INSECTS 



315 



in dry air, cannot (like aquatic animals) breathe by the whole surface of 

 the body or by portions of it (gills of fishes, crustaceans, etc.). Their 

 organs of respiration must be lodged inside of the body, where 

 structures composed of soft membranes are protected against desiccation. 

 These respiratory organs consist of numerous fine tubes, the so-called 

 air-tubes, or trachea. They, as a rule, form two large trunks, which 

 extend along the whole length of the body, and ramify like a tree into 

 finer and finer branches, which penetrate even into the antennae, legs 

 and wings, and enmesh as in a net all the internal organs of the body. 

 Eespiration proper, i.e., the discharge of carbonic acid and reception of 

 oxygen (see Part L, p. 6), takes place in the finest of these ramifications. (In 

 transparent aquatic insect larvae the tracheae may be distinctly observed, 

 even under a small magnifying power.) The 

 tracheal tubes take 'their origin from small 

 apertures of the chitinous skeleton (see illus- 

 trations, pp. 311, 312), which are known as 

 stigmata. (Show them -in a cockchafer or 

 cockroach.) 



(b) The manner in which respiration is 

 effected may be observed in any large insect 

 (especially in a cockchafer which is prepar- 

 ing to fly away). The abdomen is then 

 seen to be alternately contracted and ex- 

 panded from above downwards. The con- 

 traction is effected by muscles, and by the 

 pressure thus exercised upon the tracheal 

 tubes their contained air is expelled through 

 the stigmata. (By putting an insect under 

 water the escaping air may be seen to rise in bubbles to the surface.) 

 With the cessation of the pressure, the tracheal tubes expand once 

 more, their walls being strengthened by a very elastic chitinous fibre 

 (spiral thread) shaped like the steel spring in cushioned furniture. 

 (Compare the cartilaginous rings in the trachea of mammals.) The dila- 

 tation of the tracheal tubes in its turn produces a vacuum within them, 

 so that air is again forced into them through the stigmata (compare 

 with a bellows). The movements of the abdomen also explain why the 

 somites of this division cannot, like those of the head and thorax, form 

 a continuous rigid whole. 



5. Circulation. Since respiration in insects is not located at special 

 regions of the body (lungs or gills), but takes place over the whole body, 

 these animals do not require bloodvessels for conducting the blood laden 

 with carbonic acid gas to the organs of respiration and returning it to 



A TRACHEAL TUBE. 

 (Strongly magnified.) 



