188 OUR HOUSEHOLD INSECTS 



forming a sort of brushwood undergrowth to the large 

 ones. As these bristles readily break off, they can only 

 be seen in their perfection in comparatively fresh speci- 

 mens. It is a curious fact that the large bristles are 

 very constant in their arrangement and position, so that 

 names have been assigned to them, and the plan of their 

 distribution has been called by Osten-Sacken "chseto- 

 taxy," i.e., bristle arrangement. There are several 

 groups of flies that are entirely devoid of these bristles ; 

 and it has been pointed out that those flies that possess 

 them are\far less cautious in their flight than those that 

 are without them ; and, as an instance of this, we may 



FIG. 60. Profile view of head from above. /, face ; e, eyes ; o, ocelli. 



notice the headlong way in which a bluebottle dashes 

 about, flinging itself often with great violence against 

 the window-panes. On the other hand, a bristleless fly 

 would proceed much more cautiously, and would be 

 prone, which a bluebottle is not, to poising itself in the 

 air on the wing. The best example of this may be seen 

 in the beautiful yellow-banded " hoverer flies," or " wasp 

 flies," as they are sometimes called (Syrp/ridce), which 

 delight to hover round flowers, maintaining themselves 

 in one position by an extremely rapid vibration of the 

 wings, which gives them the appearance of a distinct 

 dark body, surrounded by a sort of misty halo. Now 



