TRACHEATES 203 



are folded together, and can only be opened with the 

 aid of forceps at the extremity of the abdomen. In 

 the beetles, only the hind wings are used for flight ; 

 the front wings form a cover for them. There is 

 equally endless variety in the legs of insects. In the 

 mole-cricket the front extremities have been converted 

 into shovels ; in the grasshoppers they have become 

 a powerful leaping apparatus. 



The skeleton of the articulates, with all these peculi- 

 arities, is of the greatest theoretical value. It provides 

 ample material for the refutation of the Lamarckian 

 principle, as it is Weismann's merit to have shown. 

 We saw above the way in which insects grow. Under- 

 neath the shell the skin secretes a new one, but this 

 is soft and elastic, and only hardens when the old one 

 has been cast off. All the characteristics of the coat — 

 its thickness, its different kinds of hairs and other out- 

 growths — are already formed before the old coat is cast 

 off. When this is done, the new one appears in full 

 development ; it hardens and grows no further, as our 

 bones do, because it is an excretory product of the 

 underlying skin. As soon as the shell comes to light, 

 the skin which has produced it loses its connection with 

 it; it has to begin immediately the work of secreting 

 a new coat. 



wasps, though not deeply, and become pupse there. From the pupae 

 issue the insects that have become males, and fly away with their large 

 bird wings. The females do not leave the pupa-covering j they are 

 without wings or legs, and like grubs, and remain in their wasp until 

 a male comes to fertilise them. The larvae develop from the fertilised 

 ova in the mother's body, and then break out of the mother's back, 

 and in their turn make their way into wasps. 



