408 



GENERAL BIOLOGY 



thin skinned and permeable, are able to get oxygen from 

 the water, and have become strictly aquatic. 



Among aquatic insect larvae (properly so-called) are 

 found three respiratory types: 



1 . Those without gills. 

 .These are minute larvae, 

 like those of the biting 

 midges (Ceratopogon, fig. 

 239) that live in floating 

 masses of filamentous 

 algae, where liberated 

 oxygen is abundant, or, 

 if of larger size, as in the 

 case of some stoneflies 

 (Perlidae) they live in 

 rapid and well aerated 

 water. The larger of 

 these although lacking 

 gills have an abundant 

 development of fine air 

 tubes in the thin mem- 

 branes joining the 

 thoracic segments of the 

 body on the ventral 

 side. 



2. Those with blood 



gills. These most nearly approximate aquatic vertebrate 

 larvae in their mode of respiration. Blood gills are 

 protrusions of the body wall through which the blood 

 flows; the exchange of gases in respiration takes place 

 between the blood inside and the water without. Blood 

 gills are developed in many dipterous larvae, and oftenest, 

 about the posterior end of the alimentary canal (fig 239^). 

 In dipterous larvae the tracheae are often somewhat reduced. 



FIG. 238. The larva of a swale fly (Sepedon 

 fuscipennis). a, Pulling away from the 

 surface film, the guard hairs surrounding 

 the breathing pores convergent at tips; b, 

 end of body as seen when resting on the 

 surface, hairs outspread. 



