39S THE BIOLOGY OF INSECTS 



that live temporarily or permanently as parasites on or in 

 the bodies of large animals and at their expense, and then 

 examples of creatures, usually of minute size, which live as 

 internal or external parasites of insects, or in ver\' close 

 association with them. 



In a pasture on a surmner day the grazing cattle are 

 commonly surrounded by swarms of flies, mostly Diptera. 

 Some of these belonging to the house-fly group are attracted 

 by the odour arising from the skin, and Hck up at intervals 

 the drops of sweat and other surface exudations. The 

 abundance of such flies attracts a number of worker wasps 

 which, fl}"ing around the cattle, pounce on some of the flies 

 and cany^ them off to feed the grubs in the nest. The 

 beasts show considerable patience through the unwelcome 

 attention of their insect visitors, but occasional or frequent 

 lashings of the tail afford evidence of irritation. Besides 

 the muscoid flies there may be seen various members of the 

 blood-sucking tabanid family, provided with a batter}- of 

 lancets, as previously described (p. 21 , Fig. 7), for piercing the 

 skin of the cattle in order to reach the desired food-supply. 

 The smallish grey " cleggs " (Haematopota) alight on the 

 beasts' hairs and inserting their st}-lets into the skin, proceed 

 to draw at leisure their fill of bovine blood. To such attacks 

 a calf or heifer responds by contraction of the cutaneous 

 muscle so that the skin moves jerkily over the underlying 

 tissue, and the clegg may be shaken off. The large breeze- 

 flies (Tabanus and Therioplectes) have stronger piercers 

 than the cleggs, and inflict wounds that may be regarded as 

 painful. When a great Tabanus approaches grazing 

 animals, its flight announced by a loud hum, the cattle 

 often run about the field in agitation and apparent fear, as 

 though they recognise in this insect a formidable foe. Yet 

 it is known that these flies often var\- their diet of blood by 

 Ucking up juices from the surface of plants, and it is likely 

 that many of them, like a large proportion of the gnats and 

 mosquitoes of sparsely inhabited countries, never have in 

 all their lives an opportunity^ of sucking blood. The larvae 

 of Tabanidae do not feed in any way at the expense of cattle, 



