Page:Hardwicke's Science-Gossip - Volume 1.pdf/269

Recently an article appeared in these pages on the Breeze-fly (page 194) in which some interesting particulars were detailed of the species of Breeze-fly, with which the writer had been brought into pleasant contact. On the present occasion we have selected a portion from our common species (Tabanus bovinus) to illustrate a for observations on the feet of flies in general. How can a fly walk upon the ceiling is a question which has often been asked, and a great many successive efforts have been made to answer it. The most recent researches in this direction are those of Mr. Tuffen West, which were published in the "Transactions of the Linnean Society, for 1861," profusely and beautifully illustrated, and we shall freely avail ourselves of the results of those researches.

The foot of the fly differs a little in minor particulars in the house-fly, the blow-fly, and the drone-fly, but there is a still greater divergence from this type in the foot of the breeze-fly. In the former instances the fifth tarsal joint is terminated by a double pad, but in the present the pad is treble. On the upper surface at the base of these pads is a pair of strong claws. Somehow or other the fly manages to walk, head downwards by the aid of these pads or these claws, or both; but how this is accomplished we can scarcely understand without a more minute examination of the parts already named. It must be premised that in our illustration the upper surface of the foot is shown.

The pad, cushion, or pulvillus is deeply cleft into what seems to be three pads, but which is in reality a single pad with three lobes. The under surface is clothed with minute hairs, which are somewhat trumpet-shaped, expanding at the outer extremity into a membranous elastic disk. It has been supposed that each of these hairs is hollow, and when the fly moves over a highly-polished surface a minute drop of fluid passes down it, to assist in adhesion, by the more effectually excluding the air. There was at one time a notion extant that the whole surface of the pad was a kind of "sucker," and that by means thereof the fly sustained itself, as a schoolboy lifts a stone by means of a moistened disk of leather at the end of a string.

The pair of claws, shown in the figure, are moveable, and both to them and the pad, flexor and extensor muscles are attached. The mode in which these parts are employed, is thus graphically described by Mr. West:—"When a fly is not making use of its pads (pulvilli), as on a surface sufficiently rough to afford it foothold with its claws alone, these only are made use of. On a smooth surface, perpendicular or horizontal, the pulvilli are brought down, and the tenent (holding) hairs applied to such surface; a slight push forwards of these, succeeded by a gentle draw backwards, at each application, removes the air between their soft elastic expansions and their plane of motion, and thus a firm hold is gained. Access of air is prevented by the minute quantity of moisture which exudes from the expanded tips of the tenent appendages; and thus a vacuum is formed. When the fly wishes to move a leg from its place of attachment, the claws are brought down and pressed against the surface; from their position they raise the hinder part of the pulvillus, where the tenent hairs^[1] are least developed, first, and so on forwards. I think a fly when once stuck fast, if it had no claws might remain so."

"That the pressure of the atmosphere is the main agent by which a fly is enabled to adhere to perfectly

smooth surfaces, cannot, I conceive, be doubted. Careful experiments on the weights of numerous