Fig. 101.—Internal skeleton of Lucanus cervus, ♂, head: A, antenna; f, mandible; d, mentum; 2, 4, tendons of mandible; f, u, t, parts of the tentorium; 3 e, labial muscles. Thorax: 2, prothorax; 3, 4, meso- and metathorax fused solidly together; 3 r, acetabulum of prothorax, into which the coxa is inserted; 2 s, sternum; 3t, acetabulum of mesothorax, 4r, of metathorax; 3 s, mesothoracic sternum fused with that of the metathorax (4g); 4 s, apodeme.—After Newport.
The acetabula.—These are the cavities in which the legs are inserted. They are situated on each side of the posterior part of the sternum, in each of the thoracic segments. They are, in general, formed by an approximation of the sternum and epimerum, and sometimes, also, of the episternum, as in Dyticus (Fig. 97, A). This consolidation of parts, says Newport, gives an amazing increase of strength to the segments, and is one of the circumstances which enables the insect to exert an astonishing degree of muscular power.
| Tabular View of the Segments, Pieces, and Appendages of the Thorax | ||
| Name of Segment | Pieces (Sclerites) | Appendages |
|---|---|---|
| 1. Prothorax | Pronotum, sometimes differentiated into | |
| Scutum | 1st pair of legs | |
| Scutellum | Patagia | |
| Episternum | ||
| Epimerum | ||
| Sternum | ||
| Antefurca | ||
| 2. Mesothorax | Præscutum | |
| Scutum | 2d pair of legs | |
| Scutellum | 1st pair of wings | |
| Proscutellum | Tegulæ | |
| Episternum | Squamæ (Alulæ) | |
| Epimerum | Peritreme | |
| Sternum | ||
| Mesofurca | ||
| Mesophragma | ||
| Apodemes | ||
| 3. Metathorax | Præscutum | |
| Scutum | 3d pair of legs | |
| Scutellum | 2d pair of wings | |
| Postscutellum | (Halteres of Diptera) | |
| Episternum | ||
| Epimerum | ||
| Sternum | ||
| Postfurca | ||
| Metaphragma | ||
| Apodemes | ||
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. In certain flies (e.g. Leptogaster) the empodium is wanting (Kolbe). By some writers the middle lobe is called the empodium and the two others pulvilli.
Fig. 103.—Side view of meso- and metathorax of Mantispa brunnea, showing the upper and lower divisions of the epimerum (s. em′, s. em″, i. em′, i. em″); s. epis, i. epis″, the same of the episternum.
