Why is this 1 you ask. Look, and you will see why. From the summit of the thorax project two little horns, which, under the microscope, are seen to be clear trumpet-shaped tubes with open mouths, cut as it were obliquely off. These enter the thorax close to the bases of the wings ; and w-hen we confine the animal in a glass cell, exercising a gentle pressure upon the thorax, we see bubbles of air alternately projected from the trumpet mouths of the tubes and sucked in again. These, then, are the spiracles, the orifices of the air-tubes, where the vital fluid enters the body, and whence it is carried to every part of the system.

There is something curiously beautiful about the structure of these spiracular tubes, of which I cannot attempt to explain the object. With a high magnifying power, their whole exterior surface is seen to be covered witli regular rounded scales, overlapping each other, an 1 very closely resembling those of a fish.

Chapter VII. Insects: their Feet

I have here inclosed a small window-fly in the live-box of the microscope, that you may examine the structure of its feet as it presses them against the glass cover ; and thus not only get a glimpse of an exquisitely formed structure, but acquire some correct ideas on the question of how a fly is enabled to defy all the laws of physics, and to walk jauntily about on the under surface of polished bodies, such as glass, without falling, or apparently the fear of falling. And a personal examination is the more desirable because of the hasty and erroneous notions that have been promulgated on the matter, and that are constantly disseminated by a herd of popular compilers, who profess to teach science by gathering up and retailing the opinions of others, often without the slightest knowledge whether what they are reporting is true or false.

The customary explanation has been that given by Derham in his "Physico-theology"; that " divers flies and other insects, 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 a stone." Bingley, citing this opinion, adds that they are able easily to overcome the pressure of the air " in warm weather, when they are brisk and alert ; but towards the end of the year this resistance becomes too mighty for their diminished strength; and we see flies labouring along, and lugging their feet on windows as if they stuck fast to the glass : and it is with the utmost difficulty they can draw one foot after another, and disengage their hollow cups from the slippery surface." *

But long ago another solution was proposed : for Hooke, one of the earliest of microscopic observers, described the two palms, pattens, or soles (as he calls the indvUl%), as "beset underneath with small bristles or tenters, like the Avire teeth of a card for working wool, which, having a contrary direction to the claws, and both pulling different Avays, if there be any irregularity or yielding in the surface of a body, enable the fly to suspend itself very firmly." He supposed that the most perfectly polished glass presented such irregularities, and that it was moreover always covered with a " smoky tarnish," into which the hairs of the foot penetrated.

The " smoky tarnish," is altogether gratuitous ; and Mr. BlackAvall has exploded the idea of atmospheric pressure, for he found that flies could Avalk up the interior of the exhausted receiver of an air-pump. He had explained their ability to climb up A'ertical polished bodies by the mechanical action of the minute hairs of the inferior surface of the palms; but further experiments haA Ting shown him that flies cannot Avalk up glass Avhich is made moist by breathing on it, or which is thinly coated with oil or flour, he was led to the conclusion that these hairs are in fact tubular, and excrete a viscid fluid, by means of which they adhere to dry polished surfaces ; and on close insjDection with an adequate magnifying power, he was always able to discover traces of this adhesive material on the track on glass both of flies and various other insects furnished with pul-cUli, and of those spiders which possess a similar faculty, f

In the earlier editions of Kirby and Spence's " Introduction to Entomology," Mr. Kirby had adopted the

" Anim. Biogr." t "L : nn. Trans.," xvi. 490, 768.

insects: their feet. Ill suctorial hypothesis. But in late one he made an allusion to Mr. Blackwall's opinion, and added the following interesting note :—

