Fig. 398.
1. Foot and leg of Ophion; 2. Foot and leg of Flesh-fly; 3. Foot and leg of Drone-fly, with pad or sucker appendage.
The wax used in the formation of cells is a secretion that exudes through certain portions of the body of the bee, since it is found in little pouches situated on the under part of the body, but it is not brought home ready for use. The walls of the cells are strengthened when completed by a kind of varnish, already referred to as the propolis, collected from the buds of poplar and lime trees, and this is spread over the walls of the cell by that wonderful pair of broad spatulæ, represented in the drawing.
Many interesting variations will be found in the legs and feet of flies, as well as in those of other orders of insects (Lepidoptera). One or two typical forms are represented in [Plate VI]., and in [Fig. 398].
Fig. 399.—Sucker on the leg of Water-beetle. (The dot in the circle represents the object natural size.)
The tarsus, or foot of the fly ([Fig. 398]), consists of a deeply bifid, membranous structure, pulvillus; anterior to its attachment to the fifth tarsal joint, or the upper surface, are seated two claws, or “tarsal ungues”; these are freely movable in every direction. These ungues differ greatly in their outline, size, and relative 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 insects. Flexor and extensor muscles are attached to both ungues and flaps; the flaps are either corrugated or arranged on the ridge and furrow plan, in other cases they are perfectly smooth on their free surface, while others are covered with minute scale-like hairs. The thickness of the divided membrane on the blow-fly does not exceed the 1⁄2000th of an inch at the margin; they somewhat increase in thickness towards the point of attachment. Projecting from the flap are organs which have been termed “hairs,” “hair-like appendages,” “trumpet-shaped hairs.” These are doubtless the immediate agents in holding on to a smooth surface, as that of glass, and are termed “tenent-hairs,” in allusion to their office. The under surface of left forefoot of Musca vomitoria is shown with tenent-hairs ([Plate VI]., No. 140); a and b are more magnified hairs, a from below, b from the side. No. 142 is the left forefoot of Amara communis, showing the under surface and form of tenent appendages, one of which is seen more magnified at a; No. 143, under surface of left forefoot, Ephydra riparia. This fly is met with in immense numbers on the surface water in salt marshes. It does not possess the power of climbing glass; this is explained by the structure of the tenent-hairs; the central tactile organ is also very peculiar, the whole acting as a float, one to each foot, to enable the fly to rest on the surface of the water; a is one of the external hairs, No. 135, under surface of left forefoot of Cassida viridis (tortoise-beetle), showing the bifurcate tenent appendages, one of which is given at a more magnified. These, in ground beetles, are met with only in males, and are used for sexual purposes. The delicacy of the structure of these hairs in the fly and the elastic membranous expansion of the foot are marvellous. When the fly is climbing, a minute quantity of some glutinous fluid is exuded, so that the tubular nature of the tenent-hairs hardly admits of a doubt.
“At the root of the pulvillus, or its under surface, is a process, which in some instances is short and thick, in others long and curved, and tapering to its extremity (Scatophaga), setose (Empis), plumose (Hippoboscidæ), or, in one remarkable example (Ephydra), closely resembling in its appearance the very rudimentary pulvillus with which it is associated. Just at the base of the fifth tarsal joint, on its under surface, there is present, in Eristalis, a pair of short, very slightly curved hairs, which point almost directly downwards.”[82]
Tenent-hairs are usually present in some modification or other. It is really difficult to name a beetle which has not some form of them; the only one I yet know that seems to me really to possess nothing of the kind is a species of Helops, living on sandy heaths. I suppose the dense cushion of hairs on the tarsi to be for the protection, simply, of the joints to which they are attached. I have detected them on the tarsal joints of species of Ephydra, and on the first basal tarsal joint of the drone of the hive-bee. A very rudimentary form of tenent-hairs is present on the under surface of some of the tree-bugs (Pentatomidæ), which have in addition a large, deeply-cleft organ at the extremity of the tarsus; this appears to be a true sucker.
When walking on a rough surface, the foot represents that of a Coleopterous insect without any tenent appendages. 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, 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 degree, retractile to almost its fullest extent within the fifth tarsal joint—a joint modified to an extraordinary degree for special purposes.