88o FRESH-WATER BIOLOGY 



condition ; it indicates only that, in these relatively primitive forms, 

 structures developed to a considerable extent upon the immature 

 stages have, in the rapid and incomplete transformation these under- 

 go, been carried over in rudimentary form into adult life. 



Among aquatic insects are many beautiful and interesting 

 forms. The keys and figures in the following pages should enable 

 anyone who has learned the parts of the body of a grasshopper, 

 or who has mastered such elementary knowledge of insect anatomy 

 as every textbook of zoology or of entomology affords, to identify 

 most of the insects he will find in the water. There are many gaps 

 in our knowledge of all the groups; even the adult insects are not 

 well known except in the showier groups, which have always been 

 more attractive to the collector ; and so many immature forms are 

 still unknown, it has been found impracticable to attempt to give 

 keys even to the genera in two orders, Plecoptera and Trichoptera. 

 Limitations of space have compelled restriction to the larger 

 groups among the Diptera. In most of the groups having com- 

 plete metamorphosis, the characterizations of the immature stages 

 have been adapted from the accounts of European writers, very 

 little having as yet been done on them in America. Here is an 

 attractive field in which the amateur and the isolated student may 

 still find pioneer work to do. 



It is the purpose of this chapter to assist the student toward 

 acquaintance with such insects as he may find in the water. The 

 limitations of space allow but brief notice of the natural history of 

 any of the groups and restrict the keys to dealing with families and 

 genera. The aim is to supplement the general works on entomology 

 and not to duplicate any part of them. Keys to the orders of 

 adult insects are available in a number of manuals and textbooks, 

 hence there is need here only to point out the readier recognition 

 marks of those orders which commonly occur in the water, and to 

 give a key to the immature stages. 



RECOGNITION CHARACTERS 



There are but nine orders of insects commonly found in water 

 in any stage: Plecoptera, Odonata, Ephemerida, Hemiptera, Neu- 

 roptera, Trichoptera, Lepidoptera, Coleoptera and Diptera. 



