222 



BUIvI^ETlN 01?^ THE BUREAU I^ISHERIES. 



came to the laboratory building (loo yards from pond iD) and picked house flies, May- 

 flies, and the like off the window screens. In view of the present widespread movement 

 against the house fly conducted by boards of health and hygienists everywhere in the 

 United States, this fly-eating habit of the odonates ought to receive every encouragement. 



In this connection also it is worth noting that Dr. G. D. Carpenter, during his inves- 

 tigation of the sleeping sickness in Africa, observ^ed one species of damselfly and two 

 species of dragonflies feeding upon the dreaded tsetse fly, the damselfly even picking 

 them off the clothing of the collector. (Campion, 1914, pp. 498 and 500.) 



Another record in this same paper and one by Poulton (1906, p. 399) credit the odo- 

 nates with eating horseflies (Tabanidae). 



ENEMIES OF ODONATE IMAGOS. 



Most authors state that the imagos do not suffer much from natural enemies except 

 during the teneral period, and this appears to be true. But during this teneral period, 

 which lasts for a varying length of time after their emergence from the nymph skin, 

 they are so weak and limp that they fall an easy prey to even the humblest enemies. 



1. Accidents. — ^A small percentage always fails to emerge properly, and in 

 collecting exuviae one will occasionally be found with the imago only partially emerged. 

 Something prevented it from getting clear of the nymph skin, and it perished in the 

 effort. Again, one or two of the wings may fail to expand properly after the imago has 

 gotten safely out of the skin, and it is then unable to fly and soon perishes. Sometimes 

 the teneral is forced to try its powers of flight too soon, and it falls into the water and 

 drowns. The number of these accidents is probably larger than appears at first, for 

 such drowned imagos easily disappear. 



Rain sometimes catches the tenerals before they have become sufficiently hardened 

 to withstand it. Kennedy (191 7, p. 530) makes a note of this: 



With many western species the most serious cause of premature death among imagos seemed to 

 be the occasional cold rains which come even in desert regions. On Satus Creek (Yakima County, 

 Wash.) I have seen Ophiogomphus severus practically wiped out for the first day or two after a rain and 

 regaining its numbers only after more had emerged. 



In the Mississippi Valley a heavy thundershower will sometimes produce the same 

 effect upon the tenerals, the rain fairly sweeping them off their perch and drowning 

 them in the gutters. 



Usually those which perish in these different ways, however, form but a small 

 percentage when compared with the innumerable hosts that pass through the meta- 

 morphosis successfully. 



2. Birds. — Lihellula luctuosa emerges mostly in the early morning, and for a long 

 time hundreds of teneral wings of this species, easily recognizable by their markings and 

 varnished appearance, were found every forenoon lying on the ground and among the 

 vegetation on the embankments of the ponds. At length the culprits were caught in 

 the very act of seizing and devouring the imagos, and they proved to be English sparrows. 

 They flocked to the embankments at daybreak and hunted through the herbage until 

 they found a teneral; they then seized it, beat its wings off, and either swallowed it or 

 carried it to their young. In this way they destroyed large numbers every day and kept 

 it up as long as the species continued to emerge. When its wings are once hardened, 

 the sparrow can no longer catch the imago, and it is thereafter free from this enemy. 



