98 UNDERHILL: 



sion of but few of these eggs is successfully accomplished, 

 and in compensation they must be produced in enormous 

 numbers, well protected from the many elements of destruc- 

 tion which they encounter. The mode of reproduction is 

 one of the principal factors determining the conditions of 

 parasitism, and, while the above modifications pertain more 

 to those dwelling pennanently within the bodies of their 

 hosts, we have in the Oistridu\ among the dipterous insects, a 

 cycle involving internal parasitism during the larval stage ; a 

 familiar example being the common horse bot fly ( Gastrophihis 

 fgui). In the warmest hours of the day, during the late 

 summer months, the female of this fly hovers about the head, 

 shoulders and fore legs of the horse, then darts down, deposits 

 its Qgg upon a hair, and at once flies away, soon to return 

 and repeat the process, until hundreds of eggs may be found 

 on the same horse. In about fourteen to twenty days these 

 eggs are hatched, a vivacious larva emerging, which, crawl- 

 ing on the skin, causes a slight irritation, impelling the horse 

 to lick, the larva reaching the mouth in this way, or possibly 

 still within the egg. Being carried to the stomach with the 

 food, it becomes fixed upon the mucous membrane by means 

 of two buccal hooks, and, with head plunged deep into an 

 alveolus, subsists upon the inflammatory products of the 

 small wound which it makes. After a sojourn here of about 

 ten months, the larva becomes detached and is passed along 

 with the intestinal contents, to be finally expelled from the 

 body of its host. It then becomes concealed in the ground, 

 and is soon enclosed in its pupal case, from which, after the 

 elapse of about thirty to forty days, it emerges as the perfect 

 insect. It is plain that a very small percentage of the eggs 

 deposited can reach the horse's mouth, and that having got 

 thus far, many of the larvae must be destroyed or pass 

 entirely through the intestinal tube without having succeeded 

 in becoming fixed to the mucous lining. For this nature 

 seems to have comix-nsated in the large number of eggs depo- 

 sited by the persistent female. 



