To complete our account of the structure of the thorax it is necessary to mention certain hard parts projecting into its interior, but of which there is usually little or no trace externally. A large process in many Insects projects upwards from the sternum in a forked manner. It was called by Audouin the entothorax; some modern authors prefer the term apophysis. Longitudinal partitions of very large size, descending from the dorsum into the interior, also exist; these are called phragmas, and are of great importance in some Insects with perfect flight, such as Hymenoptera, Lepidoptera, and Diptera. There is no phragma in connection with the pronotum, but behind this part there may be three. A phragma has the appearance of being a fold of the dorsum; it serves as an attachment for muscles, and may probably be of service in other ways. More insignificant projections into the interior are the little pieces called apodemes (Fig. 57, e); these are placed at the sides of the thorax near the wings. The apophyses are no doubt useful in preserving the delicate vital organs from shocks, or from derangement by the muscular movements and the changes of position of the body.
Fig. 57.—Transverse section of skeleton of metathorax of Goliathus druryi, seen from behind: a, metanotum; b, metasternum; c, phragma; d, entothorax (apophysis or furca); e, apodeme; f, tendon of articulation. (After Kolbe.)
The appendages of the thorax are (a) inferior, the legs; (b) superior, the wings. The legs are always six in number, and are usually present even in larvae, though there exist many apodal larvae, especially in Diptera. The three pairs of legs form one of the most constant of the characters of Insects. They are jointed appendages and consist of foot, otherwise tarsus; tibia, femur, trochanter, and coxa; another piece, called trochantin more or less distinctly separated from the coxa, exists in many Insects. The legs are prolongations of the body sac, and are in closer relation with the epimera and with the episterna than with other parts of the crust, though they have a close relation with the sternum. If we look at the body and leg of a neuropterous Insect (Fig. 58) we see that the basal part of the leg—the coxa—is apparently a continuation of one of the two pleural pieces or of both; in the latter case one of the prolonged pieces forms the coxa proper, and the tip of the other forms a supporting piece, which may possibly be the homologue of the trochantin of some Insects. In some Orthoptera, especially in Blattidae, and in Termitidae, there is a transverse chitinised fold interposed between the sternum and the coxa, and this has the appearance of being the same piece as the trochantin of the anterior legs of Coleoptera.
Fig. 58.—Hind leg of Panorpa: a, episternum; a′, epimeron; b, coxa; b′, coxal fold of epimeron; c, trochanter; d, femur; e, tibia; f, tarsus.
Beyond the coxa comes the trochanter; this in many Hymenoptera is a double piece, though in other Insects it is single; usually it is the most insignificant part of the leg. The femur is, on the whole, the least variable part of the leg; the tibia, which follows it, being frequently highly modified for industrial or other purposes. The joint between the femur and the tibia is usually bent, and is therefore the most conspicuous one in the leg; it is called the knee. The other joints have not corresponding names, though that between the tibia and the tarsus is of great importance. The spines at the tip of the tibia, projecting beyond it, are called spurs, or calcares. The tarsus or foot is extremely variable; it is very rarely absent, but may consist of only one piece—joint, as it is frequently called[[23]]—or of any larger number up to five, which may be considered the characteristic number in the higher Insect forms. The terminal joint of the tarsus bears normally a pair of claws; between the claws there is frequently a lobe or process, according to circumstances very varied in different Insects, called empodium, arolium, palmula, plantula, pseudonychium, or pulvillus. This latter name should only be used in those cases in which the sole of the foot is covered with a dense pubescence. The form of the individual tarsal joints and the armature or vestiture of the lower surface are highly variable. The most remarkable tarsus is that found on the front foot of the male Dytiscus.
It has been suggested that the claws and the terminal appendage of the tarsus ought to be counted as forming a distinct joint; hence some authors state that the higher Insects have six joints to the feet. These parts, however, are never counted as separate joints by systematic entomologists, and it has recently been stated that they are not such originally.
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; 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.