MR 
417 
But there is a yet closer relationship — a true homology. 
The lobe&pulvillus of a Ply is the homologue of a tarsal joint ; it is a sixth tarsal joint, 
which for special purposes remains membranous, unimpregnated 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 he 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, 1(5, 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, membranous, 
flexible, elastic in the highest decree, retractile to almost its fullest extent wit bin the fifth 
'b" vov "~o 
tarsal joint— a joint modified to an extraordinary degree for special purposes I 
I have not had time to look much about for examples amongst other Insects in support 
of these views; but the pkmtula 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 entile under surface 
of the last tarsal joints in Pholeus (fig. 69), the condition of the parts in the hind hmbs 
of Notonecta in both its mature and earlier condition, as well as in Sarcoptes, Psoroptes, 
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 which 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. Kitrate 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 : m 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 ol 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 wliich 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 Flies, compared with the area 
of their pulvilli, both of the membranous portions and of the surface covered by tke 
organs of holding, show the foDowing curious facts. That atmospheric pressure if the 
area of the flaps be alone considered, is equal to just one half the weight of a Fly. 1± 
the area covered by the tenent hairs be added, an increase of pressure is gamed, 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. 
Blaekwall, Hepworth, &c. 
