Larvae of Ephemerella inermis and E. infrequens 
(Ephemeroptera: Ephemerellidae) 
Stephen C. Johnson 
Dept, of Biology, University of Utah, Salt Lake City, 84112 
The larvae of the mayfly Ephemerella infrequens McDunnough and 
the cognate species, Ephemerella inermis Eaton are very similar. To 
date, the best descriptions of the larvae are given by Allen and 
Edmunds (1965), but the characters described are highly variable and 
display enough overlap to make positive identification of most speci¬ 
mens impossible, even with large series of larvae. The larva of a third 
closely related species, E. lacustris Allen and Edmunds, known only 
from Yellowstone Lake, Wyoming, can be easily distinguished from 
the other two by the presence of distinct posterolateral projections 
on abdominal segments 3-9 and claws with 10-13 denticles, while 
larvae of E. inermis and E. infrequens have projections on abdominal 
segments 4-9 and claws with 6-10 denticles. 
Photomicrographic examination of an assemblage of reared 
material of both species generously provided by W.R. Good of the 
University of Wyoming, enabled recognition of morphological charac¬ 
ters that will separate the larvae of the two species. The specimens 
were collected by W.R. Good from two localities in Wyoming: E. 
infrequens from Nash Fork Creek, Albany Co. and E. inermis from the 
Snake River at Cattle bridge, Teton Co. A subsequent redetermination 
of the extensive collection of preserved larval E. inermis and E. 
infrequens at the University of Utah has provided more information 
concerning their range, habitats, and biology. 
Taxonomy 
Adults of the two species are distinguished by the shape of the 
male genitalia (Fig. 1), however, no reliable morphological differences 
between the larval stages have been reported by earlier authors. 
Needham (1927) described the larvae and adults of E. inermis from 
material reared on the Logan River, Utah. McDunnough (1928) pointed 
out that the genitalia figured by Needham in his description were 
those of E. infrequens, not E. inermis. It seems probable that the larvae 
were also E. infrequens and it is therefore interesting to note that 
while the larvae described by Needham had distinct paired pale spots 
on the abdominal terga, later authors (Traver, 1935; Day, 1956; Allen, 
1960, 1968; Allen and Edmunds, 1965; Jensen, 1966) indicate that 
larvae of E. inermis have distinct spots on the terga and larvae of E. 
infrequens do not. 
The Pan-Pacific Entomologist 54:19-25. January 1978 
