INSECTS : THEIR FEET. Ill 



torial hypothesis. But in a late one he made an allusion 

 to Mr. Blackwall's opinion, and added the following inte- 

 resting 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 ; 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 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 cer- 

 tainly 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 



