TRANSACTIONS OF THE LINN^AN SOCIETY. 447 



Blackwall, Esq., F. L. S. — The question of the structure of 

 the pulviUi of flies, though often agitated, can hardly yet be 

 considered satisfactorily settled. Mr. Blackwall, after enume- 

 rating various opinions pubhshed regarding it, has indeed 

 clearly disproved the of late generally received one of Dr. 

 Derham and Sir E. Home, that those parts act as suckers by 

 atmospheric pressure ; — for, on enclosing a house-fly in the 

 exhausted receiver of an air-pump, he found, that so long as its 

 vital powers were unimpaired, it was still able to traverse, not 

 only the sides, but the dome of that vessel. In the sentiment, 

 however, which he has himself adopted, or it may be in his 

 explanation of it, he does not appear equally happy. Like 

 Dr. Hooke, he sees the under surface of the pidvilli to be 

 covered with closely-set minute hairs, directed downwards ; 

 and he considers that insects are enabled to take hold of the 

 roughnesses, or irregularities of the surface of glass, by means 

 of these hairs, their difficulty in doing so increasing with the 

 goodness of the polish ; but he remarks, that Hooke was in 

 error in supposing the hairs to be pointed, for to him their 

 extremities seem somewhat enlarged; and if we understand 

 correctly Mr. Blackwall's reference to the preceding paper by 

 himself, he takes this dilated appearance of the ends to be 

 owing to each hair's being fringed by still finer hairs, which 

 form at its extremity a brush on its under surface. The mode 

 of adhesion he accounts to be strictly mechanical, not by the 

 aid of a glutinous secretion, and to depend in great measure 

 on the numerous points of contact presented ; — their " influ- 

 ence being from the tarsi outwards." But the manner in 

 which these points can operate in counteracting the force of 

 gravity is left unexplained ; and indeed we do not distinctly 

 comprehend the idea here intended to be conveyed. Perhaps 

 some light may be thrown on this obscure subject by intro- 

 ducing a notice of a few observations mostly made in 1825, 

 and recently repeated. 



The appearance, under moderate magnifying power, of the 

 pulviUi of flies, is well known ; their shape, and other parti- 

 culars, differing according to the species, but maintaining a 

 general resemblance throughout the family. Through a good 

 achromatic microscope, each of the two similar parts, of which 

 they consist, is seen to spread out into a thin flat membrane, 

 more or less transparent, mostly furrowed on the upper 



