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



wliicli is as yet undescribed. The specimens have got a little spoilt by the meditim in 

 which they are mounted, lout they still show a pair of organs beneath some of the finely 

 pectinate mnch bent ungues, which represent either a pair of tenent hairs or a rudi- 

 mentary pulvillns. This can hardly be determined without further specimens, in better 

 condition ; but in any light the structure is interesting as being unique, so far as I am 

 at present aware, amongst marine Articulata. 



Tenent hairs are so usually present in some modification or other, that it is really 

 difficult to name a beetle which has not some form of them ; the only one I yet know, 

 that seems to me really to possess nothing of the kind, is a species of Helops, which lives 

 on sandy heaths ; I suppose the dense cushion of hairs on the tarsi here to be for the 

 protection, simply, of the joints to which they are attached. I have detected them on the 

 tarsal joints of species of Ephyclra, and on the first basal tarsal joint of the Drone of the 

 Hive-bee, A very rudimentary form of tenent hairs is present on the under surface of some 

 of the Tree-bugs (Pentatomidce), which have, ia addition, a large deeply cleft organ at 

 the extremity of the tarsus, which appears to be a true sucker, and will be afterwards 

 described. 



To return now to our first subject, the Ely's foot. 



I do not see that it is necessary to add much to the observations which have been already 

 made, or which will have suggested themselves, on the action of the parts of which it is 

 composed. 



"When a Fly is not making use of its pulvilh, as on a surface sufficiently rough to afford 

 it foot-hold with its claws alone, these only are made use of, — remembering of course what 

 has been already said on the greatly increased hold gained by parts mutually opposed, 

 drawing towards each other. When walking on such a surface, the foot represents that 

 of a Coleopterous insect without any tenent appendages. 



On a smooth surface, perpendicular or horizontal, the pulvilli are brought down, and 

 the tenent hairs applied to such surface : a slight push forwards of these, succeeded by a 

 gentle draw backwards, at each application, removes the air between their soft, elastic 

 expansions and then- plane of motion, and thus a firm hold is gained. Access of air is 

 prevented by the minute quantity of moisture which exudes from the expanded tips of the 

 tenent appendages ; and thus a vacuum is formed, on the same principle as in the 

 "Atmosperic Hat-peg," the "Plate-holder" of the Photographer, or the "Artificial 

 Gums " of the Dentist. When the Ely wishes to move a leg from its place of attachment, 

 the claws are brought down and pressed against the surface ; from their position they 

 raise the hinder part of the pulvillus, where the tenent hairs are least developed, first, 

 and so on forwards. If the claws were attached to the fifth joint, as it has been supposed, 

 they could not act equally well in the way I have mentioned ; and I think a fly when 

 once stuck fast, if it had no claws, might remain so*. 



This introduces the curious question, " What is the pulvillus of a Ely ?" The analogy 

 between this part and the bilobed joints of some of the beetles has been noticed by previous 

 observers t ; it will readily suggest itself on a reference to the figures given in illustration 

 of the present paper. 



* See also Tyrrell and Lister, loc. cit. t Westwood ; Inmau ; imssim. 



