Fig. 162. Tracheal tube of an insect, highly magnified, showing, at a, the elastic spiral thread.

(845). There is a limit, observes Dr. Williams*, different in different structures, at which the spiral thread ceases; and at this point the membranous trachea begins. It is not the external covering which ceases, but the spiral which, growing less and less visible, graduates insensibly into a continuous tube. The diameter of the "spiral" trachea constantly decreases as it divides; that of the membranous observes, throughout its entire course, whether it multiply into a network, or wavy brushes, or into the muriform plexus which exists in the substance of muscles, a uniformity which can compare only with that of the true blood-capillaries of the vertebrate animal. The tracheae terminate differently, and form different plexuses, in different organs, according to the varying mechanical arrangements of the ultimate parts of the latter. They are evidently air-tubes throughout, even to their final extremes.

(846). The primary, secondary, and tertiary air-tubes divide and subdivide arborescently, the branches never uniting, but the ultimate ramifications dividing and subdividing in the same profuse retiform manner as the blood-capillaries of the vertebrate animal, supplying the muscles, the glands, the mucous membranes, the brain, and every other viscus. The large air-tubes which travel along the axes of the spacious blood-channels detach from their sides here and there minute wavy branches which float in the fluid, and appear to be expressly intended to aerate the fluids.

(847). In all the transparent structures of insects, such as the wings, antennae, branchiae, etc, the blood-currents travel in the same passages as the tracheae. On closer scrutiny it will be seen that a channel, such as that of the nervure of the wings, bearing in its centre a large tracheal tube, exhibits on one side a current going in one direction; on the other, another bearing in an opposite course. These are afferent and efferent, arterial and venous blood-streams. They are bounded by separate walls. The afferent current is circumscribed by its own proper coats, the efferent by its own; and the trachea is placed intermediately, having parietes quite distinct from, although contiguous with, those of the blood-channels. This coincidence between the tracheae and the blood-currents can be traced in the wings nowhere beyond the limits of the nervures into the scaly spaces that they circumscribe. The returning of the corpuscles at a certain point renders this fact quite unquestionable. Beyond this limit, only the fluid elements, not the corpuscles of the blood, penetrate. In this extra-vascular region it is cyclosis, not circulation, which governs the movements of the nutritive fluid.

If, says Dr. Williams, everywhere the blood and the air travelled together, the inference would be that the sole design of the tracheal apparatus of the insect consisted in aerating the fluids. Since, however, the blood returns much before the tracheae reach their remote penetralia, it is evident that the tracheal system in the insect fulfils some other function. What can be the meaning of those incomparable pneumatic plexuses - veritable retia mirabilia - which embrace immediately the very ultimate elements of the solid organs of the body? those microscopic air-tubes, which carry oxygen in its gaseous form, unfluidi-fied by any intervening liquid, to the very seats of the fixed solids which constitute the fabric of the organism? The intense electrical and chemical effects developed by the immediate presence of oxygen at the actual scene of all the nutritive operations of the body, fluid and solid, give to the insect its vivid and brilliant life, its matchless nervous activity, its extreme muscularity, its voluntary power to augment the animal heat.

Such contrivance, subtle and unexampled, reconciles the paradox of a being, microscopic in corporeal dimensions and remarkable for the relative minuteness of the bulk of its blood, sustaining a frame graceful in its littleness, yet capable of prodigious mechanical results.

* Loc. cit.

(848). We must now consider the mechanism by which air is perpetually drawn into the body of the insect, and again expelled. If the abdomen of a living insect be carefully watched, it will be found continually performing movements of expansion and contraction that succeed each other at regular intervals, varying in frequency, in different species, from twenty to fifty or sixty in a minute*, but occurring more rapidly when the insect is in a state of activity than when at rest. At each expansion of the abdomen, therefore, air is sucked in through all the spiracles, and rushes to every part of body; but when the abdomen contracts, it is forcibly expelled through the same openings. Burmeister even supposes that the humming noises produced by many insects during their flight must be referred to the vibration caused by the air streaming rapidly in and out of the spiracular orifices. Insects which live in water are obliged, at short intervals, to come to the surface to breathe, at which time they take in a sufficient quantity of air to last them during the period of their immersion; but if the spiracles are closed by any accident, or by the simple application of any greasy fluid to the exterior of their body, speedy death, produced by suffocation, is the inevitable result.

(849). A moment's reflection upon the facts above stated concerning the respiration of insects will suggest other interesting views connected with the physiology of these little creatures. It is evident, in the first place, that their blood is all arterial; they can have no occasion for veins, as they have no venous blood, the whole of the circulating fluid being continually oxygenized as its principles become deteriorated. The perfection of their muscular power, their great strength and indomitable are likewise intimately related to the completeness of their respiration; so that the vital energies of the muscular system are developed to the utmost, endowing them with that vigorous flight and strength of limb which we have already seen them to possess. It must likewise become apparent that, as the blood is freely exposed to the influence of oxygen in every portion of the insect to which the air-tubes reach, one great necessity for the existence of a circulatory apparatus is entirely done away with, and, as we have observed before, all those parts of the vascular system required in other animals for the propulsion of the vitiated blood through pulmonary or branchial organs are no longer requisite; so that, by dispensing with the complicated structures usually provided for this purpose, the body is considerably lightened.