398 MR. TUFFEN WEST ON THE FOOT OF THE FLY. 



flies to the objects on which they rest or walk, was confounded by Gilbert AVhite with, 

 and explained by a reference to their permanent and involuntary attachment when dead 

 or dying from the attacks of the remarkable parasite called ' Empiisa Ilusca; ' by Cohn. 



Mr. John Blackwall (1830)* sent three communications to the Linnean Society, bear- 

 ing upon this subject. In the first is given a description of the peculiar structures met 

 •with on the feet of such spiders as are capable of ascending polished perpendicular sur- 

 faces, so far as he had opportunities for making observations upon them. He at this time 

 expressed the opinion that " the minute bristles with which the tarsal cushions of many 

 insects, remarkable for their ability to walk up glass, are furnished, appear to possess an 

 organization closely analogous " to that of the feet of these Spiders f. The importance of 

 attending to the amount of development of the structu.res for holding, in connexion with 

 the relative size of the body, and to the state of physical strength at the time of the ex- 

 periments, is clearly pointed out. My limits will not permit me here to explain the way 

 in Avhich the appendages to the legs of spiders having this remarkable climbing power act ; 

 but on a future occasion I shall enter upon this branch of the subject. 



In his second communication the parts entering into the composition of the Ply's foot 

 are described. The discovery that the hairs clothiug the lower surface of the tarsal 

 cushions have their extremities enlarged in this insect was now announced. " Tlie 

 production of a vacuum between each membrane" (tarsal cushion) "and the plane of 

 position .... Avas at once seen by Mr. Blackwall, on examining the parts with a com- 

 pound microscope, to be clearly impracticable, unless the numerous hairs on the under 

 side of these organs individually perform the office of suckers." The quotation continues 

 — " there does not appear to be anything in their mechanism which in the slightest 

 degree countenances such a hypothesis. When highly magnified, their extremities, it is 

 true, are seen to be somewhat enlarged ; but whether they be viewed in action or ia 

 repose, they never assume a figure at all adapted to the production of a vacuum." 



I shall proceed to explain, in due course, my own view of these interesting facts and 

 statements. With respect to the action of these parts, an air-pump experiment is mentioned, 

 by which it was considered to have been " demonstrated, to the entire satisfaction of several 

 intelligent gentlemen present, that the House-fly, while it retains its vital powers imim- 

 paired, can not only traverse the upright sides, but even the dome of an exhausted 

 receiver ; and that the cause of its relaxing its hold and ultimately falling from the 

 station it occupies is a diminution of muscular force attributable to impeded respiration." 

 Mr. Blackwall felt the desirability of a minute examination, for purposes of comparison, 

 of analogous structures in other insects, and appears to have examined the cushions on the 

 tarsi of several beetles, respecting which he says :— " If the slender bristles on the inferior 

 surface of the pulvilli of some of the larger Coleoptera, Frionus cervicornis for example, 

 be very highly magnified, each, beside the numerous short hairs which project from its 

 sides, will be found to have a small dense brush of exceedingly minute hairs at its 

 extremity ; and as the hairs on the pulvilli of flies, and many other insects belonging to 

 various orders and genera, with which I have experimented, perform a function similar to 

 that exercised by the bristles, and also exhibit a striking resemblance to them in external 



* Transactions of the Linnean Society, vol. xvi. t Ibid. p. 471. 



