London Labour and the London Poor, Vol. 3

by Henry Mayhew

“The number of house-flies,” he adds, “might be greatly lessened in large towns, if the stable-dung in which their larvæ are chiefly supposed to feed were kept in pits closed by trap-doors, so that the females could not deposit their eggs in it. At Venice, where no horses are kept, it is said there are no house-flies; a statement which I regret not having heard before being there, that I might have inquired as to its truth.”—(Kirby and Spence’s Entom. i. 102, 3.)

This short account of flies would be incomplete without a description of their mode of proceeding when they regale themselves upon a piece of loaf-sugar, and an account of the apparatus with which the Creator has furnished them in order to enable them to walk on bodies possessing smooth surfaces, and in any position.

“It is a remark[2] which will be found to hold good, both in animals and vegetables, that no important motion or feeling can take place without the presence of moisture. In man, the part of the eye which is the seat of vision is always bedewed with moisture; the skin is softened with a delicate oil; the sensitive part of the ear is filled with a liquid; but moisture is still more abundant in our organs of taste and smell than in any of the other senses. In the case of taste, moisture is supplied to our mouth and tongue from several reservoirs (glands) in their neighbourhood, whence pipes are laid and run to the mouth. The whole surface, indeed, of the mouth and tongue, as well as the other internal parts of our body, give out more or less moisture; but besides this, the mouth, as we have just mentioned, has a number of fountains expressly for its own use. The largest of these fountains lies as far off as the ear on each side, and is formed of a great number of round, soft bodies, about the size of garden-peas, from each of which a pipe goes out, and all of these uniting together, form a common channel on each side. This runs across the cheek, nearly in a line with the lap of the ear and the corner of the mouth, and enters the mouth opposite to the second or third of the double teeth (molares) by a hole, into which a hog’s bristle can be introduced. There are, besides, several other pairs of fountains, in different parts adjacent, for a similar purpose.

“We have been thus particular in our description, in order to illustrate an analogous structure in insects, for they also seem to be furnished with salivary fountains for moistening their organs of taste. One of the circumstances that first awakened our curiosity with regard to insects, was the manner in which a fly contrives to suck up through its narrow sucker (haustellum) a bit of dry lump-sugar; for the small crystals are not only unfitted to pass, from their angularity, but adhere too firmly together to be separated by any force the insect can exert. Eager to solve the difficulty, for there could be no doubt of the fly’s sucking the dry sugar, we watched its proceedings with no little attention; but it was not till we fell upon the device of placing some sugar on the outside of a window, while we looked through a magnifying-glass on the inside, that we had the satisfaction of repeatedly witnessing a fly let fall a drop of fluid upon the sugar, in order to melt it, and thereby render it fit to be sucked up; on precisely the same principle that we moisten with saliva, in the process of mastication, a mouthful of dry bread, to fit it for being swallowed—the action of the jaws, by a beautiful contrivance of Providence, preparing the moisture along the channels at the time it is most wanted. Readers who may be disposed to think the circumstance of the fly thus moistening a bit of sugar fanciful, may readily verify the fact themselves in the way we have described. At the time when we made this little experiment, we were not aware that several naturalists of high authority had actually discovered by dissection the vessels which supply the saliva in more than one species of insect.”

“In the case of their drinking fluids, like water, saliva is not wanted; and it may be remarked, when we drink cold water it actually astringes and shuts up the openings of the salivary pipes. Hence it is that drinking does not quench thirst when the saliva is rendered viscid and scanty by heat, by fatigue, or by the use of stimulant food and liquor; and sometimes a draught of cold water, by carrying off all the saliva from the mouth, and at the same time astringing the orifices of the ducts, may actually produce thirst. Ices produce this effect on many persons. It is, no doubt, in consequence of their laborious exertions, as well as of the hot nature of their acid fluids producing similar effects, that ants are so fond of water. We have seen one quaff a drop of dew almost as large as its whole body; and when we present those in our glass formicaries with water, they seem quite insatiable in drinking it.”[3]

