2 CHARLES T. BRUES. 



duced to simpler terms. Many entomologists have of course 

 traced the very evident progress of metamorphosis, from the 

 rather gradual series of changes shown in the ontogeny of the 

 more primitive insects, to the highly adaptive, saltatory process 

 exhibited by members of several of the greatly specialized orders, 

 and have interpreted metamorphosis on a physiological basis. 



Before entering into a discussion of those characteristics of 

 insect larvae and pupae which are of interest from a taxonomic 

 standpoint, I hope I may be pardoned for calling attention to 

 several oft-repeated groups of facts relating to insects in general. 

 Insects stand apart from other animals in several particulars. 

 They include a great wealth of species, known mainly in the 

 adult stage through the labors of a large number of systematic 

 entomologists. In spite of the fact that these workers have 

 described an almost unbelievably large number of living species, 

 the insect fauna of every zoological region is still so imperfectly 

 known that, not only are many new species and genera dis- 

 covered each year, but entirely new families and even new orders, 

 are most unexpectedly brought to light. Thus, at least two 

 new orders of insects have been found within the past ten years, 

 while in the same period three very clearly denned families have 

 been added to the well-known order Hymenoptera. The great 

 variety of insects is undoubtedly due to their having appro- 

 priated practically every sort of environment. Thus, aside 

 from the large number of terrestrial forms adapted for aerial 

 life in their perfect condition, many are amphibiotic, or even 

 purely aquatic during either a part or during the whole of their 

 life-cycle. Others have become epiphytic or epizoic, with the 

 great modifications attendant upon this mode of life. Some 

 are internal parasites, most generally of other insects, either in 

 their preparatory stages, or more rarely, during their entire lil< . 

 Highly predatory species, comparable to the birds and beasts 

 of prey, are also numerous, and examples of many other striking 

 methods of life will readily occur to one after a moment's thought. 

 The comparative anatomy of insects may be studied much more 

 readily than that of most other groups of animals, since the 

 skeletal elements are external in position, where they may readily 

 be studied in the most minute details. This condition finds no 



