Whil e recently engaged in st udying some specimens of Omop!troJI americanum, my attention was attracted to the fact that in the males of this species the second joint of th e middle tarsi is ve ry distinctly dilated. The fact being new to me I was led to exa min e our other species in order to ascertain whether or not the character was co mmon to all those occurri ng within o ur faunal limits. As a resu lt I find that th.e North American species m ay be d i,•ided into two groups based upon t hi s \ , . . . . "