I METAMORPHOSES OF INSECTS 15 



development or evolution of the one self-same animal. 

 The caterpillar in the egg is already possessed of the 

 principal vital organs, which have to be modified and 

 perfected ; others appear later on ; and often early- 

 existing structures die away as growth advances. 

 Nothing will more plainly show the fact that meta- 

 morphoses are but a progressive development of the 

 immature into the mature being than a brief examina- 

 tion of some of the structures which are altered, and 

 of those which put in their appearance during the 

 successive stages of life. 



An insect may be broadly defined as an animal 

 that is formed of a series of rings or segments.* They 

 can be traced in the embryo within the egg, they are 

 seen plainly in the larva, and can be distinguished in 

 the perfect insect. The normal number of distinct rings 

 in all insects is probably thirteen. The first consti- 

 tutes the head. In most larvae the remaining twelve 

 are equally developed and differ little from each 

 other in general appearance. But an imago is 

 evidently distinctly divisible into three pieces the 

 head, the body or thorax, and the abdomen. How- 

 ever greatly the forms of these regions may vary, the 

 difference is simply due in each case to greater or 

 less development of the primary segments, and in 

 many cases the various segments can be satisfac- 

 torily numbered and recognised. A difficulty lies 

 in the fact that the changes are not brought about 

 by alteration in the consistence of the integu- 

 ments only, but by the union of several segments 

 into one or more separate consolidated portions, and 

 by the greater or less retraction of segments, each 



