390 Annals Entomological Society of America [Vol. VI ^ 



food. From them the food is passed inward to the grinding 

 surface of the molars. Upon the left molar (PI. XLIX, Fig. 34) 

 are eight deep transverse gutters. The upper ends of these 

 are enclosed by irregular teeth and the floors are marked by 

 transverse striations. The right riiolar (Fig. 35) surface 

 bears seven overlapping ridges, all but one of which is bluntly 

 toothed and enclosed at one end by a prominent jagged process. 

 When in position the ridges of the right molar fit down into 

 the gutters of the left and the terminal teeth fit into the free 

 ends of the gutters. The food brushed into the mouth by the 

 labium and maxillae is ground in this mill. 



In the Ephemerinse the greatest modification has occurred 

 in the mandibles which have become the strongest structures 

 of the head, important alike to feeding and burrowing. 



Heptagenince. 

 In the Heptageninae which have scraping mouth-parts, 

 the labrum is entirely hidden from above. It is freely movable 

 upon the clypeus and has a row of very dense, slightly incurved 

 hairs extending along its margin. The inner surface of the 

 labrum is slightly concave, and bears the epipharynx. When the 

 labrum is extended forward the short hairs upon the inner 

 surface rake in the food and are closely followed by the thick 

 brush upon the margin. Food thus gathered in the concavity 

 of the labrum falls directly between the maxillae and mandibles. 



Legs of Nymphs. 

 The legs of May-fly nymphs consist of a coxa, trochanter, 

 tibia, tarsus and a tarsal claw. These parts vary in relative 

 size and structure according to the habit of the nymph. The 

 surfaces may be bare, scaly, spinose or hairy. In all of the 

 legs which have been examined there is a small but distinct 

 plate on the inner side at the distal end of each tibia. 



Bcetin(B. 

 The Bastinae includes nymphs which have the most general- 

 ized legs, such as those of Siphlurus, Callibastis, Ameletus and 

 Chirotonetes. All of these nymphs can move about upon a 

 heterogeneous footing (Figs. 3 and 5). The legs of Siphlurus 

 are of the most generalized type. They are long and slender 

 and the three pairs are of equal length. The surfaces are 

 sparsely covered with inconspicuous hairs. The tarsal claw 



