Popular Science Monthly/Volume 25/May 1884/How Flies Hang on
|HOW FLIES HANG ON.|
IT was believed at one time that flies and some other insects owe the faculty of running over smooth bodies like glass to the numerous hairs with which their feet are provided catching in the pores of the material. The absurdity of this supposition is readily apparent on examining glass with the microscope; and no naturalist can be found in these days to uphold it. Another theory, which has been frequently advanced, explains the fact by affirming that the feet terminate in little suckers, by the application of which to the smooth surface the insect is able to adhere by the force of the pressure of the air, in the same manner that the street-boy fastens his leather sucker tightly to the flagging. Blackwall's investigations have demonstrated that such a contact as is here supposed does not take place. He has seen flies running over the inner sides of the bell-glass receiver of an air-pump from which the air had been exhausted. If we examine the foot of a fly through the microscope, we shall find that there are no suckers on it, but that the foot-cushions are furnished with very fine hairs that prevent all close contact with the glass. The theory in question which invokes the pressure of the air was first broached by Dr. Derham, and was accepted by most of his contemporary entomologists. Other observers, among them Dr. Hooke, were of the opinion that the insects were able to attach themselves to the glass by virtue of some sticky matter in or on the hair. Blackwall explained the fact by saying that a viscous substance flowed from each hair; and probably the majority of the later entomologists have accepted this explanation. In answer to it, we may say that, if there really were a flow of a viscous fluid from the hairs, the flies would not be able to move after they had rested in one spot for a little while, for the liquid would have dried or hardened so as to detain them; but we know that the insect can always fly away instantaneously, even if it has remained in the same place for hours without moving.
I have concluded from my experiments that it is not the pressure of the air nor the power of an adhesive liquid that gives flies the faculty of running over smooth bodies, but that the power should be attributed to the molecular action between solid and liquid bodies; or, in other words, to capillary adhesion.
If we examine the under part of the pulvilli (Fig. 1) with a microscope, we shall see distinctly that it is furnished with numerous hairs, regularly distributed. These hairs terminate, at their lower end, in a
kind of bulb, the form of which varies, whence flows an oily liquid that dries slowly and does not harden for a long time. The minute drops left on the glass by the hairs may be taken away, even after two or three days have passed, without our having to moisten them, by simply rubbing a piece of fine paper over them. I have devised an apparatus for collecting these drops by cutting a hole in a piece of board over which I fix a glass slide. Turning the board over so that the glass shall be at the bottom, I have a little cell with a glass floor. With the aid of a piece of paper gummed to the wings, I introduce a fly into this cavity in such a manner that the pulvilli shall rest upon the floor. Then, putting the board under the microscope with the glass slide uppermost, we have the fly's feet under our eyes. The insect, struggling for liberty, places his pulvilli against the glass, and leaves after each effort, traces that may be observed very distinctly, for they are perfectly visible in a good light (Fig. 2).
We may discover, whenever the feet of the fly come again into contact with these tracks or minute drops, that they are composed of a very liquid substance, for they spread quite readily on the glass. We can not admit, as some naturalists assume, that the liquid can hold the club-shaped hair-ends by suction. If this were the case, the ends
would change shape during the suction, and would take the form of a disk. The fly puts its feet down and lifts them up with an incomparable facility that would not exist if the limb were really acted upon by the pressure of the air.
There is no evidence here of an adhesive substance; such a substance would harden after two or three days, and would dry or at least become viscous, like Venetian turpentine or sirup.
The power which we are investigating can be due only to capillary action; for the liquid and the hairs are the only parts that touch the polished surfaces. The idea occurred to me that the faculty arose from the attraction that each minute drop exercises upon the hair with which it is in contact; and I made several experiments to demonstrate the possibility of such an effect.
I suspended a hair from a pane of glass, by means of oil of olives. Sticking the cut end of the hair into the oil, I fixed the part, by means of the oil that adhered to it, to the glass, which I had previously washed with great care. I thus succeeded in suspending from the glass a hair 16 centimetres long, with a volume of liquid not exceeding its diameter. Replacing the oil of olives with water, I obtained the same result. The hair was 0·06 of a millimetre in diameter, and the weight suspended may be calculated to have been 0·00045 of a gramme. Repeating the experiment with horse-hairs, I found that a hair 7·5 centimetres in length remained suspended under the same conditions. The hair was 0·12 of a millimetre thick, consequently the weight adhering to the glass was 0·00085 of a gramme. A hog's bristle 0·18 of a millimetre in diameter was suspended, although, being 55 millimetres long, it represented a weight of 0·00132 of a gramme.
