1913] Biology of May-Hies. 385 



In the true burrowers, Ephemera (PI. XLIV, Fig. 8) and 

 Hexagenia (PI. XLIX, Figs. 26, 27) both lamellae are broader 

 and the number of the marginal filaments is more than doubled. 

 Each lamella is stiffened by a mid-rib of chitin which overlies 

 the main trachea. By the aid of this rib the gills can be 

 held up over the back where they are not exposed to the con- 

 stant friction as they would be when trailing from the sides. 



Gills of the HeptagenincB. 



The gills of the Heptageninae (PI. XLV, Figs, 10, 11, PL 

 XLVI, Fig. 12) show a series of slightly less homogeneous ad- 

 justments. They are fitted for breathing in different degrees 

 of rapid water, and at their maximum specialization, they are 

 important aids to the nymph in clinging to surfaces. The 

 gills are double except the last one which in Heptagenia is 

 rudimentary. (PL XLV, Fig. 10). The upper division is plate- 

 like and shows greater modification and the lower part is 

 fimbriate-lamelliform or fimbriate, and varies slightly in size 

 and position among different genera. The gills of the Hep- 

 tagenia and Epeorus have the characteristic abundant trachea- 

 tion of swift water inhabitants. In Epeorus the lamellae are 

 large, richly tracheated and lie obliquely recumbent along 

 the sides of the body, (PL XLII, Fig. 4), so that the tips and 

 outer edges touch the surface upon which the nymphs rests. 

 Along this edge is a chitinized band thickly beset with spines. 

 When clinging to stones in the rapid current this edge is pressed 

 tightly down to the surface. The bases of the gills are pro- 

 tected by sharp extensions of each tergite, which project 

 backward .over them. On the inner margin of each lamella 

 near the base is a shallow notch. When the lamellae are 

 held close to the body the fimbriate gill projects through this 

 notch and receives the full wash of the water. The first pair 

 of lamellae are scoop-shaped and curve inward back of the hind 

 legs so that little water flows beneath the body. 



In Iron fragilis there is a similar, but more perfect adhesive 

 apparatus. The outer margins of the lamellae are likewise 

 banded and their position is identical with that just described 

 The first pair of lamellae are much larger, (PL XLVI, Fig. 12), 

 and their tips are held almost in contact. The last pair are 

 folded and slightly curved so that the tips of these also nearly 

 touch. When the margins of these lamellae are closely pressed 



