RESPIRATION 99 



the lumen in a corkscrew-like manner, the turns being so closely 

 compressed together that they appear to be transverse. Where each 

 length of thread ceases there is a short interval, after which another 

 arises ; the threads never pass into the branches, nor do they ever 

 bifurcate. When seen in sections of the larger tracheae the thread 

 projects a little into the lumen, giving it a ridged appearance. Th e 

 intervals between the turns of the thread are filled in by a very thin 

 layer of chitin. 



The arrangement of circular or spiral thickenings in the wall of 

 a thin chitinous tube is one admirably adapted to ensure that the 

 lumen of the tube will be kept open even if the angle at which it is flexed 

 becomes greatly altered ; in other words, to prevent occlusion by kinking. 

 Such occlusion might readily occur in the tracheae during the move- 

 ments of the body, or when the mid-gut or crop is distended with blood. 



The outer or hypodermal lamina of the tracheal wall, which is in 

 contact with the internal organs, is very thin, and is only recognized by 

 the scattered nuclei which lie on the surface of the tubes. It is con- 

 tinued to the most minute branches of the tube, after the spiral thread 

 has ceased, and as it reaches to the intercellular spaces while still in 

 continuity with the hypodermis, it forms a sort of peritoneum, within 

 which the internal organs are contained. 



The structure of the air sacs is essentially the same as that of 

 the tracheae. There is, however, no spiral thread, and the layer of 

 chitin may be greatly thickened. In the sacs of the head, and to a 

 lesser extent those of the thorax in the more specialized flies, the 

 wall is very dense and tough, and, as it is often adherent to surrounding 

 tissues, the sacs are in some cases extremely difficult to dissect away 

 from the soft parts. The sacs in the abdomen are as a rule very 

 thin-walled, and have a dead white colour and a peculiar waxy appear- 

 ance. When ruptured in saline solution the wall of the sacspreads out 

 just as do very thin paraffin sections. 



In addition to their function of conveying air to the tissues the 

 tracheae are put to another use, in that they serve to support the internal 

 organs and to retain them in position. Their flexibility enables them 

 to adapt their position to that of an organ of changing dimensions, 

 such as the mid-gut. 



The movements of respiration in insects depend on the contraction 

 of the longitudinal and vertical muscles, and are mainly restricted to 



the abdomen when the fly is in the resting position. 



. * Respiration 



The exact mechanism is not clearly understood, but 



