INTRODUCTION 23 



certain other groups of insects, such as the grass- 

 hoppers, the newly hatched young resemble the 

 adult in many ways, differing i)rincipally in the 

 absence of wings. The young Rocky Mountain 

 locust (Melanoplus spretus), for example, changes 

 its exoskeleton (molts) five times before the adult 

 condition is attained. After each molt there are 

 slight changes in color, structure, and size, the most 

 notable difference being the gradual acquirement of 

 wings. In still other orders of insects a larva 

 hatches from the egg ; this larva, on reaching its full 

 growth, changes in shape and structure, becoming a 

 quiescent pupa, from which after a rather definite 

 interval an adult emerges. 



A combination of two simple life cycles to form one 

 complex cycle occurs in certain hydroids. The 

 eggs of these species produce free-swimming em- 

 bryos which become fixed to some object and de- 

 velop into polyps. These polyps form other polyps 

 like themselves by budding, but finally give rise to 

 buds which become jelly-fishes or medusae. In- 

 stead of remaining attached to the parent colony 

 the medusae, as a rule, separate from it and swim 

 about in the water ; they later give rise to eggs which, 

 after being fertilized, develop as before into polyps. 

 There are thus in this species two life cycles com- 

 bined, that extending from the egg to the time when 

 the colony forms medusa-buds, and that beginning 

 with the medusa-bud and ending with the mature 

 egg. Such an alternation of an asexual and a sexual 

 generation is known as metagenesis. 



