THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 257 



not merely be greatly protracted and prolonged, but it will attain to a 

 higher stage of independent development than before. So also with 

 many insect larvae, and so with zoophytes. The effects of varying 

 conditions on the young and developing animal are plainly traceable. 

 It remains for us to discover what light such reflections throw on 

 some well-marked and familiar cases of metamorphosis around us. 



The insect world teems with examples of " metamorphosis " at 

 once striking and interesting. It also, however, illustrates a previous 

 remark, that in one and the same class we may find great variations 

 in development and " metamorphosis." For instance, we may find 

 no metamorphosis at all in some insects. The lice, the bird-lice, 

 and the spring-tails (Thy- 

 sanura) thus come from 

 the egg resembling in every 

 respect, save in size, the 

 perfect in sects. They sim- 

 ply cast or shed their skin 

 at each successive stage of 

 growth, but no change of 

 form is represented in their 

 development. So also with 

 manyinsects of higher.rank. 

 A kind of day-fly ( Chloeon, 

 Fig. 171) is described by 

 Sir John Lubbock as under- 

 going no fewer than twenty 

 moultings of its skin during 

 its "metamorphosis," which 

 is not, however, of marked 

 or distinct character, since the organs of the young animal are 

 simply and gradually changed into those of the adult insect. Even 

 in insects which undergo a much more typical metamorphosis than 

 the day-flies, the gradual conversion of the larval parts into the 

 organs of the adult may be witnessed. A young cricket (Fig. 176) 

 becomes the adult very gradually, and the days of its infancy are 

 not markedly separated from those of its youth, nor are these latter 

 in turn sharply defined from the period of adult life. 



Turning, however, to actual details, we find a butterfly (Fig. 172), 

 fly, and beetle respectively to exhibit the so-called " perfect " form of 

 metamorphosis. Each begins life that is, comes from the egg, after the 

 preliminary stages common to all eggs as a grub, caterpillar, or larva 

 (a), which spends the first part of its existence in the guise of a worm, 

 eating voraciously and increasing, as a rule, many times its original 

 size in bulk. Next this voracious grub settles down and becomes the 

 chrysalis, <yc pupa. Here, quiescence is the order of the day. Some- 



FlG. 171. CHLOEON. 



A, larva ; B, perfect insect. 



