INTRODUCTION 23 
certain other groups of insects, such as the grass- 
hoppers, the newly hatched young resemble the 
adult in many ways, differing principally 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 medusee.  In- 
stead of remaining attached to the parent colony 
the medusz, 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. 
