WHITE SUCKER 
181 
This is true of fish of every age, delicate sand shells of Difflugia, the brittle shell of 
Arcella, and the shells of small bivalves all passing entire into the intestine. Prob- 
ably all material listed as fragments of insects was broken in the process of tearing 
apart the food mass with dissecting needles. This raises the question as to the 
function of the pharyngeal teeth, so well adapted by their shape to crushing the 
food. From the structure of these teeth Forbes (1890) assumed that Mollusca 
constituted a large portion of the diet, and Baker reports that bivalves make up 30 
per cent of the food; but Baker’s report (1916) is based on the examination of but 
three adult specimens and is therefore hardly conclusive. I have found single 
instances where the fish has gorged itself on one only of each of the items listed 
above, but these are meaningless if taken alone. Reighard (1915) reports a similar 
occurrence, finding the diet of five suckers (43 to 50 millimeters long) to consist 
“almost wholly of a cladoceran.” 
SUMMARY 
From the point of view of the economy of the organisms that must obtain their 
sustenance in the same waters as the sucker, the sum of the food eaten by the latter 
at all ages is of importance. 
For 162 fish heretofore listed by groups only, the average percentages are as 
follows : 
Percentage 
Chironomids 
Odonata 
May fly 
Caddis fly 
Entomostraca 
Mollusks 
Diatoms 
Algae 
Protozoa 
Rotifers 
Sand 
Other insects 
Unidentified (mucus) 
32 . 9 
3 
3 . 1 
4 
6 
. 1 
11 
2 . 4 
4 . 5 
2 . 9 
13 . 3 
3 . 7 
13 . 1 
The variation in capacity of the fish is not taken into account here. Obviously 
the larger the fish the more insects necessary to make up a given percentage of the 
volume of its intestine. The greater numbers of smaller suckers in a stream would, 
however, make up in part, at least, for their smaller capacity. 
Comparison of the food taken during the three principal periods 
[Figures are average percentage] 
Food 
Top-feeding 
period; 12 
to 16 milli- 
meters, in- 
clusive 
Fingerling 
period; 17 
to 75 milli- 
meters, in- 
clusive 
Adult 
period; 3 
to 12 
inches 
Food 
Top-feeding 
period; 12 
to 16 milli- 
meters, in- 
clusive 
Fingerling 
period; 17 
to 75 milli- 
meters, in- 
clusive 
Adult 
period; 3 
to 12 
inches 
Number examined 
25 
67 
62 
Algas 
2 
2.4 
Chironomids 
62 
28 
30 
21 
.9 
Diatoms and desmids 
8 
22 
3.6 
9 
Rotifera 
7 
4 
9 
Other arthropod fragments. 
7 
1 
5.4 
Caddis-fly larvas 
13 
Entomostraca 
4 
3 
5 
Mollusca 
.2 
Protozoa 
.5 
9 
.1 
Unidentified (mucus) 
11.5 
10 
21.4 
