STRUCTURE AND DEVELOPMENT 67 



already familiar to every one. The egg of a common 

 house-fly hatches into a larva called a maggot ; in this 

 condition the body destined to become the vastly differ- 

 ent fly is composed of soft-skinned segments very much 

 alike and also similar to the joints of a worm. Com- 

 parative anatomy demonstrates that the fly and all 

 other insects have arisen from wormlike ancestors, 

 whose originally similar segments later differentiated 

 in various ways to become the diverse segments of adult 

 insects ; the embryonic history of flies of to-day corrob- 

 orates these assertions, in so far as every individual fly 

 actually does become a wormlike larva before it changes 

 into the final and complete adult insect. The other 

 kinds of insects are equally striking in their hfe-his- 

 tories. All beetles, such as the potato bug and June 

 bug, develop from grubs which, like the maggots of flies, 

 are similar to worms in numerous respects. Butterflies 

 and moths pass through a caterpillar stage having even 

 more striking resemblances to worms. All the larvse of 

 insects are therefore like one another, and like worms 

 also, in certain fundamental characters of internal and 

 external structure; so the conclusion that the whole 

 group of insects has arisen by evolution from more 

 primitive ancestors resembling the worms of to-day is 

 based upon mutually explanatory details of compara- 

 tive anatomy and embryology. 



Let us now turn back to some of the earlier pages of 

 the embryological record which we passed over in order 

 that we might translate the later portions dealing with 

 more familiar and inteUigible structures hke gills. Be- 



