AQUATIC INSECTS 



Hydrophilus. — These are medium-sized black beetles on 

 which there is a sharp spine in the middle of the thorax which 

 projects backward between the legs (Fig. 212). The adults 

 appear to feed entirely upon algae. The young larvs live 

 on small entomostracans, later they eat tadpoles, insect larvae 

 and other beetles. They come to the surface to breathe. 

 Projecting from the sides of the abdomen are seven pairs of 

 filaments (Fig. 212), but these do not function as breathing 

 organs. Fullgrown larva, a little more than half an inch long. 



Fig. 213. — Berosus: i, adult, and 2, larva showing 

 long gills (after Wilson). 



Berosus. — The adults are small, yellowish beetles (Fig. 213) ; 

 both adults and larvse feed upon green algse. Wilson writes 

 of one species, "These larvae are very sluggish; they cannot 

 swim at all but crawl about slowly over the" vegetation, their 

 long tracheal gills standing out rigidly on either side like rods. 

 They cover themselves with green algae and lie inert for hours 

 at a time, only coming out after f ood. ' ' They breathe through 

 their lateral gills, which contain tracheae (Fig. 213), and do not 

 need to come up for air like Hydrophilus. Wilson kept larvas 

 in an aquarium for a long time, supplying them with small 

 crustaceans, yet they would take nothing but vegetable food, 

 algae, Chara and Nitella. He found them very hard to kill, 



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