242 INSECTS AND MAN 



On the legs of house flies, small reddish-brown chelifers 

 or false scorpions (fig. 64) are often found. Armed with 

 a pair of relatively formidable pincers, they are enabled to 

 obtain a firm hold of their host, but whether they so 

 obtain their food is not known. When not attached to a 

 host their habitat is the manure heap. 



A number of internal protozoon parasites are known to 

 occur in the alimentary tracts of flies ; they belong to the 

 genera Crithidia and Herpetomonas, and in some respects 

 resemble the trypanosomes which are responsible for 

 sleeping sickness in man ; they appear to have little or no 

 effect on their hosts. 



A nematode worm, Hdbronema muscoe, has also been 

 found infesting various domestic flies, in this country and 

 abroad, without ill effect. It usually takes up its abode 

 in the head or proboscis. Ransom found " that the worm 

 lives in the stomach of the horse, and that the embryos 

 which pass out in the faeces enter the fly larvae. When 

 the flies emerge the larvae are full grown, but have to pass 

 into the intestinal canal of the horse before reaching 

 maturity." 



Hymenopterous parasites, of various species, are known 

 to attack the house-fly maggots, but the subject has not, 

 as yet, received the study it deserves. The adults are 

 caught and killed by various robber flies, and in the United 

 States a small centipede, Scutigera forceps, kills large 

 numbers. Lizards, toads, and birds destroy both larvae 

 and adults. Predatory beetles and, in the tropics, ants 

 cause considerable havoc among pupae and larvae, but man 

 is the house fly's greatest enemy, and man alone can 

 materially lessen the numbers of this death-dealing insect. 



Two PESTS OF CHEESE 



Very common, and accordingly well known, household 

 pests are the cheese mites (fig. 65), Tyroglyphus longior 

 and Tyroglyphus siro. Strictly speaking, of course, these 