" On repeating Mr. Blackwall's experiments, I found, just as he states, that when a pane of glass of a window was slightly moistened by breathing on it, or dusted with flour, bluebottle-flies, the common house-flies, and the common bee-fly (Eristalis tenax), all slipped down again the instant they attempted to walk up these portions of the glass j and I moreover remarked that, each time, after thus slipping down, they immediately began to rub first the two fore tarsi and then the two hind tarsi together, as flies are so often seen to do, and continued this operation for some moments before they attempted again to walk. This last fact struck me very forcibly, as appearing to give an importance to these habitual procedures of flies, that has not hitherto, as far as I am aware, been attached to them. These movements I had always regarded as meant to remove any particle of dust from the legs, but simply as an affair of instinctive cleanliness, like that of the cat when she licks herself, and not as serving any more important object; and such entomological friends as I have had an opportunity of consulting tell me that their view of the matter was precisely the same ; nor does Mr. Blackwall appear to have seen it in a different light, since, though so strongly bearing on his explanation of the wxay in which flies mount smooth vertical surfaces, he never at all refers to it. Yet, from the absolute necessity which the Hies on which I experimented appeared to feel, of cleaning their indvilli immediately after being wetted or clogged with flour, however frequently this occurred, there certainly seems ground for supposing that their usual and frequent operation for effecting this, by rubbing their tarsi together, is by no means one of mere cleanliness or amusement, but a very important part of their economy, essentially necessary for keeping their pulvilli in a fit state for climbing up smooth vertical substances by constantly removing from them all moisture, and still more all dust, which they are perpetually liable to collect. In this operation the two fore and two hind tarsi are respectively rubbed together for their whole length ; whence it might be inferred that the intention is to remove impurities from the entire tarsi ; but this I am persuaded is not usually the object, which is simply that of cleaning the under side of the pulvilli, by rubbing them backward and forward along the whole surface of the hairs with which the tarsi are clothed, and which seem intended to serve as a brush for this particular purpose. Sometimes, indeed, when the hairs of the tarsi are filled with dust throughout, the operation of rubbing them together is intended to cleanse these hairs ; because, without these brushes were themselves clean, they could not act upon the hairs of the under side of the pulvilli. Of this I witnessed an interesting instance in an Eristalis tenax, which, by walking on a surface dusted with flour, had the hairs of the whole length of the tarsi, as well as the pulvilli, thus clogged with it. After slipping down from the painted surface of the window-frame which she in vain attempted to climb, she seemed sensible that before the pulvilli could be brushed, it was requisite that the brushes themselves should be clean, and full two minutes were employed to make them so, by stretching out her trunk, and passing them repeatedly along its sides, apparently for the sake of moistening the flour and causing its grains to adhere ; for after this operation, on rubbing her tarsi together, which she next proceeded to do, I saw distinct little pellets of flour fall clown. A process almost exactly similar I have always seen used by bluebottle-flies and common houseflies, which had their tarsi clogged with flour by walkingover it, or by having it dusted over them ; but these manoeuvres are required for an especial purpose j and on ordinary occasions, as before observed, the object in insects: their feet. rubbing the tarsi together is not to clean them,, but the pidvilli, for which they serve as brushes. Besides rubbing the tarsi together, flies are often seen, while thus employed, to pass the two fore tarsi and the tibia?, with sudden jerks, over the back of the head and eyes, and the two hind tarsi and tibia? over and under the wings, and especially over their outer margins, and occasionally also over the back of the abdomen. That one object of these operations is to clean these parts from dust, I have no doubt, as on powdering the flies with flour they thus employ themselves, sometimes for ten minutes, in detaching every part of it from their eyes, wings, and abdomen ; but I am also inclined to believe that, in general, when this passing of the legs over the back of the head and outer margin of the wings takes place in connexion with the ordinary rubbing of the tarsi together, as it usually does, the object is rather for the purpose of completing the entire cleansing of the tarsal brushes (for which the row of strong hairs visible under a lens on the exterior margin of the wings seems well adapted), so that they may act more perfectly on the pulvillL Here, too, it should be noticed, in proof of the importance of all the pulvilli being kept clean, that as the tarsi of the two middle legs cannot be applied to each other, flies are constantly in the habit of rubbing one of these tarsi and its pidvillus, sometimes between the two fore tarsi, and at other times between the two hind ones

" Though the above observations, hastily made on the spur of the occasion since beginning this note, seem to prove that it is necessary the pulvilli of flies and of some other insects should be kept free from moisture and dust to enable them to ascend vertical polished surfaces, they cannot be considered as wholly settling the question as to the precise way in which these pidvilli, and those of insects generally, act in effecting a similar mode of progression ; and my main reason for here giving these slight I hints is the hope of directing the attention of entomological and microscopical observers to a field evidently, as yet, so imperfectly explored.

" After writing the above, intended as a conclusion of this note, I witnessed to-day (July 11, 1842) a fact which I cannot forbear adding to it. Observing a house-fly on the window, whose motions seemed very strange, I approached it, and found that it was making violent contortions, as though every leg were affected with St. Vitus's dance, in order to pull its jmlvilli from the surface of the glass, to which they adhered so strongly that though it could drag them a little way, or sometimes by a violent effort get first one and then another detached, yet the moment they were placed on the glass again they adhered as if their under side were smeared with bird-lime. Once it succeeded in dragging off* its two fore-legs, when it immediately began to rub the pulvilli against the tarsal brushes; but on replacing them on the glass they adhered as closely as before, and it was only by efforts almost convulsive, and which seemed to threaten to pull off its limbs from its body, that it could succeed in moving a quarter of an inch at a time. After watching it with much interest for five minutes, it at last by its continued exertions got its feet released and flew away, and alighted on a curtain, on which it walked quite briskly, but soon again flew back to the window, where it had precisely the same difficulty in pulling its pulvilli from the glass as before; but after observing it some time, and at last trying to catch it, that I might examine its feet with a lens, it seemed by a vigorous effort to regain its powers, and ran quite actively on the glass, and then flying away I lost sight of it. I am unable to give any satisfactory solution of this singular fact. The season, and the fly's final activity, preclude the idea of its arising from cold or debility, to which Mr. White attributes the dragging of flies' legs at the close of autumn. The pulvilli certainly had much more the appearance of adhering to the glass by a viscid material than by any pressure of the atmosphere ; and it is so far in favour of Mr. Black wall's hypothesis, on which one might conjecture that from some cause (perhaps of disease) the hairs of the pul villi had poured out a greater quantity of this viscid material than usual, and more than the muscular strength of the fly was able to cope with."*

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