WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
259 
largest of these fry was only 25 mm. in length. Both of the ponds from which 
they were taken contained an abundance of beetle adults and larvae of many species, 
and the fact that the buffalofish fry ate none of them shows .that at this early 
stage they prefer other food. The two that did eat beetle larvae were much larger 
fish, and in one of them hydrophilid larvae constituted half the entire food, 
although there were 13 other food items. 
Of the 89 bass examined from pond 3D, 16 were less than 20 mm. and 20 were 
between 20 and 25 mm. in length. The smallest bass that ate any beetle larvae 
was 22 mm. long, and the largest percentage of beetle food was found in those from 
35 to 45 mm. in length. A bass, then, begins to eat beetle larvae as it approaches 
25 mm. in length and increases the amount of this food up to at least 50 mm. 
in length. Above that we have no records, but probably beetles continue in the 
diet somewhat longer, for some of the larvae are of good size. Eventually, of 
course, the bass becomes almost exclusively a fish-eater. The preference for 
dytiscid larvae in these bass was very marked, although there were nearly as many 
species of hydrophilids in the pond, and at least four of them were abundant and 
one ( Tropisternus lateralis ) a prolific breeder. 
In the case of the bluegills also fully one-third (37) of those from pond 2D 
were under 40 mm. in length and nearly two-thirds (90) of those from pond 8D. 
The bluegill does not eat beetle larvae until it reaches a length of 40 mm., or twice 
the length of the bass; but, on the other hand, it never becomes a fish eater like 
the bass and probably continues to take at least some beetle food as long as it lives. 
In support of this an interesting observation was made by Mr. Lay in the 
summer of 1916. Pond ID contained that year 26 adult bluegills and 4,258 young 
fish 2 to 3 years old. Noting that this was the only pond upon which no gyrinids 
were to be seen, Mr. Lay, as an experiment, caught and liberated on the pond at 
different times during the summer numbers of Dineutes americanus adults. In 
every instance these were immediately eaten by the bluegills, who kept this pond 
clear of gyrinid adults all summer. That this was a matter of choice, or was due 
to the fact that the beetles attracted attention through their movements, seems 
evident, because the other kinds of food in the pond were very varied and abundant, 
insect larvae, pupae, and nymphs, Crustacea, mollusks, and a rich assortment of 
vegetation. Pond 2D, which is separated from ID by a narrow bank of earth 
only 15 feet in width, contained that summer bluegill fry and yearlings, which 
were eating freely of beetle adults and larvae, but they did not keep that pond free 
from gyrinids. Most fish do not eat gyrinid adults, perhaps on account of the 
peculiar fluid which they exude when captured, which has a disagreeable odor; 
but neither that nor anything else deterred these bluegills in pond ID. It will be 
noted, furthermore, from the table that the bluegills are impartial in their choice of 
beetle food. Pond 2D contained about an equal number of dytiscids and hydro- 
philids, and their larvae were about equally divided in the fish food. 
The beetle food of the small sunfish from pond 16B was unidentified and might 
well have contained the remains of terrestrial species. The pond, however, con- 
tained six dytiscid and eight hydrophilid species, most of which were breeding 
freely. 
