INSECTA. 



917 



i ; 3 a, prsescutum ; 3 b, scutum ; 3 c, scutel- 

 lum; 3 d, post-scutellum ; meso-sternum ; 3 \ g, ster- 

 num ; 3 h, episternum ; 3/, epimeron; 3 , crest of 

 the meso-sternum ; 3 e, parapteron ; 3 k, tcochan- 

 tin ; 4, meta notum ; 4 a, praescutum ; 4 b, scu- 

 tum ; 4 c, scutellum ; 4 d, post-scutellum ; 4 e, 

 parapteron; meta-sternum ; 4 f, episternum; 4 g, 

 meta-sternum ; 4 h, epimeron ; 4 t, crest of meta- 

 sternum ; 4 A, trochantin (?) ; 4 i, coxa ; 4m, tro- 

 chanter ; 4 n, femur; o, tibia; p, tarsus; q, un- 

 guis. 



These segments constitute the proper thorax 

 of the insect, and the parts we have described 

 are found in nearly all the Coleoplera, the most 

 perfect species ; although, as before stated, they 

 are sometimes greatly modified in shape, and 

 varied in size and position, in order that the 

 body of the insect may be adapted to its pecu- 

 liar habits. Thus in the great water-beetle, 

 Hydrous piceus, (Jig. 387,) which in its general 

 appearance and mode of life very nearly resem- 

 bles the Dyticus, and not only burrows deeper 

 into the mud at the bottom of stagnant waters, 

 but is also accustomed to float among the 

 weeds on the surface to bask in the sun, the 

 form of the sternum is admirably adapted to 

 its habits. The sterna of the meso-thorax and 

 meta-thorax are not only both armed with a 

 strong keel like a boat, but the two are firmly 

 articulated together, which enables the insect 

 more securely to float on the surface of the 

 water, and thus afford additional strength to its 

 whole body for the accomplishment of its ob- 

 ject. But in the Dyticus, to which it is of 

 the utmost consequence to be able to swim 

 with the greatest rapidity, and turn with facility 

 in the water, in the pursuit of its living prey, 

 the pro-sternum and meso-sternum only are 

 slightly keeled, while the meta-sternum is 

 smooth, and the sides of the body are acute, 

 and offer the least possible resistance to its 

 movements. In addition to this, to afford suf- 

 ficient strength to the body, together with faci- 

 lity of motion, the sternum of the meta-thorax 

 is produced in front into a short spine, which is 

 inserted into a notch in the posterior part of 

 the meso-sternum ; while the coxae of the poste- 

 rior pair of legs upon which the chief efforts in 

 swimming depend, although enormously en- 

 larged to afford sufficient space for the inser- 

 tion of the muscles, are flat and smooth like the 

 rest of the under surface of the body, in order 

 that they may not oppose the slightest impedi- 

 ment to the motions of the insect. The different 

 forms of the coxae (7) and of the acetabula (4 A:), 

 into which they are inserted, have also a refe- 

 rence to the habits of the species. The large 

 posterior coxae of the Dyticus are immoveably 

 united by suture to the posterior margin of the 

 meta-sternum, because, in this insect, the pos- 

 terior pair of legs being especially designed for 

 swimming, and their motions consequently 

 being almost wholly in one direction, addi- 

 tional strength is afforded to these organs by the 

 immobility of the coxae. In the Hydrous, in 

 which all the legs are employed in walking, as 

 well as in swimming, the coxae are freely 

 articulated in their respective acetabula, 

 and each one is supported in part by the tro- 



chantin (?), (ft), which is more developed than 

 in the other insect. 



The strength of the body depends much upon 

 the size of the thoracic segments, and the firm- 

 ness of union which exists between them. Thus 

 in those species which are more especially em- 

 ployed in walking, in flying, or in swimming, 

 the meso- and meta-thoracic segments are the 

 largest. If the insect be aquatic, the largest 

 parts, as we have seen, are the sternal surface of 

 the meta-thorax, and its coxae ; but if, on the 

 contrary, the habits of the insect be aerial, then 

 the dorsal surface of the segment is larger than 

 the sternal. In those insects which are mostly 

 employed on the ground in running or walking, 

 as the Carabidte, Geotrupida, Copridz, and 

 Lucanidtf, the meso- and meta-thoracic seg- 

 ments are often anchylosed together, to give 

 greater strength to the whole body. This is 

 particularly the case in Lucanus cervus (fig. 

 388), in which the small sternum of the meso- 

 thorax (3 g) is firmly anchyclosed to the enor- 

 mously enlarged sternum of the meta-thorax. 

 The reason for this is not merely to afford 

 greater stability to the meta-thorax and its 

 wings, upon which entirely devolves the labour 

 of supporting this unwieldy insect during 

 flight, but also to give greater strength to the 

 whole body, during the efforts of the insect to 

 strip off the bark from the smaller roots and 

 branches of trees, to obtain a flow of the juices 

 upon which it subsists. That such is the reason 

 for this anchyclosed condition of its segments 

 is evident from the circumstance, that it occurs 

 not only in those insects which require great 

 muscular power during flight, but also in those 

 which are much accustomed to laborious efforts 

 in tearing, in burrowing, or in running. In 

 these, also, the acetabula (2 r, 3 r), are exceed- 

 ingly deep, and almost entirely enclose the 

 coxae within them, so that while the limb can 

 be rotated freely in almost every direction, a 

 dislocation of it is utterly impossible. The ace- 

 tabula are situated on each side of the poste- 

 rior part of the sternum, in each of the three 

 thoracic segments, and in general are formed 

 by an approximation of the sternum and epi- 

 meron, and sometimes, also, of the epister- 

 num, as in the Dyticm (fig. 384, A). When, 

 as in this instance, the episternum enters 

 largely into the formation of the acetabulum, 

 the epimeron is carried backwards, and forms 

 the postero-lateral boundary, the episternum 

 the antero-laieral, and the sternum the anterior 

 boundary, so that the acetabulum is formed by 

 the junction of three articulating sutures, and 

 completely surrounds the coxa. This consoli- 

 dation of parts gives an amazing increase of 

 strength to the segment in which it occurs, and 

 is one of the circumstances which enables the 

 insect to exert a degree of muscular power 

 which is sometimes truly astonishing. It oc- 

 curs in general in the pro-thoracic segment, as 

 in Lucanus, (388, 2,) Geotrupes, Ateuchus, and 

 other Lamellicornes. A similar condition of the 

 acetabula of the meso-thorax exists also in the 

 same insect (3 r). But instead of the posterior 

 wall of the cavity being formed by the epi- 



