MR. TUFFEN WEST ON THE FOOT OF THE FLY. 407 



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 abeady 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 the 

 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 have 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 shall 

 intercalate my own observations at the same time. 



The foot of a Tly, then, consists of a deeply bifid, membranoiis structure (fig. 1, PI. XLL), 

 to which the term "pulvUlus*" 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 closely 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 their 

 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 

 are common on some part of the integument of all true insects ; these decrease in size in 

 passing forwards towards the points of the claws, on approaching which they cease some- 

 what abruptly. Of the tmgues, 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 one 

 of the best instances of it that could be adduced. It may probably be connected, in some 

 degree, with the greater risk of injury from extraneous causes to the outer, as compared 

 with the inner aspect, of a limb. Plexor and extensor muscles are attached to both the 

 ungues and the flaps. These flaps, corrugated or arranged on the ridge-and-furrow plan, 

 are in 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 



* The statements of authors do not appear to me to be clear, as to whether they apply this terra in the singular 

 or plural number to this appendage to each tarsus. Examination of the parts, conducted with due care, shows, how- 

 ever, at once that it is, as I have stated, a single, deeply-cleft organ. 



