132 
HALF HOURS WITH INSECTS. [Packard. 
facturer, may be studied in the jaws of the young of these 
same diving beetles. The larval 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 frise 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 
4 
