FEET OF INSECTS. 



577 



especially of the Fly kind, the foot is furnished with a pair of 

 membranous expansions, termed pulvilli (Fig. 294); and these 

 are beset with numerous hairs, 

 each of which has a minute FIG. 294. 



disk at its extremity. This 

 structure is evidently connected 

 with the power which these in- 

 sects possess, of walking over 

 smooth surfaces in opposition 

 to the force of gravity; yet 

 there is still considerable un- 

 certainty as to the precise mode 

 in which it ministers to this 

 faculty. Some believe that the 

 "pulvilli" act as suckers, the 

 insect being held up by the 

 pressure of the air against their 

 upper surface, when a vacuum 

 is formed beneath; whilst Foot of F i y , 



others maintain that the adhe- 

 sion is the result of the secretion of a viscid liquid from the 

 under side of the foot. The careful observations of Mr. Hep- 

 worth have led him to a conclusion which seems in harmony 

 .with all the facts of the case ; namely, that the minute disks at 



FIG. 295. 



A. Foot ofDytiscus, showing its apparatus of suckers ; a, 6, large suckers; o, ordinary suckers : 

 B, one of the ordinary suckers more highly magnified. 



the extremity of the individual hairs act as suckers, and that 

 each of them secretes a liquid, which, though not viscid, serves 

 to make its adhesion perfect. 1 And this view of the case derives 



1 See Mr. Hepworth's communications to the " Quart. Journ. of Microsc. Science," 

 ,vol. ii, p. 158, and vol. iii, p. 312. 



37 



