Micrographia, by Robert Hooke

Observ. xxxviii. Of the Structure and motion of the Wings of Flies.

The Wings of all kinds of Insects, are, for the most part, very beautifull Objects, and afford no less pleasing an Object to the mind to speculate upon, then to the eye to behold. This of the blue Fly, among the rest, wants not its peculiar ornaments and contrivances; it grows out of the Thorax, or middle part of the body of a Fly, and is seated a little beyond the center of gravity in the body towards the head, but that Excentricly is curiously balanc'd; first, by the expanded Area of the wings which lies all more backwards then the root, by the motion of them, whereby the center of their vibration is much more backwards towards the tail of the Fly then the root of the wing is. What the vibrative motion of the wings is, and after what manner they are moved, I have endeavoured by many trials to find out: And first for the manner of their motion, I endeavoured to observe several of those kind of small Spinning Flies, which will naturally suspend themselves, as it were, pois'd and steady in one place of the air, without rising or falling, or moving forwards or backwards; for by looking down on those, I could by a kind of faint shadow, perceive the utmost extremes of the vibrative motion of their wings, which shadow, whil'st they so endeavoured to suspend themselves, was not very long, but when they endeavour'd to flie forwards, it was somewhat longer; next, I tried it, by fixing the leggs of a Fly upon the top of the stalk of a feather, with Glew, Wax, &c. and then making it endeavour to flie away; for being thereby able to view it in any posture, I collected that the motion of the wing was after this manner. The extreme limits of the vibrations were usually somewhat about the length of the body distant from one another, oftentimes shorter, and sometimes also longer; that the formost limit was usually a little above the back, and the hinder somwhat beneath the belly; between which two limits, if one may ghess by the sound, the wing seem'd to be mov'd forwards and backwards with an equal velocity: And if one may (from the shadow or faint representation the wings afforded, and from the consideration of the nature of the thing) ghess at the posture or manner of the wings moving between them, it seem'd to be this: The wing being suppos'd placed in the upmost limit, seems to be put so that the plain of it lies almost horizontal, but onely the forepart does dip a little, or is somewhat more deprest; in this position is the wing vibrated or mov'd to the lower limit, being almost arrived at the lower limit, the hinder part of the wing moving somewhat faster then the former, the Area, of the wing begins to dip behind, and in that posture seems it to be mov'd to the upper limit back again, and thence back again in the first posture, the former part of the Area dipping again, as it is moved downwards by means of the quicker motion of the main stem which terminates or edges the forepart of the wing. And these vibrations or motions to and fro between the two limits seem so swift, that 'tis very probable (from the sound it affords, if it be compar'd with the vibration of a musical string, tun'd unison to it) it makes many hundreds, if not some thousands of vibrations in a second minute of time. And, if we may be allow'd to ghess by the sound, the wing of a Bee is yet more swift, for the tone is much more acute, and that, in all likelihood, proceeds from the exceeding swift beating of the air by the small wing. And it seems the more likely too, because the wing of a Bee is less in proportion to its body, then the other wing to the body of a Fly; so that for ought I know, it may be one of the quickest vibrating spontaneous motions of any in the world; and though perhaps there may be many Flies in other places that afford a yet more shrill note with their wings, yet 'tis most probable that the quickest vibrating spontaneous motion is to be found in the wing of some creature. Now, if we consider the exceeding quickness of these Animal spirits that must cause these motions, we cannot chuse but admire the exceeding vividness of the governing faculty or Anima of the Insect, which is able to dispose and regulate so the the motive faculties, as to cause every peculiar organ, not onely to move or act so quick, but to do it also so regularly.

Whil'st I was examining and considering the curious Mechanism of the wings, I observ'd that under the wings of most kind of Flies, Bees, &c. there were plac'd certain pendulums or extended drops (as I may so call them from their resembling motion and figure) for they much resembled a long hanging drop of some transparent viscous liquor; and I observed them constantly to move just before the wings of the Fly began to move, so that at the first sight I could not but ghess, that there was some excellent use, as to the regulation of the motion of the wing, and did phancy, that it might be something like the handle of a Cock, which by vibrating to and fro, might, as 'twere, open and shut the Cock, and thereby give a passage to the determinate influences into the Muscles; afterwards, upon some other trials, I suppos'd that they might be for some use in respiration, which for many reasons I suppose those Animals to use, and, me thought, it was not very improbable, but that they might have convenient passages under the wings for the emitting, at least, of the air, if not admitting, as in the gills of Fishes is most evident; or, perhaps, this Pendulum might be somewhat like the staff to a Pump, whereby these creatures might exercise their Analogous lungs, and not only draw in, but force out, the air they live by: but these were but conjectures, and upon further examination seem'd less probable.

Schem. 23.
Fig. 4.
Schem. 26.
Fig. 2.
The fabrick of the wing, as it appears through a moderately magnifying Microscope, seems to be a body consisting of two parts, as is visible in the 4. Figure of the 23. Scheme; and by the 2. Figure of the 26. Scheme; the one is a quilly or finny substance, consisting of several long, slender and variously bended quills or wires, something resembling the veins of leaves; these are, as 'twere, the finns or quills which stiffen the whole Area, and keep the other part distended, which is a very thin transparent skin or membrane variously folded, and platted, but not very regularly, and is besides exceeding thickly bestuck with innumerable small bristles, which are onely perceptible by the bigger magnifying Microscope, and not with that neither, but with a very convenient augmentation of sky-light projected on the Object with a burning Glass, as I have elsewhere shew'd, or by looking through it against the light.

In steed of these small hairs, in several other Flies, there are infinite of small Feathers, which cover both the under and upper sides of this thin film as in almost all the sorts of Butterflies and Moths: and those small parts are not onely shap'd very much like the feathers of Birds, but like those variegated with all the variety of curious bright and vivid colours imaginable; and those feathers are likewise so admirably and delicately rang'd, as to compose very fine flourishings and ornamental paintings, like Turkie and Persian Carpets, but of far more surpassing beauty, as is evident enough to the naked eye, in the painted wings of Butterflies, but much more through an ordinary Microscope.

Intermingled likewise with these hairs, may be perceived multitudes of little pits, or black spots, in the exended membrane, which seem to be the root of the hairs that grow on the other side; these two bodies seem dispers'd over the whole surface of the wing.

The hairs are best perceiv'd, by looking through it against the light, or, by laying the wing upon a very white piece of Paper, in a convenient light, for thereby every little hair most manifestly appears; a Schem. 23.
Fig. 4.
Specimen of which you may observe drawn in the fourth Figure of the 23. Scheme, AB, CD, EF whereof represent some parts of the bones or quills of the wing, each of which you may perceive to be cover'd over with a multitude of scales, or bristles, the former AB, is the biggest stem of all the wing, and may be properly enough call'd the cut-air, it being that which terminates and stiffens the formost edge of the wing; the fore-edge of this is arm'd with a multitude of little brisles, or Tenter-hooks, in some standing regular and in order, in others not; all the points of which are directed from the body towards the tip of the wing, nor is this edge onely thus fring'd, but even all the whole edge of the wing is covered with a small fringe, consisting of short and more slender brisles.

This Subject, had I time, would afford excellent matter for the contemplation of the nature of wings and of flying, but, because I may, perhaps, get a more convenient time to prosecute that speculation, and recollect several Observations that I have made of that particular. I shall at present proceed to

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