330 



INTRODUCTION TO ZOOLOGY 



CHAP. 



The Foot. 



not in use, the proboscis is withdrawn, lying in a little 

 hollow on the under side of the head. 



As in all the family, the thorax bears only one 

 ■ pair of small membranous wings for flying, and 

 the " halteres " or " balancers," each of the latter being 

 partially covered by a lobe of the front wing, the " squama." 

 Below the thorax are attached the three pairs of hairy 

 jointed legs, with which the fly may be seen frequently 

 cleaning every part of its body. Each leg ends in a five- 

 jointed tarsus, and the last joint bears two well- 

 developed claws, under each of which is a pad 

 covered with fine hairs. These 

 hairs exude a sticky fluid when 

 the pad is pressed against a flat 

 surface, and this enables the fly to 

 run up slippery window - panes 

 with ease ; the pads are first 

 pressed against the pane, causing 

 the hairs to adhere, and are then 

 drawn up again obliquely, so that 

 the attachments of the hairs are 

 f^^^^^Wi HSi^^^A snapped one after the other. 

 I 3^«iJ^ (^^^^ H ■ Over a hundred 



history, ^m^^'l ^^^'^^^ eggs are 

 laid by the fly on any 

 damp collection of debris on which 

 the little white legless maggots 

 can feed when they hatch, which 

 they will do after a day or two. In less than seven days, 

 after two moults, the larvae pupate inside the larval skins, 

 which shrink and become oval, brown, shell-like structures 

 (see Fig. 250 for similar stages in the Blow-fly i). Inside this, 

 the curious metamorphosis (see p. 328) occurs, and in about 

 a fortnight the perfect fly will emerge. The time taken to 

 complete the metamorphosis varies with the temperature. 

 Many pupae are said, however, to persist through the winter, 

 the flies emerging the following spring. Most of the adult 

 flies die in the autumn, but some seem to hibernate in dark 

 corners, and may become active again any mild day in winter.^ 



^ Hewitt, q.J.M.S., 51, 1907, p. 39,5. 



''■ For short notes on othev House-flies see Bouse. Garilev. and Fwld, liy 

 Professor Miall. 



Fig. 248.— The Foot of a Fly. 



