│Sternum │ │Postfurca │ │Metaphragma │ │Apodemes │ ───────────────────────┴───────────────────────┴─────────────────────── [Illustration: FIG. 102.—External anatomy of the trunk of _Hydröus piceus_: _A_, sternal—_B_, tergal aspect; 2, pronotum; 2 _a_, prosternum; 2 _f_, episternum; 3 _a_, præscutum; 3 _b_, scutum; 3 _c_, scutellum; 3 _d_, postscutellum; 3 _g_, mesosternum; 3 _h_, episternum; 3 _f_, epimerum; 3 _i_, crest of the mesosternum; 3 _a_, parapteron; 3 _k_, coxa; 4 _a_, metapræscutum; 4 _b_, metascutum; 4 _c_, metascutellum; 4 _d_, postscutellum; 4 _e_, tegula; 4 _f_, episternum; 4 _h_, epimerum; 4 _g_, metasternum; 4 _i_, crest of metasternum; 4 _k_ and _l_, coxa; 4 _m_, trochanter; _n_, femur; _o_, tibia; _p_, tarsus; _q_, unguis; 7–11, abdominal segments.—After Newport. ] _b._ The legs: their structure and functions The mode of insertion of the legs to the thorax is seen in Figs. 90, 97, 101, and 103. They are articulated to the episternum, epimerum, and sternum, taken together, and consist of five segments. The basal segment or joint is the _coxa_, situated between the episternum and trochanter. The coxa usually has a posterior subdivision or projection, the _trochantine_; sometimes, as in Mantispa (Fig. 103), the trochantine is obsolete. We had previously supposed that the trochantine was a separate joint, but now doubt whether it represents a distinct segment of the leg, and regard it as only a subdivision of the coxa. It is attached to the epimerum, and is best developed in Panorpidæ, Trichoptera, and Lepidoptera. In the Thysanura the trochantine is wanting, and in the cockroach it merely forms a subdivision of the coxa, its use being to support the latter. The second segment is the trochanter, a more or less short spherical joint on which the leg proper turns; in the parasitic groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two pieces, though there are some exceptions. The trochanter is succeeded by the _femur_, _tibia_, and _tarsus_, the latter consisting of from one to five segments, the normal number being five. Tuffen West believed that the pulvillus is the homologue of an additional tarsal joint, “a sixth tarsal joint.” The last tarsal segment ends in a pair of freely movable claws (ungues), which are modified setæ; between the claws is a cushion-like pad or adhesive lobe, called the _empodium_ or _pulvillus_ (Fig. 105, also variously called _arolium_, _palmula_, _plantula_, _onychium_, its appendage being called _paronychium_ and also _pseudonychium_). It is cleft or bilobate in many flies, but in Sargus trilobate. All these parts vary greatly in shape and relative size in insects of different groups, especially Trichoptera, Lepidoptera, Diptera, and Hymenoptera.