(777). But in other regions there is an absolute necessity for a mode of communication intermediate in character between the two kinds mentioned above, having neither the firmness of the one nor the mobility of the other. This is more especially the case in the junction between the head and the anterior segment of the thorax, and also between the last-named segment and the middle piece of the thorax, in those cases where these two parts are not joined by suture. The joint employed in this case is of very beautiful construction, resembling in some respects that formed by a ball and socket: a conical prolongation of one segment is admitted into a smooth cavity excavated in the corresponding margin of the other, and secured in this position by muscles and an external ligament. Such an articulation is of course capable of being firmly fixed by muscular action, but at the same time admits of sufficient freedom of motion to allow rotation in all directions.

(778). The legs of insects, as we have already stated, are invariably six in number, one pair being attached to each of the three thoracic segments. Considered separately, every leg may be seen to consist of several pieces, connected together by articulations of different kinds, which require our notice. The first division of the leg, or that in immediate connexion with the thorax, to which it is united by a kind of ball-and-socket joint enclosed in a strong membranous capsule and possessing very various degrees of motion in different insects, is called the hip (coxa); and upon this, as upon a centre, the movements of the limb are performed. To the extremity of the coxa a small moveable piece is attached, called the trochanter; to which succeeds the thigh (femur), which is the thickest and most robust of all the divisions of the limb. The next piece, called the shank (tibia), is occasionally of considerable length, and is connected to the last by a hinge; to its extremity is appended the foot (tarsus), composed of a consecutive series of small segments, varying in number from five to one, the last of which is armed with claws, or other appendages, adapted to different kinds of progression.

These divisions of the leg the reader will easily recognize; they are for the most part united together by articulations so constructed as to allow simply of flexion and extension, which will be best understood by inspecting, in some large insect, the junction between the femur and the tibia, or the knee-joint, as we might term it. Upon the upper extremity of the tibia the observer will find on each side a precise semicircular furrow, behind which is a concentrical but smaller ridge, and still further back a circular depression or fossulet. On examining the corresponding surfaces of the femur, he will detect a ridge accurately corresponding to the above-mentioned furrow; behind this, a furrow corresponding to the preceding ridge; and still further back, a minute elevation adapted to the fossulet of the tibia, wherein it is fastened by a minute but very strong ligament. Such ridges and grooves, when fitted into each other, form a joint evidently admitting of a free and hinge-like motion, while, from its structure, dislocation is almost impossible.

(779). The above general description of the leg of an insect will prepare us to examine various modifications in outward form and mechanical arrangements by which these simple organs are adapted to progression under a great diversity of circumstances. When, indeed, we reflect how extensively this class of animals is distributed, and the variety of situations in which insects live, we are led to expect corresponding adaptations in the construction of their instruments of locomotion; and in this our expectations will not be disappointed.

(780). In the generality of terrestrial species, the last segment of the tarsus or foot is provide with a pair of strong horny hooks, which are available for many purposes, being used either for creeping upon a moderately rough surface, for climbing, or for clinging to various substances.

(781). Such simple hooks, however, would not always serve. In the case of the louse (Pediculus) for example, that is destined to climb slender and polished hairs, such prehensile organs could be of little use.

The structure of the foot is therefore modified: the tarsus in this insect terminates in a single moveable claw, which bends back upon a toothlike process derived from the tibia, and thus forms a pair of forceps, fitted to grasp the stem of the hair and secure a firm hold.

(782). Many insects, especially those of the Dipterous order, are able to ascend the smoothest perpendicular planes, or even to run with facility, suspended by their feet, in an inverted position, along substances which, from their polished surfaces, could afford no hold to any apparatus of forceps or hooklets. In the common flies (Muscidce), the exercise of this faculty is of such everyday occurrence, that, wonderful as it is, it scarcely attracts the attention of ordinary observers. The foot of the House-fly, nevertheless, is a very curious piece of mechanism; for, in addition to the recurved hooks possessed by other climbing species, it is furnished with a pair of minute membranous flaps (fig. 150, c), which, under a good microscope, are seen to be covered with innumerable hairs of the utmost delicacy: these flaps, or suckers, as they might be termed, adhere to any plane surface with sufficient tenacity to support the whole weight of the fly, and thus confer upon it a power of progression denied to insects of ordinary construction.

Feet of insects:   A, F, Dytiscus. B, Bibio febrilis. c, Musca damestica. D, Cimbex lutea.

Fig. 150. Feet of insects: - A, F, Dytiscus. B, Bibio febrilis. c, Musca damestica. D, Cimbex lutea.

E, Abyssinian Grasshopper.

(783). In Bibio febrilis (fig. 150, b), the sucking disks appended to the foot are three in number, but in other respects their conformation is the same.