THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 269 



obvious," continues Huxley, "that a metamorphosis in this sense (e.g. 

 the butterfly or moth) is a secondary complication superinduced upon 

 the direct and gradual process of development exhibited by such insects 

 as the cockroach (Fig. 173)." It is also laid down as an axiom of 

 zoology that insects which, like butterflies, undergo a complete meta- 

 morphosis (Fig. 1 72) are more differentiated and better specialised 

 in a word, are the products of a higher phase of evolution than 

 those which undergo no metamorphosis. So also we are duly warned 

 that insects " which never possess wings are less differentiated or 

 more embryonic than those which are winged. And, finally, insects 

 with the parts of the mouth in the condition of ordinary gnathites 

 (or jaws) are less differentiated than those in which such gnathites 

 are changed in form and function or become confluent." Now, on 

 this view of matters, a butterfly is bound to be regarded,' as we 

 have seen on the grounds of its develop- 

 ment, as a highly modified insect, far 

 removed from the primitive type. On 

 the other hand, "the insects which in 

 this view of their morphological rela- 

 tives occupy the lowest position in the 

 group, are the Collembola and Thy- 

 sanura." To these we may perhaps add 

 the true lice and bird-lice (Mallophaga), 

 because these also undergo no meta- 

 morphosis and possess no wings. 



What, then, are these Collembola 

 and Thysanura, in whose personnel and 

 development we may expect to find the 

 primitive form of the insect type ? The 

 Thysanura, of which the Lepisma and 

 Campodea (Fig. 181) are good examples, 

 are small insects, living in dark situa- 

 tions, such as amongst damp moss and 

 under stones. The body is either hairy 

 or (as in Lepisma) covered with minute 

 scales, which constitute objects used for 

 testing the defining powers of micro- 

 scopes. On the whole, the Thysanura very closely resemble the 

 young of the cockroach. The tail or abdomen is composed of 

 some ten segments, and bears paired appendages, from seven to 

 nine in number. They possess breathing tubes, but, as already 

 remarked, want wings and exhibit no metamorphosis. The Col- 

 lembola differ from the preceding group in possessing a tail con- 

 sisting of six joints only, and a curious tube or sucker, by the 

 viscid secretion of which they attach themselves to fixed objects. 



FIG. 181. CAMPODEA. 

 A, larva ; B, perfect insect. 



