458 MOTIOl^S OF INSECTS. 



Dipterous insects, •wliich in general have these organs, and some three 

 on each foot^, are not exclusively gifted with them ; for various others 



could walk up the sides of an exhausted receiver, denies that their suckers have any 

 such power 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 actioa 

 of the minute hairs which clothe the inferior surfaces of the suckers, nearly as 

 Dr. Hooke had 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 surfiices ; 

 and on close inspection with an adequate magnifj-ing 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 

 climbing polished surfaces, such as Saltlcus scenicus, &c. (^Linn. Trans, xvi. 490. 

 768. ; compare also Entom. 3Iag. 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 flour, blue-bottle flies, the common house-flies, and the common bee-fly {Eris- 

 talis tenax) all slipped down again the instant they attempted to walk up these 

 portions of the glass ; and I moreover remarked that each time after thus slipping 

 down, they immediately began to riib 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 soma 

 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 tar as I am aware, been attached to them. These move- 

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

 but simply as an aflfair 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 diflerent 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 ou which I experimented appeared 

 to feel of cleaning their pulvilli imniediatelj' after being wetted or clogged with 

 flour, however frequently this occurred, there certainly seems ground for suppos- 

 ing that their usual and frequent operation for eff'ecting this by rubbing their 

 tarsi together is by no means one of mere cleanliness or amusement, but a very 

 important point of their economy, essentiallj' necessary for keeping their pul- 

 villi 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 surfjce 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 Avhole length of the tarsi, as well as the pulviUi, 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 



1 Philos. Trans. 1816, 325. t. xviii. f. 8—11. 



