132 



HALF HOUKS WITH ENSECTS. [Packard. 



facturer, may be studied in the jaws of the young of these 

 same diving beetles. The hirval Corethra, a fly allied to the 

 mosquito, is an excellent areometer, and other philosophical 

 instruments have their counterparts in the special organs of 

 insects adapting them for an aquatic life. 



Cases of protective mimicry are afforded by the Caddis 

 flies (Fig. 92), which move over the bottom, carrying about 

 with them a movable chevaux de /rise of sticks, behind 

 which lurk a nimble pair of jaws ; or they mimic innocent 

 sticks, or build their cases of bits of moss and move about 



Fig. 92. 



Different forms of Case Worms. 



bearing, as it were, subaqueous Birnam forests ; or when 

 their tubes are built of sand imitate the irregularities of the 

 bottom over which they creep, and thus living in ambush 

 all the while, are protected in their turn by these disguises. 

 As the Ranatra (Fig. 93) is noticed moving about slowly at 

 the bottom of a pool, it would be easily mistaken for an ear 

 of wheat moved accidentally. It is not so harmless as it 

 looks, for its fore legs are held up in an attitude that will at 

 least be deemed by its victims a striking one. When it 

 strikes out those legs, the long claws close with a firm grasp 



