112 THE BIOLOGY OF INSECTS 



kind new to the country on a comparatively small area, had 

 provided so favourable an environment for the springtails 

 that a species, formerly so scarce as to be overlooked, forced 

 itself on the attention of the cultivators by multiplying so 

 fast as to become a " pest." Here the insects in their 

 thousands all responded in the same way to the stimulation 

 of their new surroundings by feeding on the plants with 

 which they were thus for the first time brought into touch. 

 Another similar case of a more remarkable kind has been 

 furnished through the introduction of great flocks of sheep 

 into Australia during the last century, and the reaction of 

 this introduction on some Australian flies during the last 

 thirty years. It is well known that in the British Islands 

 and in other countries of Western Europe a greenbottle fly 

 {Liicilia sertcata), belonging to a group the usual behaviour 

 of whose members is to lay eggs on carrion, has the habit 

 of depositing eggs on the wool of live sheep into whose 

 skin and flesh the maggots when hatched eat their way. 

 (See G. H. Carpenter, 1902, and R. S. Macdougall, 

 1909.) Such abnormal behaviour is a response which this 

 species has been making for centuries in England to the 

 presence of thousands of sheep in the fields and on the hills ; 

 the female fly is attracted by the odorous secretions and 

 excrements of the sheep, and her approach to the animals 

 for the purpose of egg-laying may be regarded as a typical 

 chemotropic action. The student of insect behaviour 

 cannot but wonder why one and only one kind out of the 

 scores of nearly related flies should commonly adopt this 

 habit, at once horrifying and interesting. In Australia, 

 where the sheep-grazing areas have been constantly extended 

 with the advance of human settlement into the interior of 

 the continent, a precisely similar response to the presence 

 of flocks of sheep has been made by at least five or six of the 

 native species of the bluebottle and greenbottle group of 

 flies. W. W. Froggatt (191 5-18) has found that besides 

 Lucilia sertcata and L. caesar, presumably introduced from 

 Europe, four Australian species of Calliphora, an Ophyra, 

 and a Sarcophaga act as '' Sheep-maggot flies " on the vast 



