MOTIONS OF INSECTS. 493 



fly, you see, does no more than the leech has been long known to do, when 

 moving in a glass vessel. Furnished with a sucker at each extremity, by 

 means of these organs it marches up and down at its pleasure, or as the 

 state of the atmosphere inclines it.^ 



■ 1 ■ — ■ ; '~-~~ 



' Mr. Blackwall, in a paper " On ihe Pulvilli of Insects," having found that flies could 

 walk up the sides of an exhausted receiver, denies that their suclcers have any such powjer 

 of forming a vacuum as is above ascribed to them, and explained their ability to climb up 

 vertical polished bodies, such as glass, by the mechanical action of the minute hairs which 

 clothe the inferior surfaces of the suclcers, nearly as Dr. Hooke has suggested; but further 

 experiments having shown him that flies cannot walk up glass which is made moist by 

 breathing on it, or 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 inspection 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 insects with pulvilli, and of those spiders which have the same power of climb- 

 ing polished surfaces, such as Sallicus scenicus, &cc. {Linn. Trans, xvi. 490. 768. j compare 

 also Entom. Ma^. i. -557.) 



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 flower, blue- 

 bottle flies, the common house-flies, and the common bee-fly (Em/afc /cwaz) all slipped 

 down again the instant they attempted to walk up these portions of the glass ; and I more- 

 over 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 move- 

 ments I had always regarded as meant to remove any panicle of dust from the legs, but 

 simply as an affair of instinctive cleanliness, like that of the cat when she licks herself (see 

 Letter XXIII. p. 482.), and not as serving anymore important object ; and sucli entomologi- 

 cal 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 difl'eient light, 

 since, though so strongly bearing on his explanation of the way in which flies mount smooth 

 vertical surfaces, he never at all refers to it. Yet, from the absolute necessity which the 

 flies on which I experimented appeared to feel of cleaning their pulvilli 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 im- 

 portant point of their economy, essentially necessary for keeping their pulvilli in a flt state 

 for climbing up smooth vertical surfaces 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 par- 

 ticular purpose. Sometimes, indeed, when the hairs of the tarsi are filled with dust through- 

 out, 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 EristaUs tcnnx, 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 clear, 

 and full two minutes were employed to make them so by stretching out her trunk, and pass- 

 ing 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 down. A process almost 

 exactly similar I have always seen used by blue-bottle flies and common house-flies which 

 had their tarsi clogged with flour by walking over it, or by having it dusted over them ; but 

 these manoeuvres are required for an especial purpose, and on ordinary occasions, as before 

 observed, the object in rubbing the tarsi together is not to clean them, but the pulvilli, 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 tibiae with sudden jerks over the back of the 

 head and eyes, and the two hind tarsi and tibiae 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 often to clean these parts from dust I have no doubt, as on 

 powdering flies with flour they thus employ themselves, sometimes for ten minutes, in de- 



42 



