FEET 



1001 



termed pulvilli (fig. 745) ; and these are beset with numerous hairs, 

 each of which has a minute disc at its extremity. This structure is 

 evidently connected with the power which these insects possess of 

 walking over smooth surfaces in opposition to the force of gravity ; 

 yet there is still considerable uncertainty as to the precise mode in 

 which it ministers to this faculty. Some believe that the discs 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 others 

 maintain that the adhesion is the result of the secretion of a viscid 

 liquid from the under side of the foot. The careful observations of 

 Mr. Hepworth have led him to a conclusion which seems in harmony 

 with all the facts of the case namely, that each hair- is a tube con- 

 veying a liquid from a glandular sacculus situated in the tarsus, 

 and that when the disc- is applied to a surface the pouring forth of 

 this liquid serves to make its adhesion perfect. That this adhesion 

 is not produced by atmospheric pressure alone is proved by the 

 fact that the feet of flies continue to hold on to the interior of an 

 exhausted receiver ; whilst. 

 on the other hand, that the 

 feet pour forth a secreted 

 fluid is evidenced by the 

 marks left by their attach- 

 ment on a clean surface of 

 glass. Although, when all 

 the hairs have the strain 

 put upon them equally, the 

 adhesion of their discs suf- 

 fices to support the insect, 

 yet each row may be de- 

 tached separately by the 

 gradual raising of the tarsus 

 arid pulvilli, as when we 

 remove a piece of adhesive 

 plaster by lifting it from 

 the edge or corner . Flies are 



often found adherent to window-panes in the autumn, their reduced 

 strength not. being sufficient to enable them to detach their tarsi. 1 

 A similar apparatus on a far larger scale presents itself on the foot 

 of the Dytiscus (fig. 746, A). The first joints of the tarsus of this 

 insect are widely expanded, so as to form a nearly circular plate, 

 and this is provided with a very remarkable apparatus of suckers, 

 of which one disc (a) is extremely large, and is furnished with strong 

 radiating fibres ; a second (b) is a smaller one formed on the same 

 plan (a third, of the like kind, being often present) ; whilst the 

 greater number are comparatively small tubular club-shaped bodies, 

 each having a very delicate membranous sucker at its extremity, as 

 shown on a larger scale at B. These all have essentially the same 



1 See Mr. Hepworth's communications to the Quart. Joiirn. Microsc. Sci. vol. ii. 

 1854, p. 158, and vol. iii. 1855, p. :-512. See also Mr. Tuffen West's memoir ' On the 

 Foot of the Fly,' in Trans. Linn. Soc. xxii. p. 393; Mr. Lowne's Anatomy of 

 the Blow-fly ; H. Dewitz in Zoologischer Anzeiger, vi. p. 273 ; and G. Simmer- 

 macher in Zeitschr.f. iviss. Zdol. xl. p. 481. 



FIG. 745. Foot of fly. 



