MR. TUFFEN WEST ON THE FOOT OF THE FLY. • 417 



But there is a yet closer relationsliip — a true liomology. 



The \ohedipulvillus of a Ply is the homologue of a tarsal joint ; it is a sLvth tarsal joint, 

 which for special purposes remains membranous, unimpregnatecl with chitine. This joint 

 is attached to the fifth tarsal joint at an obtuse angle, and then, at some part of its 

 length (it may be a third or one-half), it is bent upon itself, so as to bring the tenent 

 hairs, with which this part is covered, into a proper position for their taking hold of a 

 surface when the insect is- in motion or at rest. 



The ungues are always attached to the last joint of an insect's tarsus. They are not 

 attached to the fifth tarsal joint of a Dipterous insect (see PL XLI. figs. 1, 10, 16, 19) ; 

 neither are they attached to the fifth tarsal joint of a Hymenopterous insect, but to the 

 terminal sucker, which, again, in this great Order, is a sixth tarsal joint, membranouSj 

 flexible, elastic in the highest degree, retractile to almost its fullest extent within the fifth 

 tarsal joint — a joint modified to an extraordinary degree for special purposes ! 



I have not had time to look much about for examples amongst other Insects in support 

 of these views ; but the plcmtula of Lucanus, with its pair of minute claws, at once 

 occurred as a case strictly in point. 



The ungues are hairs modified for special purposes *. They have the structure of true 

 hairs. The sustentacula of Epeira, the analogous structures on the entire under surface 

 of the last tarsal joints in Pholcus (fig. 69), the condition of the parts in the hind limbs 

 of Notonecta in both its mature and earlier condition, as well as in Sarcoptes, Fsoroptes, 

 and some other Acari (figs. 70-72), all contribute to the proof of this fact. 



I believe the fluid emitted to be neither more nor less than the ordinary sudor. It is 

 admitted that it has two components, the one of winch is watery and evaporates imme- 

 diately, the other is greasy and remainsf. Perspiration from the tip of a finger, pressed 

 on clean glass, behaves in precisely a similar way ; and the marks bear much resem- 

 blance, under the microscope (allowing for the great difference in size between the two), 

 to the corresponding marks left by a Ply in walking over glass. Nitrate of silver in 

 weak solution, I do not doubt, though I have had no time for trying it with the requisite 

 degree of care, would probably equally prove the presence of a chloride in each : in human 

 perspiration we know this to be chloride of sodium ; and, reasoning from strict analogy, it 

 is not likely to be any other salt in an Insect. The slight degree of viscidity possessed by 

 weak animal fluids, as saliva and sudor, would satisfactorily account for another set of the 

 phenomena presented by the fluid from a Ply's foot. The increased amount of secretion, 

 on exertion, in both cases, hardly requires to be pointed out. 



That the pressure of the atmosphere is the main agent by which a Ply is enabled to 

 adhere to perfectly smooth surfaces cannot, I conceive, after all that has been said, be 

 doubted. Careful experiments on the weights of numerous Plies, compared with the area 

 of their pulvilli, both of the membranous portions and of the surface covered by the 

 organs of holding, show the following curious facts. That atmospheric pressure, if the 

 area of the flaps be alone considered, is equal to just one half the weight of a Ply, If 

 the area covered by the tenent hairs be added, an increase of pressure is gaiaed, equal 

 to about one-fourth the weight of a Ply. This still leaves one-fourth to be accounted for 



* This was perceived by Inman, and has no doubt been noticed by other observers as well, 

 t Blackwall, Hepworth, &c. 



