HOUSEHOLD INSECTS 241 



done by man, and much remains to be done, but the sub- 

 ject is somewhat beyond our sphere. Let us consider for 

 a moment what natural agencies are at work in the control 

 of house flies. Towards autumn many flies are to be seen, 

 in perfectly natural positions, attached to walls, etc. A 

 careful examination of these dead insects will reveal the 

 fact that they are more or less covered by a white fungus. 

 They have been killed, in fact, by the fungus, Empusa 

 muscce, belonging to a group of non-green plants, the 

 various members of which confine their attacks to insects, 

 and collectively are responsible for a considerable mortality. 

 The exact means by which the flies become infected is 

 not known, but it appears to be the only disease which 

 normally attacks house flies, though individuals which 

 have become infected with plague bacilli have been 

 shown to be short-lived. 



Various mites may be found attached to the bodies of 

 house flies, some of them truly parasitic, others merely 

 using the insect as a means of transport. Minute, red 

 rnites, belonging to the family Trombidiidw, are often found 

 clinging at the base of the wings and sucking up the juices 

 of their host with their thread-like mandibles : the fact 

 of their being six-legged shows that they are immature. 

 Later, they leave their hosts, moult, and appear as bright 

 red, hairy, free living adults. Mites, of the family Tar- 

 sonemidce, are sometimes found clinging to the abdomens 

 of house flies, though whether as true parasites or merely 

 as passengers is not known. Cheese mites in the hypopus 

 stage, described elsewhere, are also often fixed by their 

 suckers to flies, and in this respect these insects must be 

 considered as frequent carriers of food-destroying mites. 

 Other mites, belonging to the group Gamasidce, frequent 

 rubbish heaps in the larval stage, and are therefore fre- 

 quently transported by flies. The small, flat brownish 

 creatures are, on occasion, parasitic, and adhere by their 

 mouths to the ventral surface of their hosts (fig. 63). 



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