CLASS INSECT A 293 



which they pierce plant tissues and deposit eggs. Either poison injected 

 at the time of the laying of the egg or irritation arising from the growth 

 of the larva causes a hypertrophy, or overgrowth, of the plant tissues 

 and results in the production of conspicuous swellings on the stems or 

 leaves. These growths are galls. Galls are also produced in other ways, 

 and the study dealing with them is a subject by itself known as cecidiology. 

 The Diptera (dip' ter a; G., dipteros, two-winged), or flies, have only 

 the forewings developed and functional. Among them are the mos- 

 quitoes, which are annoying because of their bites but which are more 

 important because of the part they play in the transmission of diseases. 

 Mosquitoes (Fig. 197) lay their eggs on the surface of the water, the larvae 

 feeding upon organic matter contained in it. In this way they help 

 to clear up the water of stagnant pools. The larvae, or wrigglers, have 

 an air tube at the posterior end 

 of the body through which they 

 can obtain air for breathing. At 

 frequent intervals they wriggle 



their way to the surface and thrust 

 this tube through the surface film 

 to take in oxygen and throw off ^ ^^^ ^^ . 



carbon dioxide. The oxygen lasts ^^^ igg.— House fly, Musca domestica 



them for a time, but its exhaustion Linnaeus. A, larva. B, pupa. C, adult. 



1 .u 4. • 4. +U^ +^v. X 4. {From Howard, Farmers' Bull. 459, 



makes another trip to the top Q ^ ^^^^ ^^^^ 

 necessary. The pupa spends much 



more of its time at the surface, breathing through two tubes at 

 the anterior end. When the adult emerges it rests upon the 

 surface of the water until its wings are expanded and dried, and then it 

 flies away. Only the females, which may be recognized by their simple 

 antennae, suck blood. The males, which have feathered antennae, are 

 not known to feed at all. In the absence of blood the females make use 

 of nectar or juices of plants, and it is possible that the males may at times 

 use the same food. The larvae of mosquitoes may be destroyed by 

 coating the surface of the water with oil, which gets into the breathing 

 tubes and kills them by suffocation. The numbers of mosquitoes may 

 also be reduced by draining off water standing in barrels, cans, or other 

 receptacles, which offers opportunity for the development of the larvae. 



The house fly (Fig. 198), which also belongs in the Diptera, is an 

 abundant insect and exceedingly dangerous, since it carries disease germs 

 upon its feet and its habit of ahghting upon moist masses of all kinds 

 results in its transferring them to human food. Germs are also deposited 

 on food by defecation and regurgitation. It is thus known to transmit 

 typhoid fever and tuberculosis. The larva feeds in manure and other 

 filth. The insect may be controlled by covering the receptacles for such 

 material and by the use of poisons and traps. It should be kept out of 

 houses and away from food by the use of screens. 



