278 Adjustment to Conditions of Aquatic Life 



rapids of streams and the slender larv^ of the punkie 

 Ceratopogon, that live where algae abound. 



The gills of insect larvae are of two principal sorts: 

 blood-gills and tracheal gills. Blood-gills are cylindric 

 outgrowths of the integument into which the blood 

 flows. Exchange of gases is between the blood 

 inside the gills and the water outside. Such gills 

 are most commonly appended to the rear end of the 

 alimentary canal, a tuft of four retractile ana] gills 

 being common to many dipterous larvse. Bloodworms 

 have also two pairs developed upon the outside wall of 

 the penultimate segment of the body (see fig. 236 on 

 p. 393). Such gills are most like those of vertebrates. 



Tracheal gills are more common among insect larvae. 

 These are similar outgrowths of the skin, traversed by 

 fine tracheal air-tubes. In these the exchange of gases 

 is between the water and the air contained within the 

 tubes, and distribution of it is thro the complex system 

 of tracheae that ramify throughout the body. The 

 trachea where they enter such a gill usually split up 

 into long fine multitudinous tracheoles that form 

 recurrent loops, rejoining the tracheal branches (fig. 

 170). 



Tracheal gills difler remarkably in form, position and 

 arrangement. In form they are usually either slender 

 cylindric filaments, or small flat plates. Filamentous 

 gills are more common, only this sort occurring on stone- 

 fly nymphs (fig. 11 1 on p. 204), and on caddis-worms. 

 Lamelliformor plate-like gills occur on the back of may- 

 flies (fig. 113), and on the tail of damselflies (fig. 115). 

 Either kind may grow singly or in clusters. Filament- 

 ous gills are often branched. In the stonefly, Taeniop- 

 teryx, they are unbranched but composed of three some- 

 what telescopic segments. Both filamentous and 

 lamelliform gills occur on many mayfiies. 



