134 RESPIRATORY MECHANISMS 



which is narrowed in this case by the diffusion distance being 

 the whole length of the body. 



This type of respiration is supposed to exist in many 

 mosquito larvae. Generally the main tracheal trunks serve 

 also as a reservoir for air, allowing the animal a dive of a 

 number of minutes. 



Mechanical ventilation at the surface. In many larger insects 

 returning at intervals to the surface to breathe, the main 

 tracheal trunks or a system of air sacs are ventilated me- 

 chanically, as described above for the larvae of the large 

 Dytiscidce. In this group we find also the larvae of the Hydro- 

 philina and of several Diptera, especially the Syrphidce {Eristalis) 

 and Stratiomyidce. The imagines of the larger Dytiscidce and 

 Hydrophilince and all stages of the genera Nepa and Ranatra 

 also utilize this type of mechanism. Many of the imagines 

 carry a supply of air also outside the tracheal system between 

 the wings and the abdomen (Dytiscus, Hydrophilus, Nepa), and 

 in the case of Hydrophilus also on the underside of the thorax 

 where it is held by hydrofuge hairs. A quantitative study of 

 the ventilation in many of these forms could be carried out 

 without much experimental difficulty and could scarcely fail 

 to give interesting results both oecologically and with regard to 

 the regulation mechanisms. 



In the large larvae of Stratiomyia, Kuster (1933) describes a 

 contractile "air chamber," outside the deeply sunk posterior 

 spiracles which appears, according to his description, to have 

 taken over the ventilation of the tracheal system proper, a 

 most unusual arrangement which from the relative dimensions 

 of the parts could scarcely be effective. 



In Eristalis, Alsterberg (1934) describes a set of retractile 

 rectal "gills" to which he ascribes the function of eliminating 

 C0 2 directly to the water. This may be so, but seems rather 

 unlikely in view of the powerful and generally continuous 

 ventilation of the tracheal system. 



Gill function of air stores. A number of insects carrying a 

 store of air on the outside of the body supplement the oxygen 



