INSECTS : THEm FEET. 127 



6lio licks lierst'lf, and not as serving any more important 

 object ; and such entomological friends as I liave had 

 an opportunity of consulting tell me that their view of 

 the matter was precisely the same ; nor does Mr. Black- 

 wall appear to have seen, it in a difi'erent 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 neces- 

 sity which the flies on which I experimented appeared 

 to feel of cleaning their 2^^^^ villi 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 efliecting 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 ver- 

 tical substances by constantly removing from them all 

 moisture, and still more all dust, which they are per- 

 petually 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 ob- 

 ject, 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 

 bnish for this particular purpose. Sometimes, indeed, 

 when the hairs of the tarsi are filled with dust through- 

 out, the operation of rubbing them together is mtended 

 to cleanse these hairs ; because, without these brushes 

 were themselves clean, they could not act upon the 



