12 OUR INSECT FRIENDS AND FOES 



swellings (ganglia), usually closely united 

 together, in each segment of the body. The 

 respiratory system is very remarkable and in- 

 teresting, consisting of the external breathing 

 apertures, the stigmata or spiracles, minute oval 

 slits along the sides of the body, from which 

 arise internally tubular structures called tracheae, 

 which branch in all directions through the body. 

 These tracheal tubes are very remarkable struc- 

 tures, composed of an inner layer of mucous 

 membrane and an outer layer of serous mem- 

 brane, the membranes being separated by a 

 spiral band, the coils of which give the walls of 

 the tubes a striated appearance. This spiral 

 band, which is extremely elastic, consists of 

 inward foldings and thickenings of the chitinous 

 wall. The distribution of the tracheae, and 

 position of the spiracles, varies in different 

 insects, for although, as a rule, the spiracles are 

 placed along the sides of the abdomen and 

 thorax, they may, as, for example, in the aquatic 

 larva of the Gnat, be situated on the tail. As 

 might be supposed, the circulatory system of 

 insects is not very highly developed, the need 

 for definite blood-vessels being greatly lessened 

 on account of the very thorough way in which 

 all the organs are supplied with oxygen by 

 means of the ramifications of the tracheae. It 

 consists of a large dorsal vessel or heart, extend- 

 ing from near the apex of the abdomen to the 

 right of the brain, containing a series of cavities, 

 the ventricles, which have openings on each side, 

 and communicate with each other by a valve. 

 The blood is colourless, and the speed of the 



