132 EVENINGS AT THE MICROSCOPE 



they adhere to dry polished surfaces; and on close inspec- 

 tion 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 other insects fur- 

 nished with pulvilli, and of those spiders which possess 

 a similar faculty. 1 



In the earlier editions of Kirby and Spence's "Intro- 

 duction to Entomology," Mr. Kirby had adopted the suc- 

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

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

 esting 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 com- 

 mon 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 



1 "Linn. Trans.," xvi. 490, 768. 



