TRACHEAL RESPIRATION 139 



Boving (1906), who studied the morphology of the larvae 

 very carefully, is of opinion that these thorns are completely 

 closed and that air must diffuse into them through a very thin 

 chitinous membrane. The metabolism, determined by Ege 

 (1915) as 100 ml/kg/hour in a larva weighing 45 mg, is how- 

 ever much too high to allow this, and it is safe to assume that 

 air penetrates from the plant through very narrow slits in the 

 chitin. In view of the often very low 2 percentage in the 

 plant roots and the size of the animal, it seems doubtful 

 whether diffusion alone can be sufficient. The larvae have 

 a number of spiracles (shown in Fig. 80), each provided with 

 elaborate closing mechanisms. These spiracles open to the 

 water and allow air to pass out, as seen when a larva is put 

 into hot water (Boving). They can serve only for mechanical 

 expiration, and it is assumed both by B0ving and by Ege that 

 active inspirations take place from the plant roots and expira- 

 tions to the water, but it is a serious difficulty that the tracheal 

 system shows, according to B0ving, no air sacs and tracheae 

 only with a circular cross section. No respiratory movements 

 have been observed in nature. 



Spiracles adapted to insertion in the roots of aquatic plants 

 have been found recently in several other insect larvae. 

 Varley (1937) describes the arrangement in the syrphid fly 

 Chrysogaster, in the ephydrid flies Hydrellia and Notiphila, and 

 in the curculionid beetle Linorhaptrus. Perhaps one of these 

 might be better suited for experiments than Donacia. Several 

 genera of mosquitoes beside Mansonia also utilize the air in 

 aquatic roots. The peculiar structures must have been inde- 

 pendently developed in the several families, and Varley men- 

 tions the case of the mosquito Aediomyia which may represent 

 a possible developmental stage, putting the siphons, which 

 show no special modifications, into gas bubbles and attempting 

 to stick them into plants. 



Tracheal gills. A considerable number of aquatic insect 

 larvae have become completely independent of direct access 

 to the air by the development of tracheal gills, characterized 



