FISH CONTROL 



175 



drifts along, fish evidently mistake it for an inanimate object. The 

 slightest movement, however, on the part of the wriggler apparently 

 never goes unseen, and it is instantly seized and devoured by the fish. 

 It often happens that a mosquito larva placed in open water drifts 

 toward places of protection before it is discovered by the minnows, and, 

 if no fish are very near, it moves toward it with a remarkable rate of 

 speed and quickly places itself over the object.' 7 



THE TOP MINNOW (Gambusia affinis) 



The top minnow, Gambusia affinis Baird and Gerard, so far as 

 is known at present, is probably the best fish for general anti- 

 mosquito work, within its habitat, of any of the North American 

 pisces. It is known on the Atlantic from Delaware to Mexico 

 and in the Mississippi Valley from Illinois to Louisiana. It 

 inhabits both fresh and brackish water, while an occasional 



*>z - •.■'• ?-'* 



Fig. 118. — Gambusia affinis, female. (After U. S. Bun an of Fisheries.) 



straggler is taken in strictly salt water. This fish is viviparous — 

 that is, it gives birth to its young. It, therefore, requires no 

 special environment for depositing and hatching its eggs. 



The female, which is larger than the male, rarely measures 

 more than 40 to 45 millimeters in length, although specimens 

 have been taken measuring as much as 65 millimeters in length. 

 The average length of the adult male is 25 millimeters. The 

 young, at the time of birth, are from 8 to 10 millimeters in length. 

 They grow rapidly at first, but several months elapse before 

 they reach full size. 



The sexes in the young cannot be distinguished externally, 

 but the anal fin in the male gradually becomes modified into a 

 long, sharp process which in the sexually mature fish serves as 

 an intromit tent organ. This process is the characteristic 

 identification mark of the adult male Gambusia. Gravid female 

 Gambusia may be identified by a black spot on each side of the 



