The Insect World. Figuier Louis
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bury themselves in the soil. These vessels are called tracheæ. Their communications with the air are established externally in different ways, according to the character of the medium in which the insect lives.
It is well known that a vast number of insects live in the air. The air penetrates into the tracheæ by a number of orifices placed at the sides of the body, which are termed spiracles. On close examination these may be seen in the shape of button-holes in a number of different species. Let us dwell for a moment on the breathing apparatus of the insect, that is to say, on the tracheæ.
This apparatus is sometimes composed of elastic tubes only, sometimes of a collection of tubes and membranous pouches. We will first treat of the former.
The coats of these breathing tubes are very elastic, and always preserve a cylindrical form, even when not distended. This state of things is maintained by the existence, throughout the whole length of the tracheæ, of a thread of half horny consistency, rolled up in a spiral, and covered externally by a very delicate membranous sheath. The external membrane is thin, smooth, and generally colourless, or of a pearly white. The cartilaginous spiral is sometimes cylindrical and sometimes flat. It only adheres slightly to the external membrane, but is, on the other hand, closely united to the internal one. This spiral thread is only continuous in the same trunk; it breaks off when it branches, and each branch then possesses its own thread, in such a way that it is not joined to the thread of the trunk from which it issued, except by continuity, just as the branch of a tree is attached to the stem which supports it. This thread is prolonged, without interruption, to the extreme points of the finest ramifications.
The number of tracheæ in the body of an insect is very great. That patient anatomist, Lyonet, has proved this in his great work on the Goat-moth Caterpillar, Cossus ligniperda. Lyonet, who congratulated himself with having finished his long labours without having had to destroy more than eight or nine of the species he wished to describe, had the patience to count the different air-tubes in that caterpillar. He found that there were 256 longitudinal and 1,336 transverse branches; in short, that the body of this creature is traversed in all directions by 1,572 aeriferous tubes which are visible to the eye by the aid of a magnifying glass, without taking into account those which may be imperceptible.
The complicated system of the breathing apparatus which we are describing is sometimes composed of an assemblage of tubes and membranous pouches, besides the elastic tubes which we have already mentioned. These pouches vary in size, and are very elastic, expanding when the air enters, and contracting when it leaves them, as they are altogether without the species of framework formed by the spiral thread of the tubular tracheæ, of which they are only enlargements.
Fig. 13 is explanatory of these organs of respiration.
The respiratory mechanism of an insect is easily understood. "The abdominal cavity," says M. Milne-Edwards, "in which is placed the greater part of the respiratory apparatus, is susceptible of being contracted and dilated alternately by the play of the different segments of which the skeleton is composed, and which are placed in such a manner that they can be drawn into each other to a greater or less extent. When the insect contracts its body, the tracheæ are compressed and the air driven out. But when, on the other hand, the visceral cavity assumes its normal size, or dilates, these channels become larger, and the air with which they are filled being rarefied by this expansion, is no longer in equilibrium with the outer air with which it is in communication through the medium of the spiracles. The exterior air is then impelled into the interior of the respiratory tubes, and the inspiration is effected."
The respiratory movements can be accelerated or diminished, according to the wants of the animal; in general, there are from thirty to fifty to the minute. In a state of repose the spiracles are open, and all the tracheæ are free to receive air whenever the visceral cavity is dilated, but those orifices may be closed, and the insect thus possesses the faculty of stopping all communication between the respiratory apparatus and the surrounding atmosphere.
Some insects live in the water; they are therefore obliged to come to the surface to take the air they are in need of, or else to possess themselves of the small amount contained in the water. Both these methods of respiration exist under different forms in aquatic insects.
To inhale atmospheric air, which is necessary for respiration, above the water, certain insects employ their elytra [2] as a sort of reservoir; others make use of their antennæ, the hairs of which retain the globules of air. In this case it is brought under the thorax, whence a groove carries it to the spiracles. Sometimes the same result is obtained by a more complicated arrangement, consisting of respiratory tubes which can be thrust into the air, which it is their function to introduce into the organisation.
Insects which breathe in the water without rising to the surface are provided with gills—organs which, though variable in form, generally consist of foliaceous or fringed expansions, in the midst of which the tracheæ ramify in considerable numbers. These vessels are filled with air, but it does not disseminate itself in them directly, and it is only through the walls of these tubes that the contained gas is exchanged for the air held in suspension by the surrounding water. The oxygen contained in the water passes through certain very permeable membranes of the gill, and penetrates the tracheæ, which discharge, in exchange, carbonic acid, which is the gaseous product of respiration.
Fig. 14 represents the gills or breathing apparatus in an aquatic insect. We take as an example Ephemera. [3] It may be observed that the gills or foliaceous laminæ are placed at the circumference of the body, and at its smallest parts.
We have now seen that the respiratory apparatus is considerably developed in insects; it is, therefore, easy to foresee that those functions are most actively employed by them. In fact, if one compares the oxygen they imbibe with the heavy organic matter of which their body is composed, the amount is enormous.
Before finishing this rapid examination of the body of an insect, we shall have to say a few words on the nervous system.
This system is chiefly composed of a double series of ganglions, or collections of nerves, which are united together by longitudinal cords. The number of these ganglions corresponds with that of the segments. Sometimes they are at equal distances, and extend in a chain from one end of the body to the other; at others they are many of them close together, so as to form a single mass.
The cephalic ganglions are two in number; they have been described by anatomists under the name of brain. "This expression," says M. Lacordaire, "would be apt to mislead the reader, as it would induce him to suppose the existence of a concentration of faculties to control the feelings and excite the movements, which is not the case." [4] The same naturalist observes, "All the ganglions of the ventral chain are endowed with nearly the same properties, and represent each other uniformly."
The ganglion situated above the œsophagus gives rise to the optic nerves, which are the most considerable of all those of the body, and to the nerves of the antennæ. The ganglion beneath the œsophagus provides the nerves of the mandibles, of the jaws, and of the lower lip. The three pairs of ganglions which follow those placed immediately below the œsophagus, belong to the three segments of the thorax, and give rise to the nerves of the feet and wings. They are in general more voluminous than the following pairs, which occupy the abdomen.
Fig. 15 represents the nervous system of the Carabus auratus: A is the cephalic ganglion; B, the sub-œsophagian ganglion; C, the prothoracic ganglion; D and E are the ganglions of the mesothorax and metathorax. The remainder, F F, are the abdominal ganglions.
