The fly's foot, with horn-like projections and two pads by which the insect can cling to surfaces.
E.J. Spitta (Edmund Johnson Spitta, 1853-1921) (photograph)
Original source: <i>Marvels of the universe. A popular work on the marvels of the heavens, the earth, plant life, animal life, the mighty deep, with an introd. by Lord Avebury and with contributions by leading specialists, etc.</i>, published in 2 vols in London 1911-1912. Page 238 <a rel="nofollow" class="external autonumber" href="https://archive.org/details/marvelsofunivers01lond/page/238">[1]</a>.