HOUSEHOLD INSECTS 239 



though it is slightly larger ; the larvae also are similar, but 

 they may be distinguished by the fact that, instead of a 

 double row of lateral spines, they have rows of feather- 

 like appendages. The eggs are commonly laid in human 

 excrement, and in about eighteen hours the larvae emerge ; 

 in a little over a week they are full grown, and the pupal 

 stage is of approximately the same duration. Pollenia 

 rudis, the cluster fly, despite its reddish-grey colour and 

 stout build, is often mistaken for the house fly. It frequents 

 houses in spring and autumn, is very sluggish in its move- 

 ments, and has a habit of collecting in numbers where it is 

 unlikely to be disturbed ; hence its popular name. 



Some other less frequent visitors to houses may be 

 mentioned, because their occasional appearance indoors 

 gives rise to considerable conjecture as to their identity. 

 Often, especially in outhouses, a long-bodied, black, sluggish, 

 hump-backed fly, with yellowish legs and wings, may be 

 found on the windows ; it is the window fly, Scenopinus 

 fenestralis. Its larvae are much more elongated than those 

 of the other flies we have mentioned, quite snake-like in fact. 

 Verrall says : " The larva was at one time supposed to feed 

 on stable clothing and old carpets, especially when thrown 

 into a heap and neglected, whence the perfect insect 

 obtained the name of 'carpet fly/ It is now, however, 

 known to be predaceous and to feed on the larvae of the 

 clothes moth, Tinea pellionella, or of the Pulicidce, fleas, 

 which are the real culprits, and consequently is a benefactor 

 instead of being injurious." 



Fruit flies, Drosophila fenestrarum, and its relatives, are 

 attracted to houses by the smell of over-ripe and decaying 

 fruit. They are very small, thick set, yellowish flies, and 

 some very interesting experiments have been carried out 

 with them, with a view to testing their sense of smell. 

 Space, or lack of it, forbids any account of these experi- 

 ments ; reference is made to them in the bibliography ; it is 

 interesting to note, however, among other things, that the 



