SURROUNDINGS OF GROWING INSECTS 217 



these countries and other parts of Western Europe, a species 

 of green-bottle (Lncilia sericata), is so notorious for its habit 

 of laying eggs on the wool of live sheep, that it has become 

 known as the " Sheep-maggot Fly ". The maggots (Fig. 75) 

 when hatched begin to bite their way into the skin of the 

 sheep ; if present in large numbers, and unchecked, they may 

 burrow deeply into the muscular and fatty tissue, causing, 

 finally, the animal's death. Years ago the attacks by this fly 

 on sheep were said to have been confined to low-lying, rich 

 pastures, but now flocks on mountainous highland grazings 

 are frequently troubled. x The area in which the habit prevails 

 is therefore extending, and it is of interest in this connexion 

 to notice that since the introduction of immense flocks of sheep 

 into the inland districts of Australia, several species of muscoid 

 flies, native to that continent, have taken to the same method 

 of egg-laying. 2 Such behaviour on the part of these insects, 

 repulsive though it be, is of much interest to the naturalist 

 because it affords examples of a widespread change of habit 

 in a species or group of species, whose members are abandon- 

 ing the scavenging method of larval feeding, and are taking 

 instead to a parasitic life in the bodies of large animals. And 

 it is highly instructive to recognize that this change of habit 

 follows as the result of a marked change in the surroundings 

 of the insects in certain parts of their geographical range. 

 Great flocks of sheep form, for example, a new element in the 

 environment of insects native to Australia. 



The recently acquired parasitic habit of these sheep-maggots 

 is clumsy and disharmonious, leading to great suffering and 

 frequently to death on the part of its host. There are, 

 however, many kinds of flies whose larvae live in the bodies 

 of large animals in such a manner that the parasitism is 

 specialized ; these cause the host comparatively slight incon- 

 venience unless they be present in abnormally large numbers. 

 Naturally the life-histories of those parasitic on domesticated 

 animals have been especially studied, and it is well to remember 

 that domestication, involving the comparative crowding of 

 many beasts of the same kind on a restricted area, tends to 



1 R. S. MacDougall : " Sheep-Maggot and Related Flies ". Trans. 

 Highl. Agric. Soc. 1909. 



2 W. W. Froggatt : " Sheep-Maggot Flies ". Bull. Dept., Agri., N.S. 

 Wales. 1915-18. 



