MR. TUFFEN WEST ON THE FOOT OF THE FLY. 107 
the knowledge of the subject, and yet, from one cause or another, still left the account of 
the structure incomplete. It was not necessary to my purpose to mention the opinions 
of those who merely wrote without making observations on which to found their opinions, 
or who, although they did make observations, have yet added nothing to our knowledge of 
.the structures now being treated of. 
Having thus cleared the way, and ascertained distinctly what has been already done, I 
shall proceed to mention the results of my observations. 
It will have been seen that scarcely anything was left to be discovered respecting tho 
Fly's foot, and that what was really needed was to extend the inquiry to the structure 
of the feet in other insects. 
As the additions to the knowledge of the Ply's foot have been given above in scattered 
notices, when mention was made of the labours of the successive observers who ha\ e con- 
tributed to the building of it up, it will be necessary that I should collect them now. This 
shall be done in the briefest manner possible; and in order to save repetition, I -hall 
intercalate my own observations at the same time. 
The foot of a Ply, then, consists of a deeply bifid, membranous structure (fig. 1, PI. X LI .), 
to which the term "pulvillus*" has been applied; anterior to the point of attachment of 
this part to the fifth tarsal joint, on the upper surface, are seated two claws, or " tarsal 
ungues," which are freely moveable in every direction, and may be clos. ly approximated 
or widely separated. These ungues differ greatly in their outline, size, and relative degree 
of development to the tarsi, and to the bodies of the insects possessing them, and in then- 
covering ; most are naked over their entire surface, having, however, a hexagonal network 
at their bases, which indicates a rudimentary condition of minute scale-like hairs, such as 
common on some part of the integument of all true insects 
these deer 
passing forwards towards the points of the claws, on approaching which they cease some- 
what abruptly. Of the ungues, the outer one is always a little the stronger; the spines 
terminating the tibiae, or the calcaria attached to the same part in insects where these are 
present, show proofs, equally with these ungues, of the universal prevalence of the law, 
that of two corresponding processes on any limb, the outer is invariably the more robust. 
To the human or comparative anatomist, examples of this remarkable fact will occur so 
directly, that it will be unnecessary for me to remark further respecting it, than that the 
difference in the relative degrees of development of the femoral trochanters furnishes on- 
of the best instances of it that could be adduced. It may probably be connected, in >me 
degree, with the greater risk of injury from extraneous causes to the outer, as compared 
*-ith the inner aspect, of a limb. Plexor and extensor muscles are attached to both the 
ated or arranged on the ridge-and-furrow plan, 
and the flaps. These flaj 
gclt^ -x «™- 
some cases perfectly smooth on their superior surface, in others this surface is 
covered with minute scale-like hairs. The thickness of the flaps in the Blow-fly does not 
exceed -0002 inch at the margin; thence they increase rapidly in thickness in passing 
inwards and backwards towards the point of attachment. Projecting from their inferior 
* 
<rular 
or plural number to this appendage to each tarsus. Examination o\ 
ever, at once that it is, as I have stated, a single, deeply-cleft organ. 
ducted 
