INSECT METAMORPHOSIS 



the sum of the departure of the young and the departure of 

 the adult from what would have been the normal line of devel- 

 opment if neither had become structurally adapted to a special 

 kind of life. 



We may express this idea graphically by a diagram 

 (Fig. 138), in which the line nm represents what might 

 have been the straight course of evolution if neither the 

 adult (/) nor the young (L) had departed along special 

 lines of their own. But, when the adult and the young 

 have diverged from some point (a) in their past history, 

 the line LI, which is the sum of nm to L and of nm to /, 

 represents the change which the young is bound to make 

 in reverting to the adult form. The young must, there- 

 tore, prepare itself tor this event in proportion as the 

 distance LI is short or long. 



Where the structural disparity between the young and 

 the adult is not great, or is mostly in the external form 

 ot the body, the young insect changes directly into the 

 adult, as we have seen in the case of the grasshopper 

 (Fig. 9) and the cicada (Fig. 118). But with many in- 

 sects, either because of the degree of difference that has 

 arisen between the young and the adult, or for some 

 other reason, the processes of transformation are not ac- 

 complished so quickly and require a longer period for their 

 completion. In such cases, the creature that issues at 

 the last shedding of the skin by the young insect is in a 

 very unfinished state, and must yet undergo a great 

 amount ot reconstruction before it will attain the form 

 and structure of the fully adult insect. This happens in 

 all the groups of the more highly evolved insects, including 

 the beetles; the moths and butterflies; the mosquitoes and 

 flies; the wasps, bees, ants; and others. The newly 

 transformed insect must remain in a helpless condition 

 without the use of its legs and wings for a period of time 

 varying in length with different species, until the adult 

 organs, particularly the muscles, are completely formed. 



In the meantime, however, the soft cuticular layer of 



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