INSECTS 



normal procedure of growth by direct development from 

 the embryo to the adult. 



It must appear that the nymph, or young of an insect 

 with incomplete metamorphosis, is merely an aberrant 

 development of the normal form of the young as it 

 occurs in an insect without metamorphosis. This is 

 evident from the fact that the nymph has external wings, 

 fully developed compound eyes, and in general the same 

 details of structure in the legs and other parts of the body 

 as has the adult. Most larvae, on the other hand, have 

 few or none of the structural details 

 of the adult that might be expected to 

 occur in a normal postembryonic ado- 

 lescent form; but they do have many 

 characters that appear to belong to a 

 primitive stage of evolution and that 

 we might expect to find in an em- 

 bryonic stage of development. The 

 caterpillar, for example, has legs on 

 the abdomen (Fig. 136, AbL), an 

 embryonic feature possessed by none 

 of the higher insects in the adult 

 stage; it has only one claw on its 

 thoracic legs, a character of crusta- 

 ceans and myriapods, but not of adult 

 winged insects or of nymphs. Like- 

 wise, there are certain features of the 

 internal structure of the caterpillar 

 that are more primitive than in any 

 adult insect or nymph; and the same 

 evidence of primitive or embryonic 

 characters might be cited of other 

 larvae. On the other hand, the structural details of some 

 larvae are very much like those of the adults, and such 

 larvae differ from the adults of their species principally 

 in the lack of the compound eyes and of external wings. 

 Now, if all the insects with complete metamorphosis 



[248] 



Fic. I40. A bristle- 

 tail, Thermobia, a mem- 

 ber of the order Thy- 

 sanura, another primi- 

 tive group of wingless 

 insects, (Twice natu- 

 ral size) 



