38 



THE CANADIAN ENTOMOLOGIST 



coupled with a primitive venational scheme, viz., reduction of the mandibles 

 to a weak lamina, (in the case of Perlid(2 followed by an inturning of the clypeus 

 and labrum under the frontal shelf, so that neither of these parts is visible 

 from above), and, in the larva, either absence of gills or replacement of the 

 original segmental gill-appendages of the abdomen by secondary gill-tufts around 

 the bases of the legs and on the first two abdominal segments. Within this 

 complex, the Pteronarcidae keep the more primitive form of venation, very 

 similar to that of the Austroperlidae; like these latter, they have lost both the 

 archedictyori of the anal fan and also the original complete contour of the outer 

 margin of the hindwing. They are also specialized in a unique manner by the 

 approximation of the fore coxae. On the other hand, the Perlidse have a some- 

 what more advanced venational scheme, though some of the original cross- 

 venation still persists in the Perlodinae; the fore coxae remain widely separated, 

 but the joints of the tarsi become specialized, both first and second joints being 

 very short, and the third much longer than both these two together. The 

 Perlodinae difTer only from the Perlinae in the more complete cross-venation of 

 the distal portion of the wing, and therefore cannot be granted at the most 

 more than subfamilv rank. 



PTEHOmRC/M. 



 EI/STHENIIDA 

 ( 



MsnoPEnw/t 



LEPTOPERLIM 



CAPNIID£ 

 NEMOiiniS/E. 



Fig. 2. — Phylcgenetic diagram to show the relationships of the various families of the Order 

 Perlaria. The Eustheniidse are the remains of the original stock, without any specialized 

 characters. The main line of evolution leads first to the Austroperlidae, from them to 

 the Leptoperlidse, and culminates in the Capniid^e and Nemouridae. From far back along 

 the Eustheniid line, an evolutionary sidebranch gave origin to another distinct group, 

 out of which arcss the Pteronarcidte and the Perlidse. 



The differences of the various families may be clearly set out in the following 

 table, in which characters marked A are to be regarded as archaic, those marked 

 B as specialized, while the addition of the letter U to either indicates that it is 

 unique for the family. In the last line, the percentage of archaic characters 

 present for the most archaic members of each family is calculated, the number 

 so obtained giving a fairly reliable indication of the position of the family in 

 the line of evolution. It should always be borne in mind that there are two 

 culminating points for the family, viz., the Perlidae on the one hand, as the 

 end of a side-branch of evolutionary effort, and the Nemouridae on the other, 

 as the end of the main line of ascent of the Order. This idea is indicated in 

 the Phylogenetic Diagram given in Text Fig. 2. 



