12. Coleoptera, Hemiptera, Hymenoptera, 100 — 



Arachnida, Mollusca, Nemotoda, 

 Hirudinea, Pisces (ova). 

 1 1. Crustacea. 



1H>— 



10. Plecoptera 

 9. Diptera (imagines) other than Simu- 



liidae 

 8. Diptera (larvae and pupae) other than 



Simuliidae. 80 — 



7. Simuliidae (imagines). 



6. Simuliidae (pupae). 



5. Simuliidae (larvae). 



4. Trichoptera (imagines). 



3. Trichoptera (larvae and pupae). 



2. Ephemeroptera (sub-imagines and 

 imagines). 



70- 



00— 



10- 



30— 



20— 



1. Ephemeroptera (nymphs). 



10— 1 



9,10,1 1,1 



5 



FFTrl 10,11,12 



41 

 Usinger: Introduction 



1 I 'l lO.ll.l Wv ■ u_ii_i —100 



nun nii n na n u 



90 



. HI 



— 70 



— 60 



50 



40 



— 30 



— 20 



10 



April 



Jun 



July 



Intro, fig. 57. Composition of food material month by month. The sections of each column 

 represent, superimposed in the order shown on the left, the percentage of the month's food 

 material formed by the different groups drawn on (Neil 1 , 1938). 



make up 1 per cent or more of the fish's diet will 

 be considered as food organisms. Sufficient numbers 

 of both bottom samples and fishes' stomachs shall 

 be taken to keep the standard error within 10 per 

 cent of the mean. 



Another factor to consider in food preference studies 

 is the rate of digestion of different organisms eaten 

 by fish. Hess and Rainwater (1939) found that soft- 

 bodied organisms, such as many dipterous larvae, 

 are digested and passed through the alimentary tract 

 much more rapidly than heavily chitinized forms such 

 as stonefly nymphs. Differences in rates of digestion 

 of various organisms and differences in time of expo- 

 sure to digestion in different stomachs (owing to 

 delays in dissecting stomachs, gathering fish from 

 traps or gill nets, etc.) therefore should be considered 

 in interpreting food preference data. 



It is probably too early in the development of the 

 science of aquatic entomology to outline an ideal 

 procedure for the analysis of fish-food organisms 

 because too many of the basic techniques are inade- 

 quate and too little information is available on insect 



life histories. However, Neill (1938) has conducted 

 a comprehensive study on the River Don in Scotland 

 that might well serve as a model for future investiga- 

 tions: 1. The general characteristics of the site were 

 recorded on a sketch map and summarized, including 

 area, gradient, current velocity, depth, nature of 

 bottom, nature of banks, influence of pollution, and 

 general nature of the biota, plant and animal, includ- 

 ing fish, and, as far as possible, synchronous field 

 observations were made both in regard to day and 

 hour (noon); 2. Daily observations were made of air 

 temperature, barometric pressure, light intensity, 

 rainfall, wind, water temperature, depth, current speed, 

 water samples for pH, alkali reserve, carbon dioxide 

 content, oxygen content, bottom samples (using a 

 cylindrical sampler somewhat like the Hess modifica- 

 tion of the Surber sampler), plankton samples, surface 

 samples, and fish captures, and preservation of stom- 

 achs; 3. The bottom fauna for the limited area during 

 spring and summer (intro. fig. 54) was compared with 

 the food material (intro. fig. 55) taken from the stom- 

 achs of fish feeding in the same area over the same 



