112 



CIVIC BIOLOGY 



family, the tachina flies, many of which look much like com- 

 mon house flies, feed upon other insects and are among our 

 most effective helpers in holding certain insects in check. 

 Tachina flies are being imported from Europe to destroy gypsy 

 and fcrown-tailed moths. Syrphus flies are another large family 

 which feed upon other insects. Tachina and syrphus flies are 

 found about rank vegetation in which other insects abound. 

 Of flies caught in and about houses the typhoid fly gener- 

 ally numbers over 90 per cent. It is distinguished by the 

 " elbow " on the fourth vein as it curves up to the third 

 vein near the tip of the wing (Fig. 55). The proboscis is an 



extensible trunk adapted for 

 lapping up liquids, and cannot 

 be used for either biting or 

 piercing. The foot is provided 

 with claws for climbing over 

 rough surfaces, and also with 

 two pads (pulvilli) covered 

 with sticky, tubular hairs, 

 which enable the fly to walk 

 on ceilings and windowpanes. 

 No more effective mechanisms 

 for collecting dust could be designed than a fly's feet and 

 proboscis, a combination of six feather dusters and thirteen 

 damp sponges. The constant " cleaning " movements of flies 

 are clearly designed to rub off and scatter the adhering germs 

 everywhere they go. 



The " little house fly " (Fannia canicularis), smaller than the 

 common fly, is often seen in swarms hovering under chandeliers. 

 In breeding and feeding habits it resembles the house fly. 

 Other flies found about houses are the following : 

 Bluebottles, greenbottles, and flesh flies, or blowflies, which 

 so frequently lay their eggs on meat. These flies are scav- 

 engers, but we can dispose of dead animals in much more 



FIG. 55. Wings of (a) house fly, 

 (6) stable fly, (c) little house fly, 



(d) horn fly 

 Photograph by I. A. Field 



