1913] Biology of May-flies. 391 



is long, slender and without teeth. The tibial plate is well 

 developed, consisting of a thick, flattened projection of the 

 tibia, which bears transverse ridges. The attachment of the 

 legs allows free movement and the nymphs are capable of 

 running very swiftly. The middle and hind legs of Chiroton- 

 etes are similar to those which have been described, but the 

 first pair has been modified for food gathering and respiration. 

 At the base of the coxa, there is a large tuft of forked gill 

 filaments. From the tibia an elongate flattened spur extends 

 for more than half the length of the tarsus, and along the inner 

 margin of femur, tibia, and tarsus is a regularly arranged 

 Vow of very long, stiff hairs. When the legs are sharply bent, 

 these hairs, together with the tibial spur form the bottom of 

 the plankton basket already referred to. 



In Casnis, Tricorythus and Ephemerella the legs do not 

 lift the bodies at all. Nymphs of Caenis and Tricorythus clamber 

 upon very uneven surfaces so that the legs do not extend straight 

 out from the body as they do in some of the Ephemerellas later 

 noted. The strain of pulling and climbing comes evenly upon 

 every segment of the leg and there is little difference in their 

 size. In both of these genera the tarsal claws are in constant use 

 and are correspondingly well developed. The same evenly 

 distributed development may be seen in the legs of certain 

 Ephemerellas, which constantly crawl over mud, dead leaves, 

 and small debris. In others in which there are well established 

 clinging habits (PI. L, Figs. 39, 43), the fore femora are enor- 

 mously developed by the constant pulling incident to their 

 position. In these legs the hinder part of the femora is greatly 

 thickened by the muscular development, but the front edge 

 is thin and blade-like and often jaggedly toothed or serrate. 



EphemerincB. 



Two stages of modification are shown in the legs of Pota- 

 manthus and Ephemera. In Potamanthus, which crawls 

 upon the bottom in a manner similar to the Heptageninee, 

 the legs sprawl out from the body in the same way. The 

 tibia . is prolonged into a flat spine which overlaps the first 

 third of the tarsus. The structure of this fore leg appears 

 to be the fore-runner of the greater modification shown in the 

 fore legs of Ephemera, (PI. XLIV, Fig. 8), which are perfect 

 digging tools. 



