44'8 TRANSACTIONS OF THE LINNi^AN SOCIETY. 



surface in delicate ridges, from the base to the margin, where 

 they terminate in hairs ; and these ridges are sometimes 

 crossed by others. In some species, the marginal hairs might 

 well be mistaken, with the less accurate instrument employed 

 by Sir E. Home, for a serrature; — they in reality form the 

 edge of that remarkable covering of the under surface which 

 has so much puzzled observers. 



This covering is composed of pellucid elastic hairs, arranged 

 with beautiful regularity, which appear to have their insertion 

 along the ridges just mentioned ; all are inflected downwards, 

 and slope a little outwards ; and those near the sides of the 

 pulvillus are inclined also towards the margin, or project 

 beyond it; — each is nearly of an even thickness till near its 

 extremity, where it is bent suddenly into a knobbed or flat- 

 tened end. Hooke's comparison of them to the wire-teeth of a 

 card for working wool, was not an inapt one, except that all 

 their terminations, which he calls "tenters," are turned out- 

 wards from the tarsus. 



The under side of the pulvilli, seen in a favourable light, 

 when these terminations show as a multitude of bright spots, 

 disposed at equal distances on a flat surface, is among the 

 most striking of microscopic objects. The length and distance 

 of the hairs is not always in proportion to the size of the fly : 

 — in the broad 'pulvillus, for example, of Sarcophagus carna- 

 rius, they are as close as in that of the house-fly, {Musca 

 dojnestica,) but their number is vastly greater in the larger 

 species. In Scatophaga stercoraria, the flat, transparent, and 

 apparently glandular ends, are very distinct. 



To obtain a just idea of this apparatus, it should be viewed 

 in different positions, and by reflected, as well as by trans- 

 mitted light; — the creature should be living, and the linear 

 power employed from two hundred to five hundi-ed. 



Flies use their pulvilli, or their claws, according to the 

 nature of the surface ; and often, when walking horizontally, 

 and on a soft substance, do not touch with either, but rest 

 entirely on other articulations of the tarsus. When they 

 adhere to polished glass, in a vertical position, they may be 

 seen, from the other side, to apply to it the bent extremities 

 of the hairs described. Those of nearly the whole pulvilU 

 will be employed when the insect is full of vigour, but some- 

 times, when exhausted, it will be suspended by those of a 



