126 EVENINGS AT THE MICEOSCOPE. 



Is tliinly coated witli oil or fioiir, he was led to the con- 

 clusion that these hairs are in fact tnhular, and excrete 

 a viscid fluid, bv means of "which they adhere to dry 

 polished surfaces ; and on close inspection with an ade- 

 quate magnifying power, he was always able to dis- 

 cover traces of this adhesive material on the track on 

 glass both of flies and various other insects furnished 

 with ptdvilU, and of those spiders which possess a sim- 

 ilar faculty.* 



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

 troduction to Entomology," Mr. Kirby had adopted the 

 suctorial hypothesis. But in a late one he made an al- 

 lusion to Mr. Blackwall's opinion, and added the follow- 

 ing interesting 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 immedi- 

 ately be ganto rub flrst 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 alwavs rec^arded 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 



* Linn. Trans, xvi. 490, 7fi8. 



