40 
Psyche 
[Vol. 87 
Monthly samples of nymphs were taken from December, 1974, to 
November, 1975. The bottom substrate was disturbed and a hand 
dredge (mesh size, 0.75 mm 2 ) was placed directly downstream to 
collect the drifting nymphs. Except for the rearing experiment, all 
nymphs were preserved in 70 percent ethanol. 
The water quality parameters were measured monthly throughout 
the study period with a Hach kit, Model AL-36-WR. Dissolved 
oxygen (mg/1), carbon dioxide (mg/1), bicarbonate alkalinity (mg/1 
CaC0 3 ), total hardness (mg/1 CaC0 3 ) and pH were recorded. Water 
temperature was measured each month with a Taylor thermometer 
placed near the surface of the water. 
Length-frequency distributions were recorded for the nymphs in 
order to help determine the duration of the life cycle. The body length 
(from the base of the caudal filaments to the front margin of the head 
excluding the genal spines) was measured under a Bausch and Lomb 
dissecting microscope with a Vernier caliper to the nearest 0.1 mm. 
Both sexes were grouped together and placed in one millimeter size 
groups. 
A population range diagram was used to show the monthly 
variation in nymphal head widths. The head width measurements, 
excluding the eyes, were made with an ocular micrometer mounted in 
a dissecting microscope to the nearest 0.05 mm. Males and females 
for each month were separated and the range, mean, standard 
deviation and standard error of the mean were calculated for each 
group. 
Sex determination of the nymphs was made by the position and 
shape of the eyes. The eyes of the male were closer together and more 
angular toward each other than those of the female. A chi-square test 
was applied to determine any significant departure from the expected 
1:1 ratio at the 0.05 confidence level. 
The foregut content of five nymphs was analyzed for each month. 
The head was severed from the body with micro-dissecting scissors 
and an incision was made along the midline of the ventral body 
surface to the center of the abdomen. The foregut was removed and 
its contents emptied onto a microscope slide with water as the' 
mounting medium. The contents were viewed under a microscope 
with a Whipple ocular grid at 430X magnification. Ten fields or grids 
were selected at random and counted for each nymph. Values were 
given according to the number of small grid squares covered by the 
individual food item. Organic and mineral detritus were grouped 
