INSECTS: THEIR FEET 133 



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

 wall appear to have seen it in a different 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 amuse- 

 ment, but a very important part of their economy, essen- 

 tially necessary, for keeping their pulvilli in a fit state for 

 climbing up smooth vertical substances by constantly re- 

 moving from them all moisture, and still more all dust, 

 which they are perpetually liable to collect. In this opera- 

 tion 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 pul- 

 villi 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 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 pul- 

 villi. 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 



