444 



THE INVERTEBRATA 



There are thus primary respiratory centres, each responsible for 

 movement in its own segment, and specially localised secondary 

 centres, which can influence those movements in accordance with 

 the demands for oxygen. The site of the secondary centre varies in 

 different animals, but never appears to lie in the head. Just as 

 secondary centres respond to oxygen lack, so have they been shown 

 to respond to the influence of carbon dioxide. 



Though the above remarks would apply to the majority of insects, 

 there are many stages of reduction in the group, culminating in the 

 wingless Collembola, many of which have no tracheae at all, gaseous 

 exchange taking place through the skin. 



Aquatic insects fall into two physiological groups. The first 

 is distinguished by direct breathing, at least one pair of functional 



Fig. 311. 



Pupa of Anopheles maculipennis. After Nuttall and Shipley. 

 /. respiratory funnel. 



Spiracles being retained. In the water beetle Dytiscus the abdominal 

 spiracles communicate with a supply of air under the elytra which 

 is renewed when the beetle comes to the surface : in the larva of the 

 mosquito the spiracles are open to the air while the animal is sus- 

 pended from the surface film (Fig. 312). 



The second group includes the early stages of the Odonata, 

 Plecoptera, Ephemeroptera and Trichoptera. These have no func- 

 tional spiracles but breathe by means of tracheal gills — expansions of 

 the body wall through whose thin walls respiratory exchange between 

 the animal and the water is effected according to the laws of diflf"usion 

 (Fig. 333). They are usually external but in certain dragonfly nymphs 

 {Aeschna and Libellula) the rectal wall is raised into such gills and 

 respiration is effected by pumping water in and out through the anus. 