Rennie, in his Insect Miscellanies, after describing the pedestrian contrivances with which various insects are furnished, says,[4]—“The most perfect contrivance of this kind, however, occurs in the domestic fly (Musca domestica), and its congeners, as well as in several other insects. Few can have failed to remark that flies walk with the utmost ease along the ceiling of a room, and no less so upon a perpendicular looking-glass; and though this were turned downwards, the flies would not fall off, but could maintain their position undisturbed with their backs hanging downwards. The conjectures devised by naturalists to account for this singular circumstance, previous to the ascertaining of the actual facts, are not a little amusing. ‘Some suppose,’ says the Abbé de la Pluche, ‘that when the fly marches over any polished body, on which neither its claws nor its points can fasten, it sometimes compresses her sponge and causes it to evacuate a fluid, which fixes it in such a manner as prevents its falling without diminishing the facility of its progress; but it is much more probable that the sponges correspond with the fleshy balls which accompany the claws of dogs and cats, and that they enable the fly to proceed with a softer pace, and contribute to the preservation of the claws, whose pointed extremities would soon be impaired without this prevention.’ (Spect. de la Nat. vol. i. p. 116.) ‘Its ability to walk on glass,’ says S. Shaw, ‘proceeds partly from some little ruggedness thereon, but chiefly from a tarnish, or dirty, smoky substance, adhering to the surface; so that, though the sharp points on the sponges cannot penetrate the surface of the glass, it may easily catch hold of the tarnish.’ (Nature Displ. vol. iii. p. 98, Lond. 1823.) But,” adds Rennie, “it is singular that none of these fanciers ever took the trouble to ascertain the existence of either a gluten squeezed out by the fly, or of the smoky tarnish on glass. Even the shrewd Réaumur could not give a satisfactory explanation of the circumstance.”

“The earliest correct notion on this curious subject was entertained by Derham, who, in mentioning the provision made for insects that hang on smooth surfaces, says, ‘I might here name divers flies and other insects who, besides their sharp-hooked nails, have also skinny palms to their feet, to enable them to stick to glass and other smooth bodies by means of the pressure of the atmosphere—after the manner as I have seen boys carry heavy stones with only a wet piece of leather clapped on the top of the stone.’ (Physico-Theology, vol. ii. p. 194, note b, 11th edit.) The justly-celebrated Mr. White, of Selborne, apparently without the aid of microscopical investigation, adopted Derham’s opinion, adding the interesting illustration, that in the decline of the year, when the flies crowd to windows and become sluggish and torpid, they are scarcely able to lift their legs, which seem glued to the glass, where many actually stick till they die; whereas they are, during warm weather, so brisk and alert, that they easily overcome the pressure of the atmosphere.”—(Nat. Hist. of Selborne, vol. ii. p. 274.)

“This singular mechanism, however,” continues Rennie, “is not peculiar to flies, for some animals a hundred times as large can walk upon glass by the same means.” St. Pierre mentions “a very small lizard, about a finger’s length, which climbs along the walls, and even along glass, in pursuit of flies and other insects” (Voyage to the Isle of France, p. 73); and Sir Joseph Banks noticed another lizard, named the Gecke (Lacerta Gecha, Linn.), which could walk against gravity, and which made him desirous of having the subject thoroughly investigated. On mentioning it to Sir Everard Home, he and Mr. Bauer commenced a series of researches, by which they proved incontrovertibly, that in climbing upon glass, and walking along the ceilings with the back downwards, a vacuum is produced by a particular apparatus in the feet, sufficient to cause atmospheric pressure upon their exterior surface.

“The apparatus in the feet of the fly consists of two or three membranous suckers, connected with the last joint of the foot by a narrow neck, of a funnel-shape, immediately under the base of each jaw, and movable in all directions. These suckers are convex above and hollow below, the edges being margined with minute serratures, and the hollow portion covered with down. In order to produce the vacuum and the pressure, these membranes are separated and expanded, and when the fly is about to lift its foot, it brings them together, and folds them up, as it were, between the two claws. By means of a common microscope, these interesting movements may be observed when a fly is confined in a wine-glass.” (Phil. Trans. for 1816, p. 325.)

“It must have attracted the attention of the most incurious to see, during the summer, swarms of flies crowding about the droppings of cattle, so as almost to conceal the nuisance, and presenting instead a display of their shining corslets and twinkling wings. The object of all this busy bustle is to deposit their eggs where their progeny may find abundant food; and the final cause is obviously both to remove the nuisance, and to provide abundant food for birds and other animals which prey upon flies or their larvæ.