-body; the blood and its adjuncts of this kind being called by Wielowiejski[[56]] the blood-tissue. We shall return to the consideration of this tissue after sketching the apparatus for distributing the refined chyle, or blood as we must, using the ordinary term, call it.
There is in Insects no complete system of blood-vessels, though there is a pulsating vessel to ensure distribution of the nutritive fluid. This dorsal vessel, or heart as it is frequently called, may be distinguished and its pulsations watched, in transparent Insects when alive. It is situate at the upper part of the body, extending from the posterior extremity, or near it, to the head or thorax, and is an elongate tube, consisting as it were of a number of united chambers; it is closed behind, except in some larvae, but is open in front, and has several orifices at the sides; these orifices, or ostia, are frequently absent from the front part of the tube, which portion is also narrower, being called the aorta—by no means a suitable term. Near the lateral orifices there are delicate folds, which act to some extent as valves, facilitating, in conjunction with the mode of contraction of the vessel, a forward movement of the blood. The composition of the tube, or series of chambers, is that of a muscular layer, with internal and external membranous coverings, the intima and adventitia. Olga Poletajewa states[[57]] that in Bombus the dorsal vessel consists of five chambers placed in longitudinal succession, and not very intimately connected, and that there is but little valvular structure. In Cimbex she finds a similar arrangement, but there are ten chambers, and no aorta.