112 EVENINGS AT THE MICROSCOPE. 



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 

 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 object, which is simply that of cleaning the under 

 side of the pulvilli, by rubbing them backward and for- 

 ward along the whole surface of the hairs with which the 

 tarsi are clothed, and which seem intended to serve as a 

 brush for this particular 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 pulvilli. Of this I witnessed an inte- 

 resting 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 as the pulvilli, thus clogged 

 with it. After slipping down from the painted surface of 

 the window-frame which she in vain attempted to climb, 

 she seemed sensible that before the pulvilli could be 

 brushed, it was requisite that the brushes themselves 

 should be clean, and full two minutes were employed to 

 make them so by stretching out her trunk, and passing 

 them repeatedly along its sides, apparently for the sake of 

 moistening the flour and causing its grains to adhere ; for 

 after this operation, on rubbing her tarsi together, which 

 she next proceeded to do, I saw distinct little pellets of 

 flour fall down. A process almost exactly similar I have 

 always seen used by bluebottle-flies and common house- 

 flies which had their tarsi clogged with flour by walking 

 over it, or by having it dusted over them ; but these 

 manoeuvres are required for an especial purpose, and on 

 ordinary occasions, as before observed, the object in 



