RESPIRATION 25 



Simulium, Corixa, &c.) (Fig. 5, D). Sometimes there are specialized 

 plate-like projections rich in tracheae (Ephemcridae, Plecoptera (Fig. 

 5, E) ; or papillae from the anus, consisting of little more than massed 

 tracheal branches covered by a thin cuticle (Elmis, Col.); or a net- 

 work of tracheae inside the rectum (as in the larvae of Anisopterid 

 dragon-flies) (Fig. 5, F); or rigid cuticular filaments containing air 

 (spiracular gills), which do not collapse on drying, and can therefore 

 serve for respiration both in water and on land (as in the pupa of 

 Simulium, and other insects from mountain torrents). 



In some cases the oxygenation of these organs is ensured by a 

 highly developed respiratory centre; the gill plates of Ephemeridae 

 are violently agitated in water poor in oxygen, and the rectum of 

 Aeschna is ventilated by abdominal movements controlled by pri- 

 mary and secondary respiratory centres (p. 20). It has been shown 

 that the partial pressure of oxygen in the closed system of these 

 insects is always less than its tension in the surrounding medium, and 

 that of carbon dioxide greater; it is therefore generally believed that 

 the exchange of gases through these gills is effected solely by dif- 

 fusion. 



In addition to these undoubted gills, there are, in many aquatic 

 insect larvae, tubular out-growths from the body surface which may 

 contain blood only, or blood with a rather sparse supply of tracheae. 

 These structures are called blood gills; but what experimental evi- 

 dence exists seems to point against their having any but a subsidiary 

 function in respiration. In one case, that of the anal papillae of the 

 mosquito larva, they are concerned in the active uptake of ions 

 (sodium, potassium and chloride) from the surrounding water and 

 thus play an important part in nutrition. 



We have seen (p. 24) that the external air stores carried by aquatic 

 insects help to control their equilibrium in the water. The air in the 

 tracheal system, also, must often be of some importance for this 

 purpose; and in the pelagic larva of Corethra there are developed 

 four large air sacs which function solely as hydrostatic organs. This 

 larva can adjust its specific gravity to that of the water it is in by 

 varying the capacity of these air sacs - apparently by some active 

 change which causes expansion or contraction of their cuticular 

 walls. 



