72 



GENERAL ZOOLOGY 



The sheep botfly {(Es'trus o'vis) in a similar manner at- 

 tacks the nostrils of sheep. The appearance of one of these 

 flies in a flock of sheep is sufficient to throw them into a 

 state of panic; they run about with their noses between 

 their legs, or try to bury them in dust, to escape their tor- 

 mentor. The female of this botfly does not lay her eggs as 

 do most other insects, but retains them within her body till 



Fig. 48. Metamorphosis of horse botfly 



A, egg on hair of horse (natural size) ; B, egg on hair of horse (enlarged) ; 

 C, young larva (enlarged) ; D, young larva (much enlarged) ; E, spines ; F, full- 

 grown larva (twice natural size) ; G, female (natural size). (After Osborn, Bulletin 

 No. 5, N. S., United States Department of Agriculture, Division of Entomology) 



they hatch, and then deposits the living young, — that is 

 to say, in her method of reproduction she is viviparous. 

 Fruit Flies. The minute flies (Drosoph'ila) so often seen 

 about over-ripe fruit are commonly called fruit flies, pomace 

 flies, or vinegar gnats. Much that we now know of the laws 

 of heredity has come from the study of these minute flies. 

 They are easily reared and reproduce so rapidly that they are 

 favorite objects for laboratory study of the manner in which 

 characteristics are passed from one generation to another. 



