1 82 BULLETIN OF THE BUREAU OF FISHERIES. . 



waters as well as into artificial ponds, aquatic gardens, etc., in mosquito-infested regions 

 may, therefore, play an important role in the extermination of these pests. Experi- 

 mental work of this kind already undertaken in New Jersey suggests that the plan of 

 combating mosquitoes by the introduction of Gamhiisia and other fishes with similar 

 habits is entirely feasible. 



As was pointed out by Seal " (1908), Gambusia and the related genus Heterandria 

 possess certain habits and characters which render them superior to all other fishes 

 as mosquito destroyers. As suggested by their common name, top minnows, they feed 

 at the surface. Being of small size they readily find their way into shallow waters 

 which are inaccessible to larger fishes. Gambusia affinis is often found in large numbers 

 in water less than an inch in depth. Furthermore, it habitually searches for food among 

 the vegetation and debris along the borders of pond or stream. In Mullet Pond it is 

 rarely found in the open water, but is present in abundance among the marsh grasses 

 along the swampy borders, where it not only finds food but is also protected from larger 

 fishes. 



The small size of this species, its viviparous habits, and its hardy nature ought to 

 render its introduction and maintenance in new waters comparatively easy. It thrives 

 under a wide range of conditions. Furthermore, the young, being brought forth in an 

 advanced stage of development, are not subjected to many of the dangers which beset 

 the young of oviparous fishes. 



The breeding season continues during the spring and summer, several broods being 

 produced during the season. Seal '' (191 1), obser\'ing these fish in captivity, has demon- 

 strated that two or more generations may be born in a summer. 



The adult females vary greatly in size, ranging from 3 to 6.5 centimeters in length. 

 The males are relatively fewer in number and smaller than the females. The adult 

 males range from 1.8 to 3 centimeters in length. Nearly all of the adult females taken 

 by the writer during July, 191 2, carried either mature ova or embryos. 



The present investigation was carried on at the United States Fisheries Laboratory 

 at Beaufort, N. C, during the summer of 191 2. 



REPRODUCTIVE ORGANS. 



FEMALE. 



Ovary. — The ovary is located in the abdominal cavity just beneath the air bladder 

 and dorsal to the posterior portion of the intestine. It opens directly into the urogenital 

 sinus, which communicates with the exterior through the urogenital aperture just 

 posterior to the anal opening. It is a paired tubular organ, but, unlike the ovary of 

 many teleosts, it is not bifurcated and has no distinct median wall. The left side of the 

 ovary is always shorter than the right. (PI. xvi, fig. 7.) This disparity in the length of the 

 two sides of the ovary is due to the position of the stomach, which is located in the left 

 side of the abdominal cavity. When distended with mature ova or embryos, the ovary 

 fills the greater part of the abdominal cavity beneath the air bladder and causes con- 

 siderable distension of the abdominal walls. At the left the ovary in this distended 



a Seal. William P.: Fishes in their relation to the mosquito problem. Bulletin Bureau Fisheries, vol. xxvn. 1903. p. 831-838. 

 6 Seal, William P.: Breeding habits of the viviparous fishes Gambusia holbrookii and Heterandria formosa. Proceedings of 

 the Biological Society of Washington, vol. xxiv, p. 91-96. 



