20 
The Structure and Special Physiology of Insects 
respiratory capillaries. The tracheae are readily recognized under the micro¬ 
scope by their finely 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 viscera, which, compressing the soft air-tubes, 
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 arranged 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 dioxide 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. (After Jenkins 
and Kellogg.) 
