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OUR HOUSEHOLD INSECTS 



FIG. 35. Cast Skin of 

 Cockroach. 



light, they acquire the depth of coloration characteristic 

 of their age, the skin at the same time hardening. As 

 the skin, while soft, is some- 

 what transparent, the beating 

 of the heart, which, as usual 

 with insects, lies along . the 

 middle of the back, just under- 

 neath the skin, can be pretty 

 easily observed just after a 

 moult. Cornelius determined in 

 this way that the pulsations 

 were so rapid as eighty per 

 minute. It would hardly be 

 safe, however, to conclude from 

 this that the rate is always so 

 high, for at moulting times 

 the insects are in a very ab- 

 normal condition, and their internal economy is much 

 disturbed. 



At the last moult but one the rudiments of wings 

 appear, and the insect is, at this period of its life, some- 

 times called a " pupa." Many entomologists, however, 

 prefer the term " nymph " for this stage, reserving the 

 name " pupa " for the quiescent condition which pre- 

 cedes the final moult in beetles, bees, butterflies, and 

 flies. No such quiescent condition is observable in the 

 cockroach, and as a nymph it is undistinguishable from 

 the larval or earlier stages, except by the presence of 

 the rudiments of wings, and its life is just as active and 

 rapacious as heretofore. Sometimes the insect is called 

 a nymph throughout the whole of its immature life. 

 At the last moult it appears in its final form and of its 

 final size, the female differing little from its nymph ; but 

 the male now acquires its wings, and both are by this 



