BREEDING PLACES 147 



survival of the species is assured. All the forms so far discussed, and to them 

 belong by far the most mosquitoes in point of numbers, being dependent upon 

 seasonal conditions, show a marked periodicity. This is also true of the salt- 

 marsh mosquitoes, although their development is governed by somewhat differ- 

 ent conditions and there may be, and usually are, a number of broods during a 

 season. Here the determining factor is the flooding of the marsh, either by 

 rains or by a high tide, the higher ground along the beach preventing the escape 

 of the water. Here again the eggs lie buried in the mud or sod and hatch when 

 submerged, and here, also, a part of the eggs lie over until another flooding of 

 the marsh. On the Pacific coast, where no rains occur for many months, a crop 

 of salt marsh mosquitoes appears every four weeks following the monthly high 

 tides that overflow the upper reaches of the marshes where the eggs are. All 

 these species in which a long unfavorable period is passed in the egg state are 

 closely related and belong to the genera Aedes and PsoropJiora. The genus 

 Psorophora is most adapted to the conditions in the semi-arid tropics. Simul- 

 taneously appear, and developing with equal rapidity, the non-predaceous larvae 

 of the subgenus Janthinosoma and those of PsoropJiora proper which prey 

 upon them. 



In marked contrast to the forms just discussed are those which breed con- 

 tinuously, as long as breeding-places exist and temperature conditions are 

 favorable. Such are the forms in which the eggs are deposited upon the water 

 and which hatch soon after being deposited; these are the species of Anopheles, 

 Culex, Culiseta, Uranotcenia and Lutzia. The growth of the larvae depends upon 

 food conditions and upon temperature, so with these forms there are no well- 

 marked broods or generations; adults and larvae in all stages may be found at 

 all times until frost cuts the larvae ofF. As it is the hibernating females that pre- 

 serve the species until the following season, and many of these females die off 

 in the course of the winter, these species first make their appearance in small 

 numbers and gradually increase to the end of the season. Never, however, do 

 they reach such enormous numbers as the periodic species. To the continuous 

 breeders belong the house mosquitoes which breed in rain-water barrels or other 

 artificial receptacles. Other species of Culex breed in natural pools of a more or 

 less permanent character, in swamps, or along the margins of streams. Others 

 breed in the water of various water-bearing plants, such as the bromeliads, Heli- 

 conia and Calathea of the tropics. In these continuous breeders the eggs are 

 laid upon the water and develop at once, hatching within a few days. Certain 

 forms of Aedes take a position intermediate between the forms with well-marked 

 broods and the continuous breeders. Such are the species of Aedes and Stego- 

 conops which breed in the water in hollow trees and in similar situations. With 

 these the eggs are laid above the water line and they hatch when the water 

 rises to them. As the result of successive rises, due to rains or other causes, 

 eggs hatch at brief intervals and larvae of various sizes are always present. Thus 

 mosquitoes of these species are continuously present while there is warm 

 weather and occasional rain. 

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