434 
NORTH AMERICAN PLECOPTERA 
hal)its, egg-la}4ng and mating. In attempting the identification 
of the material upon which I was working, a great lack of detailed 
systematic knowledge was keenly felt. In many instances 
original descriptions are inadequate both on account of their 
brevit}^ and also because of the characters chosen for the basis 
of the descriptions. Coloration, which is exceedingly variable 
in the group, has often served as the chief character. Venation 
has been used also, but here again the inconstancy of certain 
characters that were selected, such as the number of tips of veins 
or the number of cross-veins, makes them of little value. The 
need for a more complete systematic working basis seemed so 
great that, at Prof. Needham’s suggestion, the ecological work 
was put aside temporarily, and such a study commenced. 
Prof. Franz Klap^lek (Prague, Bohemia), who has done per- 
haps more than anyone else on the group in recent years, has used 
with considerable success genital and venational characters as 
the basis for his desciiptions. In my own work on the North 
American forms I am also making them the essential characters 
in my descriptions. The genitalia of this order are not only 
constant in form for each species, and, therefore good characters, 
but they are so diverse, and, in many instances, so remarkable, 
that they are unusually distinctive. Although stone-flies are 
very piimitive in many ways they have reached a high degree of 
specialization in this respect. Quite apart from their value 
systematically, the genitalia are intensely interesting, on account 
of their complexity, from morphological and physiological view- 
points. 
In the order Plecoptera I recognize a single family and four 
sub-families: 
Perlidae 
Pteronarcinae 
Perlinae 
Nemourinae 
Capninae 
The present study, which is only a beginning of a monograph 
of the order, treats the Pteronarcinae and a portion of the Per- 
linae under the tribal name Perlodini. This sub-family and tribe 
can be distinguished from all other Plecoptera on the presence of 
an apical network of supernumerary cross-veins. In the Pteron- 
arcinae the network extends in the fore-wing from costa through 
the anal veins. In the Perlodini it is of greater or less extent but 
