FROM EGG TO INSECT SNODGRASS 



413 



Along with the buildino- of the tissues and organs within the body, 

 there goes on a modeling of the external form into that which the 

 creature will have at hatching. With most of the higher animals, 

 the young at hatching or at birth is recognizable as the progeny of 

 its parents, and this is true of many insects, such as grasshoi)pers 

 (fig. 28, A) and roaches (B). A young fly (E), a young bee or wasp 

 (D), or a young moth or butterfly (C), however, comes out of the 

 egg in a form whose parentage would never be guessed from its 

 physical characters; those insects must later undergo a transforma- 





Fio. 29. — Diagram of external structure of a theoretically coinpleto 

 mature insect (female), sliowing appendages of one side only 

 Ah, alxlomen, of 12 segments (I-XII) ; Ant, antenna; C'er, ccrcus ; 

 E, compound eye ; H, head ; Lu U, L,„ legs of first, second, and third 

 thoracic segments; Lh, labium (the united second maxillae, fig. 14B, 

 2Mx) \ Lm, labrum ; Md, mandible; IMx, maxilla; Dip, ovipositor; 

 ^It, spiracle; Th, thorax, of three segments (/, -', .(} ; TV^, W-.-., wings, 

 attached to second and third thoracic segments. 



tion, or metamorphosis, before they attain the form of the mature 

 insect of their species. The double lives that such insects lead en- 

 ables them to take advantage of two quite different envircmments 

 during their lifetime. In the first, or larval, period the young in- 

 sect specializes on feeding, and its anatomy is modeled to that end; 

 in the second period the mature insect devotes its energy mo.stly to 

 the business of mating and egg laying, and its construction is adapted 

 to these purposes, but, being the adult, its form is already that of its 

 species. Hence, it is clear that the form of the young is one acquired 

 by a departure from the ancestral path. To understand the advan- 

 tage of this duality to the insect, we have only to imagine how much 



