184 OUR HOUSEHOLD INSECTS 



in some way connected with respiration. Situated as 

 they are on the meta-thorax, they appear to be the 

 representatives of hind wings. 



The legs of the bluebottle consist of the usual parts, 

 and there is nothing of special interest or importance 

 till we come to the feet (Plate I.). All the tarsi 

 consist of five joints each, and are terminated by a pair 

 of curved claws, under each of which is placed a fleshy 

 pad, fringed with hairs, and between the pads is a 

 straight sharp-pointed spine. In these pads, or " pul- 

 villi " aa, they are called, lies the secret of the power 

 flies possess of running over surfaces in any position, 

 often in defiance of gravity. A fly finds no more diffi- 

 culty in running up or down a vertical window-pane, 

 or across a ceiling, than in walking on the upper side of a 

 horizontal surface. This is not the case with all insects, 

 many of which would struggle in vain to mount a 

 perpendicular glass surface; hence it cannot depend 

 entirely upon the claws, for these are developed in all 

 insects, and would therefore give all equal facilities. 

 No doubt the claws are of some help when the surface 

 is at all irregular, as, for example, on a ceiling; but 

 they can hardly be of much use in travelling over glass. 

 We therefore look to the pulvilli for the explanation. 

 It was at one time supposed that their efficiency de- 

 pended upon atmospheric pressure, and that they acted 

 like suckers, the edges being closely applied to the 

 surface, and the centre part pulled up so as to create 

 a vacuum beneath. This explanation, however, seems 

 to be negatived partly by the absence of any mechanism 

 to produce such a vacuum, and partly by the presence 

 of great numbers of minute hairs on the under surface, 

 which could hardly do otherwise than interfere with 

 such a close application of the edges of the pad to the 



