220 



BULLETIN Olf THE BUREAU 01? FISHERIES. 



things being equal, the more natural the pond and its surroundings can be kept the 

 greater will be the likelihood of success. Artificial conditions are usually difficult of 

 maintenance and should be established only when necessary. The dragonflies and 

 nymphs constitute an important factor in the environment of the ordinary fishpond, and 

 even their voracious appetite is accounted for in nature's methods of equalizing 

 things. This is shown by the fact that they eat one another with as much avidity and as 

 little compunction as is shown toward any other kind of food. Their removal, therefore, 

 would considerably disturb the balance and impose artificial conditions that might be 

 difficult to handle. The station is having much success in rearing various kinds of 

 fish, among which are the buffalofish and channel catfish, whose breeding is admitted to 

 be very difficult. Some of this success may well be due to the maintenance of a natural 

 equilibrium in the animal and vegetable life of the fishponds. 



Till yard (191 7, p. 335) has suggested that such an equilibrium may be advanta- 

 geously rhodified without in the least impairing its value : 



Not only are the dragonflies the most powerful determining factor in preserving the balance of 

 insect life in ponds, rivers, lakes, and their surroundings, but they do most certainly make war upon the 

 flies, mosquitoes, and gnats, which we all desire to see exterminated. I believe that a successful check- 

 ing of the mosquito pest in the ornamental waters of parks and gardens could be readily obtained by 

 the introduction of species whose larvae, as well as the imagines, would prey upon the nuisance. If a 

 successful planting of a colony of dragonflies in such a position were to be tried, the species selected 

 might also be chosen for its coloring, and thus add a new note of interest to the locality. The glorious 

 red Orthetrum villas ovittatum has now become well established in the Botanical Gardens at Brisbane 

 (Australia) and certainly adds a vivid touch of color to its lovely surroundings. 



The suggestions herein contained naturally lead up to the next consideration, which 

 seems worthy of a separate heading. 



Odonates as Destroyers of Mosquitoes, Gnats, and Flies. — The quiet waters 

 of an artificial fishpond furnish admirable conditions for the breeding of mosquitoes, and 

 the screens at their outlets afford similar breeding places for gnats. The mosquitoes 

 may include, in the proper geographical localities, both Anopheles and Siegomyia, the 

 carriers of malaria and yellow fever. Obviously in a fishpond these pests can not be 

 kept down by treatment vnth an oil film, neither can they be allowed to breed unhindered. 

 The consumption of the larvae by young fish might furnish an important check, but in 

 intensive fish culture very little attention can be paid to the attitude of the fish toward 

 mosquito larvae. 



Of the nine ponds in series D at Fairport six were stocked in the spring with fish 

 that would not eat mosquito larvae. In some of these ponds broods of young fish were 

 raised later in the season, but previous to their appearance the mosquitoes could breed 

 unhindered by the fish. In at least two of the six ponds no young fish were raised, or 

 they were removed before they had time to produce any effect upon insect larv^. Here, 

 therefore, so far as the fish were concerned, the mosquitoes might hold undisturbed sway 

 during the entire season. 



No fish eats adult mosquitoes; when the pupae are once safely transformed into 

 imagos they are in no danger of further molestation from that source. Hence if the 

 fishpond is to be prevented from serving as a breeding ground for these obnoxious pests 

 some other check must be provided. Mention has already been made of the fact that 

 the odonate nymph eats mosquito larvae and pupae, and the adult dragonfly is an 

 even greater enemy of the mosquito imago. Till yard (1917, p. 328) stated that he 



