»» ZOOLOGY 



are vi-saparous, the larvae being ready to pupate at the time 

 of Iiirth. The sheep-tick is one of the best-known forms. 

 Diptera alhed to the foregoing hve as parasites on the body 

 of the honey-bee. 



Fleas are likewi,se \\angless, blood-sucking parasites. The 

 body is compressed from side to side, so that it can move easily 

 amongst the hairs of its host. The hindermost legs are strongest 

 antl are used in springing. Fleas develop in dust in the cracks 

 of the floor and about the sleeping-places of domestic pets. 

 One species prefers mankind and is beUeved to disseminate 

 certain diseases such as the bul)onic plague. They may be 

 combated by means of cleanliness and Persian insect powder. 



The foregoing study of the Diptera shows how great a men- 

 ace they are to man's health. But indircctlj^ they are inju- 

 rious to man Iiy attacking domestic animals and cultivated 

 plants. Thus the tsetse-fly is a menace to the commerce of a 

 large part of a continent. The horse-fly, the horn-fly, which 

 worries cattle, the buffalo-gnat, which worries or even kills 

 domestic animals, and the dangerous bot-fl}- are all causes 

 of great loss to industry. Also the larvae of some flies infest 

 vegetables, such as cabbage, radish, cauliflower, onion, 

 as well as various fruits, and cause great damage. The 

 gall-gnats destroy clover and its seed ; and, worst of all, the 

 Hessian-fly infests wheat and Indian corn. This last-named 

 scourge, so called because of a tradition that it was imported 

 in tlie straw bedding of the troop-ships which brought over the 

 Hessian mercenaries in 177o, has spread, ^\^thin a century, 

 over the eastern lialf of the United States, and has at various 

 times injured the wheat crop to the value of millions of dollars. 

 The larvae of certain Muscidte' attack, in Europe, the stems, 



' Of the genus Chlorops. 



