398 MR. TUFFEN WEST ON THE FOOT OF THE FLY. 
flies to the objects on which they rest or walk, was confounded by Gilbert "White with, 
and explained by a reference to their permanent and involuntary attachment when dead 
or dying from the attacks of the remarkable parasite called ' Empnsa Jfnscce ' by Colin. 
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 structures 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 which 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 clothing the lower surface of the tarsal 
cushions have their extremities enlarged in this insect was now announced. "The 
production of a vacuum between each membrane " (tarsal cushion) " and the plane of 
position .... was 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 
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 whicli 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 unim- 
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 
a 
in 
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, JPrionus cervicomis 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, to!, xvi. + Ibid. P- 471. 
