4 TT. S. BUREAU OF FISHERIES. 



imdrained salt marshes of New Jersey, the tropical swamps of Cen- 

 tral America, or the plains of New Zealand. 



As is always the case with widely distributed and abundant ani- 

 mals, mosquitoes have acquired a host of enemies which prey upon 

 them at all periods of their lives. They are also afflicted with para- 

 sites, both animal and plant. Furthermore, many species of mos- 

 quitoes in their immature stages must compete for food with a va- 

 riety of animals, and the food supply of the female imagos is a more 

 or less precarious one. Also, the very perfection and complexity of 

 their specialized adaptations renders their adjustments to unstable 

 environmental conditions delicate and easily upset. 



Nevertheless, with all the great array of dangers with which the}' 

 are beset and the numberless enemies which press upon them from 

 all sides and seek to devour them in water and in air as eggs, larvae, 

 pupae, and imagos, mosquitoes are only too obviously abundant and 

 nearly omnipresent. How does this happen? Why do not their 

 enemies and competitors overcome them ? Clearly it is because dur- 

 ing the long period of their evolution, along with their associated 

 biota, mosquitoes have acquired a degree of fecundity sufficient to 

 furnish not only the toll demanded by their raveners but a surviving 

 surplus to maintain the several species at their normal but change- 

 able numerical balance. The fact that mosquitoes and animals which 

 are known to prey upon them are found in association is a necessary 

 consequence of this relation and does not mean either that the latter 

 will in time exterminate the former or that they are exerting no 

 checking influence upon them, both of which opposite assumptions 

 are sometimes made. It means only that the index of fecundity of 

 each is such as to maintain the necessary balanced relation between 

 predator and prej'^, so that the racial survival of both is secured. 

 Were this not true of mosquitoes they must have ceased to exist, and 

 doubtless species haA^e become extinct for this very reason. In the 

 interaction of the multitudinous checks and balances that operate 

 among the elements of an ecological complex there is alwaj's a large 

 reserve or factor of safety (residing mainly in reproductive ca- 

 pacity) as there is in all compensatory regulative processes in nature. 

 These relations are seldom simple and direct and may seldom be as- 

 sumed. Many examples of this might be cited. All biologists un- 

 derstand the significance of these interrelations called the balance 

 of nature, but persons without biological training often misunder- 

 stand their meaning. 



If, therefore, mosquitoes occur in such numbers as to prove a pest 

 in^ny locality, they may be attacked by so modifying an existing 

 relation or introducing a new condition as to effect changes beyond 

 their range of adaptability or regidation, thereby establishing a 

 higher rate of mortality and reducing or overcoming their factor of 

 safety in reproduction. The engineer works chiefly by modifying 

 the physical environment. The knowledge that mosquitoes pass the 

 immature stages of their lives in standing water enables him to 

 greatly restrict breeding by destroying such bodies of water. The 

 knowledge that most species of mosquito larvae and pupae respire 

 chiefly by means of tracheae, the air in which must be renewed at 

 the surface at frequent intervals, has suggested the second most gen- 



