Records and Descriptions of North American Plecoptera. Part III. Notes on Isogenoides

Abstract

For the loan of the material which made this study possible I am deeply indebted to Prof. N. Banks of the Museum of Comparative Zoology at Harvard University, to Dr. C. E. Palm and Dr. H. Dietrich of Cornell University, and to Dr. A. B. Gurney of the United States National Museum. Their kindly cooperation and the wealth of help and advice received from Dr. C. P. Alexander and Dr. G. C. Crampton of this department have made this project both interesting and profitable. Isogenus was erected as a genus by Newman in 1833. In 1904 Klapalek split off the subgenus Dictyogenus on the basis of an apical network of crossveins of the wings. In 1912 he erected Isogenoides as a subgenus of Isogenus to receive some North American species with a nail on the posterior ventral margin of the seventh abdominal segment of the male. A reanalysis of these groups shows that Dictyogenus, like other Perlodid genera based on very superficial and highly variable wing venational characters, is not valid. The few individuals at hand show, as would be expected in the Perlodidae, that the number and position of apical crossveins varies to such a great extent that intermediates are numerous and the character cannot be used to define Dictyogenus. In addition, thoracic sclerites, mouthparts, and other head structures of Dictyogenus and Isogenus are identical and the genitalia also are of a single type. Isogenoides, however, exhibits some very fundamental differences. In the thoracic sternum of this group are generic characters which are even more distinct and unique than those found bv Klapalek in the genitalia. On the basis of these discoveries I am here synonymizing Dictyogenus under Isogenus; and to give Isogenoides its justified equal standing with related groups I am raising it to generic rank. Isogenus seems to be represented in North America by numerous species, placed in the literature in the genus Perla. Of Isogenoides there are a half dozen North American species known to me. Not a single one of these species, however, has been placed in its proper genus since the original work of Klapalek (1912). Needham and Claassen in their monograph placed them in Isogenus; Banks placed some in Perla; and Frison placed one in Hydroperla. More species of Isogenoides than are discussed here may eventually be recognized as already described and improperly placed in other genera of Plecoptera.