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 amoimt 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. 



