10 THOMAS SAY FOUNDATION 



must be free to be turned in any position for study. Only 

 the color is lost in boiling. In some of the genera the 

 critical determination of the species is scarcely possible 

 without this treatment, which loosens up the connectives 

 between the segments so that hidden, retractile parts 

 may be exposed. 



DIAGNOSTIC CHARACTERS. 



Stoneflies are the most primitive of winged insects. 

 Metamorphosis is very slight, even nymphal gills in sev- 

 eral genera being carried over into adult life. There is 

 little of that specialization which in the higher insects 

 fixes color patterns and the finer details of structure along 

 sharply defined specific lines. All characters of colora- 

 tion, venation, size and form vary enormously. Even 

 secondary sexual characters, the ultimate criteria of 

 species here as in other groups, are relatively inconstant. 

 The form of the subgenital plate of the female has been 

 much used to distinguish American species and a most 

 useful character it is; but it should be used with discre- 

 tion, for it is subject to malformation and to shrinkage 

 and distortion on drying, and is altered somewhat with 

 the age of the specimens. A critical study of the nu- 

 merous species that have been based on variants of color 

 patterns and variants of form of subgenital plate, has led 

 us to the suppression of a good many names as synonyms, 

 including some of our own. He who, familiar with the 

 relative fixity of small characters in the higher orders, 

 turns to the study of the Plecoptera, will soon learn that 

 he is dealing with differences of another order and of 

 wider latitude. 



In the following pages we propose to give a brief review 

 of those characters that have proved most useful in the 

 systematic study of this order of insects ; also, to point 

 out certain limitations in the use of these characters. 



Stoneflies are soft-bodied insects less constant in struc- 

 tural details, as already stated, than are the higher 

 orders. Most readily observable and least distorted in 

 drying are the wing venation and the proportionate 

 length of the tarsal segments, and these will serve for 

 recognition of the larger groups, while here, as elsewhere, 

 the ultimate criteria for species are found in genitalia. 



The head is broad and flattened, widest across the eyes, 

 that are set far forward in Kathroperla (PI. 1, fig. 11) and 

 well to rearward in Acroneuria (PI. 1, figs. 1, 2, 3, 5 and 



