1913] Biology of May-flies. 389 



cling to the rocks with its head upstream. In doing this 

 it uses its fore legs little, but they are held up and straight 

 forward close beneath the labium. Armed with long bristles 

 as they are, they help to form an efficient plankton basket 

 which catches the food carried along in the water. 



In Caenis, Tricorythus and Ephemerella the mouth-parts 

 are often reduced. In all these the mandibles are stout with very 

 strong canines (PI. XLVII, Figs. 16, 19, and PL XLVII. Figs. 

 20, 21). Structures like the palpi which extend out from the 

 mouth are much shorter. In Tricorythus (Fig. 22), the body 

 of the labrum is strong, but the palpi are weak and stubby. 

 This reduction is carried to the limit in the maxillae of Eph- 

 emerella deficiens in which the palpi have disappeared, leaving 

 only a little peak of chitin at their attachment place (Fig. 18). If 

 one observes nymphs of Tricorythus or Ephemerella forag- 

 ing, they will see them continually thrusting their heads 

 through harsh gravel where such appendages would be in the 

 way. 



The Bcetiuce have the most generalized mouth-parts ex- 

 amined. This group includes species in which there have been 

 modifications of the mouth-parts for rakers and plankton 

 baskets, and great reduction of palpi. 



EphemerincB 



Nymphs of Potamanthus, Polymitarcys and Hexagenia 

 all gather their food in the same places and by the same means. 

 Their relative specialization has been closely correlated with 

 the extent to which they have been modified for burrowing. 

 The mandibles of Potamanthus (PI. XLVIII, Figs. 23 and 24) 

 show the beginning of this modification. The canines are 

 here in their usual position at the tip of the mandible (c), 

 but upon the outer side of each is a stout pointed process. 

 These processes are not long, and when the mandibles are in 

 natural position only their tips show beyond the labrum. 

 These processes are similar in shape and identical in position 

 with the tusks of the true burrowers. Ephemera and Hexa- 

 genia (PI. XLIX, Figs. 31, 36). In these the processes are long, 

 slightly incurving tusks which are the most conspicuous features 

 of the head. The canines are on the median side of these near 

 the base, and when the mandibles are in natural position, 

 they extend downward and can thus most efficiently grasp 



