20 The Structure and Special Physiology of Insects 



respiratory capillaries. The trachea^ are readily recognized under the micro- 

 scope by their linely transversely ringed or striated appearance (Fig. 39). 

 These transverse "rings" are really spirally arranged short chitinized 

 thread-like thickenings on the inner wall of the tube, which by their elasticity 

 keep the delicate air-tubes open. The tubes are filled and emptied by a 



rhythmic alternately contracting and expanding 

 movement of the abdomen, called the respiratory 

 movement. When the ring-muscles contract, the 

 walls of the abdomen are squeezed in against 

 the vi.scera, which, compressing the soft air-lubes, 

 force the air out of them through the spiracles; 

 when the body-walls are allowed to spring back 

 to normal position fresh air rushes in through the 

 spiracles and fills up the air-tubes, which expand 

 because of the elastic spiral thickenings in their 

 walls. Insects which live in water either come 

 up to the surface to breathe and in some cases 

 to take down a supply of air held on the outside 

 of the body by a fine pubescence like the pile of 

 velvet, or they are provided with tracheal gills 

 (Fig. 40) which enable them to breathe the air 

 mixed with, or dissolved in, the water. Gilled 

 insects do not, of course, have to come to the 

 surface to breathe. The gills may be thin plate- 

 like flaps on the sides or posterior tip of the 

 body, or may be tufts of short thread-like tubes 

 variously arrang.d over the body. Or they 

 may be, as in the dragon-fly nymphs, thin folds along the inner wall of the 

 rectum, the water necessary to bathe them being taken in and ejected again 

 through the anal opening. In all cases these insect gills differ from those 

 of other animals, as crabs and fishes, in that they are not organs for the 

 purification of the blood, i.e., effecting an exchange of carbon dio.xide and 

 oxygen carried by it, but are means for an osmotic exchange of the fresh 

 air dissolved in water for carbon-dioxide-laden air from air-tubes or tracheae 

 which run out into the gills. Probably no more blood enters these gills 

 than is necessary to bring food to them. Impure air is brought to them 

 by air-tubes, and exchanged by osmosis through the thin walls of air-tube 

 and gill-membrane for fresh air, which passes from these gill air-tubes to 

 the rest of the respiratory system of the body. 



The nervous system of insects shows the fundamentally .segmental make-up 

 of the body better than any of the other systems of internal organs, although 

 probably in the successive chambers of the dorsal vessel or heart, and certainly 



Fig. 40. — Young (nymph) of 

 May-fly showing (g.) tra- 

 cheal gills. (.-Vfter Jenkins 

 and Kellogg.) 



