314 
Psyche 
[September-December 
Head widths of the naiads were used to show the monthly 
variation in growth (Figure 2). Naiads collected in March and 
April had a mean head width of 1 .9 and 2.4 mm (maximum width), 
respectively. This represented the greatest growth rate, 49 per- 
cent, throughout the year. No naiads were found from May to 
July, and the appearance of one naiad in August, 0.62 mm head 
width, corroborates the theory of a new size class beginning. 
From November through April, the temperature steadily in- 
creased from 5 to 15 C, corresponding to the increase in mean 
head widths and greatest growth rate (March to April). When the 
new size class appeared in August, the temperature was 22.7 C 
and dropped steadily until November when the previous pat- 
tern began again. 
Naiadal Food Habits. — Of the 76 foreguts examined, 30 (40%) 
contained food and 46 (60%) were empty. Foregut analyses of 
the naiads of D. robusta indicated that dipteran larvae, primarily 
chironomids, ranked first in percentage frequency of occurrence 
in all seasons. From December through February, chironomids 
comprised the total foregut contents. There was an average of 
5.6, 4.9, and 2.8 larvae per foregut in December, January, and 
February, respectively. In March and April, the naiads fed on 
chironomids and mayflies (Baetidae). The chironomids averaged 
5.6 and 10.8 per foregut in March and April, respectively, and 
the mayflies averaged 1.5 and 1.0 per foregut in March and April, 
respectively. No naiads were collected from May to July. When 
the size class was beginning in August, the single stonefly found 
contained no food. Likewise, from September to November, 
no foregut contents were found in the naiads. Many diatoms 
were found in the foreguts of D. robusta naiads. The predomi- 
nant species were Navicula spp. and Surirella sp., and in smaller 
numbers, Meridion sp., Rhoieosphenia sp., and Fragillaria sp. 
Frison (1935) stated that Isoperla duplicata (= D. robusta) was 
carnivorous but did not mention its diet. Minshall and Minshall 
(1966) noted that two other perlodid stoneflies were carnivorous. 
They reported that Isoperla clio and Isogenus decisus fed on 
larval chironomids and mayflies (Baetidae). 
Adult Stage. — When the naiads of D. robusta are ready to emerge 
as adults, they crawl from the water to the surrounding vege- 
tation or rocks. The emergence data were calculated by counting 
exuviae found on the concrete base of a bridge. Few adults were 
