RESPIRATION IN INSECTS 39 1 



In association with the alimentary canal are various glands :-- 

 (a) The salivary glands, which open in or near the mouth. They 

 are usually paired on each side, and provided with a 

 reservour. They arise as invaginations of the ectoderm 

 near the mouth. Their secretion is mainly diastatic in 

 fimction, i.e. it changes starchy material into sugar by 

 means of a ferment. Along with these may be ranked 

 the " spinning glands " of caterpillars, etc., which also 

 open at the mouth. They secrete material which hardens 

 into the threads used for the cocoon. 

 (6) From the beginning of the mid-gut bhnd outgrowths sometimes 

 arise (in some Orthoptera, etc.), which are apparently 

 digestive. They are sometimes called pyloric caeca. In 

 other cases (some beetles) there may be more numerous and 

 smaller glandular outgrowths resembling villi in appearance. 

 In the wood-eating termites, and in certain beetles, the 

 hind-gut is crowded with symbiont Infusorians ; these 

 apparently attack the insoluble substances of the wood 

 and form simpler compounds available to the host as food ; 

 without the Protozoa, termites starve on a wood diet. In 

 the larva of the beetle Potosia, which lives in ants' nests 

 and feeds on pine-needles, there are hosts of bacteria which 

 dissolve and ferment cellulose. In other beetle larvae 

 {Anohium) yeasts live in the cells lining the gut and must be 

 supposed to aid in nutrition by supplementing the digestive 

 enzymes of the host. Very many insects, especially those 

 which suck blood or plant-juices, have their bacterial 



symbionts. 



Respiratory system.— The body of an insect is traversed 



by a system of air-tubes (tracheae), which open laterally by 



special apertures (stigmata), and by means of numerous 



branches conduct the air to all the recesses of the tissues. 



In animals which breathe by gills or lungs the blood is 



carried to the air ; in insects the air permeates the whole 



body. But how does the air pass in and out ? In part, no 



doubt, there is a slow diffusion ; in part the movements of 



the wings and legs will help ; but there are also special 



expiratory muscles. We see their action when we watch a 



drone-fly panting on a flower. Inspiration is passive, as in 



birds, and depends on the elasticity of the skin and of the 



tracheal walls ; expiration is active, and depends upon these 



muscles. They are chiefly situated in the abdomen, but in 



some beetles (at least) they are also present in the meta- 



thorax. 



