244: 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG. 436. 



this faculty. Some believe that the disks 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 con- 

 clusion which seems in harmony with all the facts of the case namely, 

 that eacn hair is a tube conveying a liquid from a glandular sacculus 

 situated in the tarsus; and that when the disk 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 attachment on a 

 clean surface of glass. Although, when all the hairs have the strain put 

 upon them equally, the adhesion of their disks suffices to support the 

 insect, yet each row may be detached separately by the gradual raising of 

 the tarsus and 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 ap- 

 paratus on a far larger scale, 

 presents itself on the foot of 

 the Dytiscus (Fig. 436, A). 

 The first joints of the tarsus 

 of this insect are widely ex- 

 panded, so as to form a 

 nearly-circular plate: and 

 this is provided with a very 

 remarkable apparatus of 

 suckers, of which one disk 

 (a) is extremely large, and 

 is furnished with strong rad- 

 iating 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 structure; 

 the large suckers being furnished, like the hairs of the Fly's foot, with secret- 

 ing sacculi, which pour forth fluid through the tubular footstalks that carry 

 the disks, whose adhesion is thus secured; whilst the small suckers form 

 the connecting link between the larger suckers and the hairs of many 

 beetles, especially Curculionidce.* The leg and foot of the Dytiscus, if 



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

 Science," Vol. ii. (1854;, p. 158, and Vol. iii. (1855), p. 312. See also Mr. Tuffen 

 West's Memoir ' On the Foot of the Fly, 'in " Transact, of Linnaean Society," Vol. 

 xxii., p. 393, and Mr. Lowne's " " 



A, Foot of Dytiscus, showing its apparatus of suckers; 

 a, b, large suckers; c, ordinary suckers: B, one of the 

 ordinary suckers more highly magnified. 



Anatomy of the Blow-fly," p. 19. 

 -called Sucke 

 Monthly Microscopical Journal," Vol. v., p. 267. 



., . , 



2 See Mr. Lowne ' On the so-called Suckers of Dytiscus and the Pulvilli of 

 Insects,' in " " 



