262 MOTIONS OF INSECTS. 



Dipterous insects, which in general have these organs, and 

 some three on each foot are not exclusively gifted with 



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 wit- 

 nessed an interesting instance in an Eristalis tenax, which by walking on a sur- 

 face 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 them- 

 selves 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 blue-bottle 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 ordi- 

 nary occasions, as before observed, the object in rubbing the tarsi together is not 

 to clean them, but the pulvilli, for which they serve as brushes. Besides rubbing 

 the tarsi together, flies are often seen, while thus employed, to pass the two fore 

 tarsi and tibia; with sudden jerks over the back of the head and eyes, and the 

 two hind tarsi and tibia; over and under the wings, and especially over their outer 

 margins, and occasionally also over the back of the abdomen. That one 

 object of these operations is often to clean these parts from dust I have no doubt, 

 as on powdering flies with flour they thus employ themselves, sometimes for ten 

 minutes, in detaching every part of it from their eyes, wings, and abdomen ; but 

 I am also inclined to believe that, in general, when this passing of the legs over 

 the back of the head and outer margin of the wings takes place in connection 

 with the ordinary rubbing of the tarsi together, as it usually does, that the ob- 

 ject is rather for the purpose of completing the entire cleansing of the tarsal 

 brushes (for which the row of strong hairs visible under a lens on the exterior 

 margin of the wings seems well adapted), so that they may act more perfectly on the 

 pulvilli. Here, too, it should be noticed, in proof of the importance of all the 

 pulvilli being kept clean, that as the tarsi of the two middle legs cannot be applied 

 to each other, flies are constantly in the habit of rubbing one of these tarsi and 

 its pulvillus sometimes between the two fore tarsi, and at other times be- 

 tween the two hind ones. I ought also not to omit stating, that having 

 taken out of a spider's net one of the minute Chalcididce just caught, and 

 pulled away the threads attached to it, it spent some time in passing its hinder 

 tarsi over its wings and abdomen, and then in passing its fore tarsi through 

 its palpi, apparently, as in the case of flies, to clean its pulvilli from any 

 remains of the spider's net; and that having surrounded a minute beetle (Meli- 

 gethes ceneus), which chanced to be on the window, with a slight circle of moisture, 

 it was unable to pass through it, and repeatedly drew its wetted fore tarsi 

 through its mouth, and rubbed the hind tarsi together ; and that precisely the 

 same results took place in the case of an Ichneumon placed in similar circum- 

 stances, only it spent much more time in rubbing both its fore and hind tarsi 

 together after being wetted, and in passing the former over its antennae and 

 through its mouth ; and when powdered with flour, it spent, like the flies before 

 mentioned, some minutes in cleaning itself by the same processes. 



Though the above observations, hastily made on the spur of the occasion since 

 beginning this note, seem to prove that it is necessary the pulvilli of flies and of 

 some other insects should be kept free from moisture and dust to enable them to 



i Philos. Tram. 1816, 325. t. xviii. f. 8—11. 



