INSECT WAYS AND MEANS. VII.--HOW INSECTS BREATHE Fig. 1.--Aphis, showing "TracheAE" (greatly magnified). Fig. 2.--"Tracheal Filaments" of Aphis (greatly magnified). Fig. 3.--"Spiracles" of Water Beetle (greatly magnified). Fig. 4.--Section of Crayfish, showing gills (magnified). Fig. 1.--Aphis, showing "TracheAE" (greatly magnified). Fig. 2.--"Tracheal Filaments" of Aphis (greatly magnified). Fig. 3.--"Spiracles" of Water Beetle (greatly magnified). Fig. 4.--Section of Crayfish, showing gills (magnified).

Animal life cannot be sustained without breathing, though, strange as it may seem, many of the lower animals have no special breathing organs. By breathing, we mean supplying the body with the life-giving oxygen contained in the air. Animals which live in the water breathe by taking in the oxygen held in solution in the water.

In the simplest animals which live in water, the body is only a small 'blob' of jelly, so small that the oxygen passes directly into the body. The bodies of some worms are so delicate that the oxygen easily passes through the outer layers and mixes with the blood within.

In more complicated animals this life-giving gas is conveyed all over the body by means of the blood, which is brought into contact with the water, or the air, by structures known as gills. In the crayfish, for example, the gills are placed above and rise from the bases of the legs, being saved from injury by a broad shield lying behind the head. (In fig. 4 this shield has been cut away so as to show the gills, marked g, which it really covers.) By means of the circulation of the blood, the crayfish breathes. This blood is carried to the gills and bathed by a constant stream of fresh water, which enters behind the covering and shield, and passes forwards till it comes out on each side of the mouth. The blood, thus refreshed by the oxygen in the water, is carried again all over the body, and in its course loses more and more oxygen, and becomes more and more charged with poisonous gases, which are got rid of on the return of the blood to the gills. The letter s in this figure marks the stump of the leg, which, for the sake of clearness, has been cut off.

In ourselves, the work of breathing, or of purifying the blood, is done by means of the lungs. The lungs are large, spongy organs in the chest, and are continually supplied with fresh air, which passes in through the nose and mouth and down the wind-pipe, by what we call the act of breathing.

Insects take in oxygen in a way quite different from that of the crayfish or mankind. In some larval insects, which live in water, as in some worms, the body is so thin that no special breathing organs are necessary; others breathe by means of gills like those of the crayfish, but arranged differently—sometimes along each side, and sometimes at the tail end of the body. But in the ordinary adult insect the work of breathing is carried on by means of a system of tubes, known as 'tracheÆ,' which run all over the body. Into these tubes the air is drawn through a number of holes on the surface of the body, called 'spiracles,' or breathing pores. The tracheÆ or tubes are everywhere bathed by the blood, which is thus constantly 'aerated,' or kept fresh.

One very remarkable thing about these tubes is the way they are kept open. A horny, spirally-twisted thread runs through them, and thus they are prevented from closing up by pressure, or by the bending of the body or limbs. In fig. 2, this thread is marked c. This plan of keeping open the passage in a tube likely to be blocked by sudden bending, has been imitated by mankind, in making rubber gas tubing, for example. As a plain rubber tube is easily bent, the gas would be in constant danger of being cut off. To prevent this, Nature's patent is usually imitated, and a coil of wire is placed along the inside of the tube. Thus, a sharp bend, such as would instantly obstruct the passage of the gas, is prevented.

The openings at the end of the breathing tubes, on the surface of the insect's body, are known, as we have said, as 'spiracles,' or 'stigmata.' They can be closed at will by special muscles, and, to prevent dust from getting into the tube, the rim of each spiracle has a more or less complicated fringe or strainer. In fig. 3 the spiracle is shown open, the opening being marked by the letter o. When closed the fringes interlock like clasped fingers.

Fig. 1 shows the position of the breathing tubes in the aphis or green fly. The spiracles or pores are marked o, the breathing tubes t.

Some insects which live in water, such as the water-beetle, breathe air in the same way as their relatives who live on land. To do this they have to come frequently to the surface of the water to take in fresh supplies of air. In the great Dyticus water-beetle this is done in a curious way. The creature, rising to the surface, first thrusts its tail up into the air, and then bending it downwards, lets the air rush in to fill the space between the body and the upper wing-cases. This done, the tail is pressed back again, and the beetle returns to the depths, where the imprisoned air is taken in through the pores into the tubes.

Besides the system of tubes just described, many insects possess a wonderful system of air-cells, which give extra help in breathing during flight. These air-cells are largest in insects which fly most. It is a curious fact that birds have an exactly similar system; in many cases, even the bones of birds are filled with air. It is generally stated, indeed, that birds with the strongest flight have the most 'pneumatic' bones. This not quite true, for the swallow, for example, has the long bones of its wing filled with marrow, and not with air. Other birds, however, like the storks, which fly much, and the owls and nightjars, have all the bones in the body thus filled with air which they obtain from the air-cells.

W. P. Pycraft, F.Z.S., A.L.S.

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