INFLUENCE OF ENVIRONMENT 253 



In the years 1914 and 1915 Dr. F. E. Chidester (4) undertook and 

 carried out an intensive study of the fish enemies of the salt marsh mos- 

 quitoes. The investigations showed clearly that the common killy (Fuiv- 

 dvlus heteroclitus) is by far the most important fish enemy of the salt 

 marsh mosquito but that the fresh water killy (Fundulus diaphanous 

 Le Sueur), the sheepshead minnow (Cyprinodon variegatus Lacepede), 

 and the top minnow (Gambusia affinis — Baird and Girard) are com- 

 monly associated with it and play a strong secondary part. 



Fig. lxxxi. Sheepshead minnow. Cyprinodon variegatus Lacepede. (From 

 Jordan and Evermann, U. S. Nat. Mus., Bui. 47). 



Dr. Chidester's studies showed that this common killy migrated to 

 the shallowest pools of the salt marshes and traveled up streams into 

 water of very low salinity, that when in land-locked pools it is able to 

 maintain itself over winter by burying itself in the mud, but that when 

 the way is open it will return to deep water and come to visit the 

 marshes when the temperature reaches 45 °F. In land-locked pools 

 these fish burrow into the mud to a depth of from six to eight inches, 

 when tlie temperature of the water reaches 45 °F. 



Dr. Chidester's work shows that the inland migration of the common 

 killy begins as early as the latter part of March and that egg-charged 

 females are found as early as April 19. In the early spring three types 

 of this species enter and spread in the marshes ; larger males and fe- 

 males, the latter of which will lay eggs in a week or two ; medium sized 

 (one-half grown) which will not lay eggs for a month; and the small 



