RESPIRATION 23 



detail in accordance with local demands for oxygen, (i) During 

 growth and moulting new tracheae and tracheoles grow out from the 

 existing system, and the new outgrowths are far more abundant in 

 regions with a deficient oxygen supply, (ii) In the intervals between 

 moults, of course, no new tracheae or tracheoles can be formed. But 

 if the epidermal cells have an inadequate supply of tracheoles they 

 give out elongate processes, sometimes as much as 100 \t long which 

 become attached to remote air-filled tracheoles and then, by con- 

 tracting, draw these tracheoles towards them. 



Respiration of aquatic insects 



The majority of insects that are aquatic in the adult stage breathe 

 gaseous air like the terrestrial forms; but they develop special struc- 

 tural modifications to enable them to do so. Since all the spiracles 

 cannot be exposed to the exterior, there is a tendency for the 

 respiratory function to become concentrated at one end of the body. 

 In Hydrophilus, the mesothoracic spiracles are the most important ; 

 in Dytiscus and its allies and in the aquatic Hemiptera, the last ab- 

 dominal spiracles predominate. In many larvae (Dytiscus, Culicidae 

 and many other Diptera) the other spiracles have become obliterated, 

 or at least functionless, and the insects breathe entirely through the 

 terminal spiracles ('metapneustic' respiration) (Fig. 5, C), In all these 

 cases, in the antennae of Hydrophilus, in the abdominal fringes of 

 hair in Notonecta, in the elytral surface itself in Dytiscus, and around 

 the tracheal orifices of the metapneustic larvae, there are hydrofuge 

 structures that enable the air-containing regions of the body to break 

 through the surface film and obtain communication with the atmos- 

 phere (see p. 6). 



In many cases (mosquito larvae, Dytiscus larvae, &c.) the tracheal 

 trunks are so capacious that they serve as stores of air, and enable 

 the insect to remain submerged for a considerable time. In other 

 insects, a store of air is carried beneath the elytra (in Dytiscus &c.) or 

 over the ventral surface of the body (in Notonecta, Corixa, &c.) by 



fig. z> 



A, simple anastomosing tracheae, with sphincters in the spiracles (p. 13); B, mechanically 

 ventilated air sacs developed (p. 18); C. metapneustic respiration; only terminal spiracles 

 functional (p. 23); D. tracheal system entirely closed: cutaneous respiration (p. 24); E, 

 the same with abdominal tracheal gills (p. 25); F, the same with rectal tracheal gills (p.25) 

 c 



