GENERAL ZOOLOGY 



of 5 or 6 days, before pupation. The pupal case, or puparium, is formed by 

 the hardened and darkened exoskeleton of the last-stage larva. The adult 

 emerges from this case about 5 days after pupation. Thus, a generation may 

 be developed every 12 days under favorable conditions; and, since every 

 female may lay about 100 eggs, the potential rate of multiplication in a single 

 season is very great. Many of the adults die with the onset of winter, but some 

 hibernate in protected places. These overwintering individuals then become 

 active again in early spring and lay the eggs from which the flies of another 

 season arise. 



Mosquitoes have assumed great importance since it was discovered, about 

 1895, that certain mosquitoes carry the protozoan parasite causing malaria, 

 and later, that the virus causing yellow fever is also transmitted by mosqui- 

 toes. Adult females, whose mouth parts are capable of piercing the skin and 

 sucking the blood of man, are responsible for the transfer of these parasites, 

 since the mouth parts of the male cannot penetrate the human skin. Ap- 

 parently the malaria parasites of importance in human malaria are carried 

 only by mosquitoes of the genus Anopheles, and the virus of yellow fever is 

 carried only by members of the genus Aedes. The most common mosquitoes 

 of temperate latitudes belong to the genus Culex, which does not serve as a 

 host for either of these parasites. Culicine mosquitoes lay eggs fastened to- 

 gether in little rafts which float upon the surface of fresh waters. The larvae 

 hatching from these eggs are the wrigglers commonly seen in standing water, 

 where they hang suspended from the surface film by tubes at the posterior 

 ends of their bodies; air reaches their tracheae through these tubes (Fig. 

 15.37). The larval stage of Culex lasts from 1 to 4 weeks, depending on the 

 species and to some extent on temperature and the abundance of the food 

 supply. The food during this period consists of minute organisms captured 

 and ingested by the mandibulate mouth parts of the larva. With their third 

 molt the larvae change into active but non-feeding pupae, in which the head 

 and thoracic regions are enlarged and the wings and antennae of the adult 

 can be seen, as in the chrysalis of a butterfly. These pupae, like the larvae, 

 must secure air from the surface, but the air tubes of the pupa are located on 

 the dorsal side of the thorax. After 2 to 5 days the pupal skin is shed, and 

 the adult emerges (Fig. 15.37). Both larvae and pupae are killed when the 

 water in which they live is covered with a film of oil, for this film prevents 

 their access to the atmospheric air which they require. Other methods of 

 control involve drainage of swamps and other bodies of stagnant water, and 

 emptying even such smaller breeding places as tin cans and rain barrels. 



A great many insects of economic, medical, and veterinary importance to 

 man belong to the order Diptera. The tsetse fly of Africa, which transmits 

 the trypanosomes of sleeping sickness; stable flies, horn flies, botflies, warble 

 flies, horseflies, and screwworm flies which variously parasitize, feed upon, or 

 otherwise annoy cattle and horses; the Hessian fly, which destroys standing 

 crops of grain; all these are but a few of the insects with which man must 

 constantly contend. 



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