MOTIONS OF INSECTS. 



261 



the space between the two claws. This may be seen by 

 looking at the movements of a fly in the inside of a glass 

 tumbler with a common microscope. 1 Thus the fly, you see, 

 does no more than the leech has been long known to do, 

 when moving in a glass vessel. Furnished with a sucker at 

 each extremity, by means of these organs it marches up and 

 down at its pleasure, or as the state of the atmosphere in- 

 clines it. 2 



1 Philos. Trans. 1816, 325. t. xviii. f. 1— 7. 



2 Mr. Blackwall, in a paper " On the Pulvilli of Insects," having found that 

 flies could walk up the sides of an exhausted receiver, denies that their suckers 

 have any such power of forming a vacuum as is ahove ascribed to them, and ex- 

 plained their ability to climb up vertical polished bodies, such as glass, by the 

 mechanical action of the minute hairs which clothe the inferior surfaces of the 

 suckers, nearly as Dr. Hooke had suggested ; but further experiments having 

 shown him that flies cannot walk up glass which is made moist by breathing on 

 it, or is thinly coated with oil or flour, he was led to the conclusion that these 

 hairs are in fact tubular, and excrete a viscid fluid, by means of which they ad- 

 here to dry polished surfaces; and on close inspection with an adequate magni- 

 fying power, he was always able to discover traces of this adhesive material on 

 the track on glass both of flies and various insects with pulvilli, and of those 

 spiders which have the same power of climbing polished surfaces, such as Salticus 

 scenicus, &c. (Linn. Trans, xvi. 490. 768. ; compare also Entom. Mag. i. 557.) 



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, blue-bottle flies, the common house-flies, and the common bee-fly 

 (Eristalis tenax) all slipped down again the instant they attempted to Avalk 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 par- 

 ticle of dust from the legs, but simply as an affair of instinctive cleanliness, like 

 that of the cat when she licks herself (see Letter XXIII. p. 243.), 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 pre- 

 cisely 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 abso- 

 lute 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 certainly 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 amusement, but a very important point 

 of their economy, essentially necessary for keeping their pulvilli in a fit state for 

 climbing up smooth vertical surfaces by constantly removing 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 re- 

 move 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 forward along the whole surface of the hairs with 

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



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