The parts of the foot at the extremity of the last tarsal joint proper are of great importance to the creature, and vary greatly in different Insects. The most constant part of this apparatus is a pair of claws, or a single claw. Between the two claws there may exist the additional apparatus referred to above. This in some Insects—notably in the Diptera—reaches a very complex development. We figure these structures in _Pelopaeus spinolae_, a fossorial Hymenopteron, remarking that our figures exhibit the apparatus in a state of retraction (Fig. 59). According to the nomenclature of Dahl and Ockler[24] the plate (_b_) on the dorsal aspect is the pressure plate (_Druck-Platte_), and acts as an agent of pressure on the sole of the pad (C, _e_); _c_ and _d_ on the underside are considered to be extension-agents; _c_, extension-plate; _d_, extension-sole (_Streck-Platte_, _Streck-Sohle_). These agents are assisted in acting on the pad by means of an elastic bow placed in the interior of the latter. The pad (_e_) is a very remarkable structure, capable of much extension and retraction; {106}when extended it is seen that the pressure plate is bent twice at a right angle so as to form a step, the distal part of which runs along the upper face of the basal part of the pad; the apical portion of this latter consists of two large lobes, which in repose, as shown in our Figure (_f_), fall back on the pad, something in the fashion of the retracted claws of the cat, and conceal the pressure-plate. The mode in which Insects are able to walk on smooth perpendicular surfaces has been much discussed, and it appears highly probable that the method by which this is accomplished is the exudation of moisture from the foot; there is still, however, much to be ascertained before the process can be satisfactorily comprehended. The theory to the effect that the method is the pressure of the atmosphere acting on the foot when the sole is in perfect apposition with the object walked on, or when a slight vacuum is created between the two, has apparently less to support it. [Illustration: FIG. 59.—Foot of _Pelopaeus_, a fossorial wasp: A, tarsus entire; B, terminal joint, upper side; C, under side. _a_, claw; _b_, base of pressure-plate; _c_, extension-plate; _d_, extension-sole; _e_, pad; _f_, lobe of pad retracted.]