NEUEOPTERA— EPHEMERID.55. 121 



2. Ephemera immobilis. 



PI. 12, Fig. 5. 



Larva. — This is the hxrgest of the ephemerid larvse, and is represented 

 by a single specimen and its reverse. The body is stout, largest at the meso- 

 thorax and metathorax, tapering rapidly and somewhat rounded in front, 

 tapering gently behind, the hinder half of the abdomen more rapidly than 

 the basal half The head is small, about as broad as the terminal segment of 

 the body, transversely rounded oval, less than half as broad as the thorax, 

 and symmetrical, being rounded in front as behind ; the mandibles, not rep- 

 resented on the plate, are not so long as the head, moderately stout, nearly 

 straight and tapering. The front legs are nearly as long as the thorax, the 

 femora and tibiae, which are of equal width, nearly or quite as broad as the 

 length of the prothorax ; the tibia is a little longer than the femur and 

 about half as long again as the tarsus, which is also somewhat slenderer. 

 The other legs are longer and a little stouter, but retain the same relations, 

 excepting that the tarsus is much longer, half as long again as the tibia 

 and toward the tip tapering. The thoracic branchiae form a pair of trian- 

 gular equilateral pads, their inner margins straight and attingent at the 

 mediodorsal line, their outer margins convex. The dorsal surface of the 

 abdomen is ornamented by a pair of approximated subdorsal, longitudinal, 

 curved, white streaks, convexities outward, reaching the posterior but not 

 the anterior border of each segment. The caudal setse are of unequal 

 length, the outer more than one-third, the middle one nearly one-fourth, the 

 length of the body. They ai"e fringed, the outer ones on the inner surface 

 only, the middle one on both sides by a delicate fringe of hairs, which 

 increases in breadth from either end toward the middle, where the fringe is 

 from a third to a fourth the width of the last abdominal segment. 



Length of body, 21°"" ; breadth of thorax, S""™ ; of head, 2.4°"° ; length 

 of fore tibia, 2.25"™ ; breadth, 0.6""" ; length of hind tibia, 2.75""°^ ; breadth, 

 O.S""" ; length of outer caudal setae, 8"" ; of middle seta, 5™". 



The species differs from the other larvae here described by its greater 

 size and the peculiar fringing of the caudal setae. 



Florissant. One specimen, Nos. 8824 and 8828. 



