290 BULLETIN OF THE BUREAU OF FISHERIES. 
Watton, Izaax. 
1653. The compleat angler; or the contemplative man’s recreation, pp. 91, 93, 97, 115. London. 
Wiu1amson, Hucu. 
1802. Onthe Ephemeron leukon, usually called the white fly of the Passaick River. Transactions, 
American Philosophical Society, vol. 5, No. VIII, pp. 71-73. Philadelphia. 
EXPLANATIONS OF PLATES. 
PLATE LXX. 
Hexagenia bilineata Say. 
Fig. 1. The mayflies on the gauge house on the big dam at Keokuk, Iowa, at 5.30 a. m. July 14, 1916. 
They were about equally numerous all along the dam, which is nearlya milein length. Photo 
by Emerson Stringham. 
Fig. 2. The burrows of the mayfly nymphs in a mud bank of the Okaw River near Sullivan, II. 
Burrows exposed by lowering of water in the stream and by undercutting of the current 
giving a vertical section. Photo by Prof. T. L. Hankinson. 
PLATE LXXI. 
Hexagenia bilineata Say; adult. 
Fig. 3. The adult male (segmentation of the tails omitted, as in all the similar figures following). 
Fig. The legs of one side, fore, middle, and hind from left to right, as in the following plates. 
Fig. 5. The end of the abdomen of the male from beneath, showing forceps, penes, and base of tails; 
middle tail rudimentary. 
> 
Pirate LXXII. 
Hexagenia nymphs. 
Fig. 6. The nymph of H. bilineata Say; dorsal view. The hind feet are turned forward for compari- 
son with the other feet, and the gills on the right are moved aside to show the markings on 
the abdomen. The 3-branched, rudimentary gills on the first abdominal segment are 
abnormal. (See fig. 13.) 
Fig. 7. Portion of head of same, more enlarged. 
Fig. 8. Comparable portion of head of H. recurvata Morgan. 
Figs. 9,10, 11. Fore, middle, and hind legs, respectively, of H. bilineata Say; the front leg adapted 
for shoveling, the hind leg for pushing. 
Fig. 12. The unbranched rudimentary first gill of H. recurvata Morgan. 
Fig. 13. The normal bifid rudimentary first gill of H. bilineata Say. 
Fig. 14. The functional second gill of same, showing its fringes of respiratory filaments. 
PLaTe LXXIII. 
Pentagenia vittigera Walsh; adult. 
Fig. 15. Adult male of the quadripunctata form. 
Fig. 16. Legs of one side. 
Fig. 17. End of abdomen of male from beneath. 
Fig. 18. End of abdomen of female from beneath. 
PLATE LXXIV. 
Pentagenia vittigera Walsh; nymph. 
Fig. 19. The nymph from above, with the gills on the right pushed aside to show the markings of the 
abdomen. 
Fig. 20. Head of same from above, more enlarged. 
Fig. 21. Mandible of same, showing serrated tusk. 
Figs. 22, 23, 24. Fore, middle, and hind legs, respectively. 
Fig. 25. A rudimentary gill of the first abdominal segment. 
Fig. 26. A functional gill of the second abdominal segment. 
