HOUSE FLIES AND BLUEBOTTLES 189 



the flies that have no bristles rarely use their legs for 

 locomotion ; they fly rather than run from place to place ; 

 whereas the bristly flies use their legs quite as readily 

 as their wings, and hence run more risk of collisions 

 with other objects than do those whose time is spent 

 chiefly on the wing. In consideration of these distinc- 

 tions, it has been suggested that the bristles are protec- 

 tive in function, and serve as a sort of buffer in case of 

 collisions. Whatever may be thought of this idea, the 

 constancy of both number and arrangement of these 

 bristles in the species in which they occur seems to 

 indicate that they have some important function to 

 fulfil in the economy of the insect. 



Below the platform on which the ocelli are situated, 

 there is, in the female, a broad flat area between the 

 compound eyes, which slopes outwards and forms the 

 ascent to the summit of a conical protuberance, which 

 is best seen when a profile view of the head is .obtained 

 from above. It is a kind of avenue, bordered on each 

 side by the rows of bristles above referred to. In the 

 male, and more particularly in those species which are 

 holoptic, this avenue is, of course, largely encroached 

 upon by the eyes, and in the latter case becomes merely 

 an elongated triangle. Below this bordered avenue is 

 a considerable depression, the outline of which is egg- 

 shaped ; at the upper and narrower end of this are 

 attached the antennae, which are of a most peculiar 

 form, quite characteristic of the particular section of 

 the fly order to which the bluebottle belongs. 



Each antenna consists of three joints, the two basal 

 ones being short and conical, and the outer one long 

 and parallel-sided, and far larger than the others. Near 

 the end of this nearest the head, springs, at an angle, 

 a sort of feathery plume, consisting of two or three 