I also experimented with a hair ending in a bulb, which I formed by holding the hair to a flame. I fixed to the glass a hair 0·06 of a millimetre in diameter, terminating in a bulb 0·12 of a millimetre in diameter, and weighing 0·00085 of a gramme, or the same as the horse-hair previously used.
The results of these experiments added weight to my supposition that the liquid does not have to be viscous to enable the flies to stick. To gain an absolute conviction, I weighed a number of flies, and found their mean weight to be 0·045 of a gramme. I then ascertained the number of hairs on the lower part of the pulvilli, and the size of the extremities which they brought to bear upon the glass. It is not an exaggeration to put the number on each pelote at 800 or 1,000; this would give the fly a total of 10,000 or 12,000 hairs, by means of which, with the assistance of a minute drop of liquid, it could support itself on a solid body. It is proper to add, however, that a fly running on a window has only three or four of its feet on the glass at a time, and that therefore only half of its hairs, or 5,000 or 6,000 of them, are serving it at once. I repeated my experiments, to determine the weight hairs are capable of supporting when suspended in the manner I have described, and found again that a hair 0·06 of a millimetre in diameter will bear a weight of 0*00045 of a gramme; of 0·12 millimetre, 0*00085 of a gramme; and of 0·18 millimetre, 0·00132 of a gramme, when the air is in motion. Then, according to my calculations, a fly would be able to walk upon glass, even if it weighed 0·020 of a gramme more than it actually does. I tested this by pasting little papers on the wings of flies to increase their weight. They still kept themselves on the glass; but they walked upward with some difiiculty when their weight was doubled.
I perceived in the course of my experiments that the flies, especially the weighted ones, ceased to adhere to the glass when it was moistened with the breath. Blackwall had essayed to explain this fact by assuming that the sticky substance by means of which he supposed they adhered mingled with the water, and was so much diluted by it as to cease to be effective. I found, by examination with the microscope, that this was not the case; no mixture or dilution took place, but rather a repulsion of the oily liquid by the water, and that that, or the contrary of what Blackwall assigned, was the reason adherence failed. Adherence likewise failed when the opposite side of the glass was moistened with ether, in consequence of the condensation of vapor occasioned by the evaporation of the ether.
Adherence also fails completely when the glass is covered with a thin wash of oil. And a fly which has been put upon a glass so covered, and is then transferred to a clean glass, will not be able to adhere to that till after some interval. An extremely thin coating of oil is enough to bring about a failure to adhere; even the rubbing of the finger on the glass is sufficient. The failure in this case is caused by the running together of the little drops of liquid on the hairs, by which the adhering surface is much reduced below the total surface presented by the little drops acting separately. Each foot then acts as a single hair, the diameter of which is equivalent to its own; and, even if its diameter were equivalent to a millimetre, the six feet bearing together upon the glass would not be competent to sustain the fly. For, according to the experiment with the horse-hair, a diameter
of 0·12 of a millimetre will bear 0·00085 of a gramme; consequently, a diameter of a millimetre will bear 0·007 of a gramme, and the six feet together 0·042 of a gramme.
It is very difficult, if not impossible, for a fly to walk on a vertical polished surface when it is thinly covered with dust. When, after it has made the effort, we examine its feet with the microscope, we shall perceive that the interspaces between the hairs are filled with dust. After it has rubbed its feet against one another for a short time, and has passed its wings over them, the dust will be found to have disappeared, and it will again be able to walk on glass. The object of this labor, which flies may be observed to be performing at every moment, is not, then, as was once supposed, to cleanse the wings, but to keep the feet in good condition to stick on smooth surfaces. The wings are supplied with a kind of rough hairs that may very well fill the place of brushes.
Blackwall believed that flies cleansed their feet for the purpose of removing the superfluous viscous liquid from their pilæ. If this were the case, all the parts of the insect that touched its feet would shortly be covered with that substance; and, if it does not dry but becomes gelatinous, the fly would collect all the dust with which it comes in contact, and would soon look like a lump of dirt. Contrary to this, we know that flies are always clean.
Other insects that can walk on glass like flies have also, like them, little hairs with club-shaped terminations on the bottoms of their feet, and adhere in the same way. The accompanying illustration (Fig. 3) represents the end of the foot of a beetle (the Polydrosus sericeus), and shows that it is provided with all the appurtenances we have been describing.
I think I have proved by my experiments that the faculty possessed by flies of walking over polished bodies should not be attributed to a viscous liquid, but simply to capillary action. Even if this liquid, which causes the hairs to adhere to the polished surface, were nothing but pure water, the flies would be able to support themselves upon it, whatever position they might be in.—Translated for the Popular Science Monthly from La Nature.