INSECTA 



393 



circulation through the main trunks is set up aiding considerably in 

 the diffusion of gas through the whole system. Air sacs in the form 

 of thin-walled diverticula of the main tracheae occur in many insects 

 (Fig. 293), particularly those with the power to fly for prolonged 

 periods such as bees, migratory locusts and houseflies. These also 

 assist considerably in the circulation of air through the tracheal 

 system owing to the ease with which they can be compressed. 



While a neuromuscular mechanism has thus been developed to 

 assist respiration in the typical flying insect with its active meta- 

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

 in the wingless CoUembola, which have no tracheae at all, gaseous 



Fig. 294. Tracheal end cell and tracheoles from silk gland of caterpillar, 

 Phalera bucephala. From Imms, after Holmgren, e. end cell; c. tracheoles; 

 t. trachea. 



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 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 suspended from the surface film (Fig. 296). 



The second group includes the early stages of the Odonata, 

 Plecoptera, Ephemeroptera and Trichoptera. These have no func- 



