INSECTA. 



915 



surface, or meso-notum. It is the segment 

 that bears the elytra and middle pair of legs. 

 The first piece, the prtfscittitw (3 </), is in ge- 

 neral narrow and transverse ; it is very readily 

 overlooked, being in most cases bent down- 

 wards to form the mcso-pfiragma, the anterior 

 boundary of the segment. The second piece, 

 the scutum (3 />), is a much broader and very 

 distinct corneous plate, and may be regarded, 

 perhaps, as the most important division ot the 

 meso-notum, since it is to this that the ante- 

 rior pair of wings, the elytra, are articulated. 

 It is followed by the scutellum (3 c), which 

 also is a very important division. Like the 

 scutum it is a broad piece, that covers the pos- 

 terior part of the meso-notum, and extends 

 on each side to the base of the elytra, the 

 a(ut<e, which are continuous with them, being 

 attached to its margin. It is developed in the 

 middle line into a remarkable elevated plate, 

 that is shaped like an armorial shield, and 

 is so exceedingly large in some species, that 

 it covers nearly the whole of the body. In 

 Dyticta, and most of the Coleoptera, it is the 

 small triangular plate which is situated be- 

 tween the elytra, at their base, and is supposed 

 to be of use in keeping these organs steady 

 during flight. The fourth and last piece of the 

 meso-notum, the pvst-scutellitm (3 rf), like the 

 prae-scutum, is narrow and inconspicuous. It 

 is situated immediately behind the scutellum, 

 and is the posterior boundary of the meso- 

 notum. These parts together form the dorsal 

 surface of the first wing-bearing segment, 

 which is developed to as great an extent on its 

 under as on its upper surface. The meso- 

 notum is most fully developed in those insects 

 in which the anterior pair of wings are the 

 largest, as in the Lepidoptera, Hymenoptera, 

 and Diptera, while in those in which the chief 

 organs of flight are the posterior wings, as in 

 the Coleoptera, it is the smallest of the three 

 thoracic segments. The meso-sternurn (Jig. 385, 

 A), like the pro-sternum, is formed by a strong 

 middle piece, the proper sternum of the seg- 

 ment, (3 ,) which is developed laterally into 

 two processes, behind which the coxae of the 

 middle pair of legs are articulated, and anteri- 

 orly and laterally the episterna (3/) and epi- 

 mera (3 h). Each episternal piece is a broad 

 elongated plate, which forms the anterior part of 

 the meso-sternum. It is attached to the ante- 

 rior margin of the lateral sternal process, so 

 that its actual position is a little altered, the 

 corresponding part of the pro-thoracic segment 

 being situated behind the process of the sternal 

 piece. This is a circumstance which occa- 

 sionally takes place in the development of 

 every part of the skeleton, the actual position of 

 one part being altered by the greater or less de- 

 velopement of another, while the relative posi- 

 tion of each part always continues the same. 

 Thus, although the epislernum is situated more 

 anteriorly in the meso- than in the pro-sternal 

 surface, it still continues to be articulated with 

 the sternum. The epimeron (3 h} is situated 

 behind the episternum. It is a narrow elon- 

 gated plate, that forms the posterior portion 

 of the meso-thorax, and is united to the anterior 



of the meta-thorax. At its superior extremity 

 it is much broader than at its inferior, which is 

 articulated with the extremity of the sternal 

 process, and also with the coxae of the middle 

 legs. At the anterior border of the episternum 

 there is a very narrow but distinct plate, the 

 parapteron (3 e). This piece, which is con- 

 nected especially with the wings, undergoes a 

 great change of form and size in the different 

 orders. In the Coleoptera it is narrow and 

 evanescent, but as we shall hereafter see, is 

 largely developed in the Lepidoptera. It is 

 evidently a normal portion of the skeleton, but 

 has only been described by Audouin as found 

 in the meso- and meta-thoracic segments. We 

 have before alluded to the existence of two 

 detached pieces in the connecting membrane of 

 the pro-thorax and head, which we regard as 

 the analogues of these pieces of the meso- and 

 meta-thorax. If this be correct, the relative 

 position of these to the other parts of the pro- 

 sternum is precisely similar to that of the same 

 parts of the meso-sternum. The wedifurca, 

 (3 s,) or ento-thorax of this segment, is at- 

 tached to the internal surface of the sternal 

 piece, as in the pro-thorax. It is formed by 

 two ascending rami, which are larger and 

 longer than those of the pro-thorax, but like 

 them are developed into two expanded por- 

 tions, which are approximated together and 

 form an arch, under which the nervous cord 

 passes in its course to the meta-thorax. 



Fig. 386. 

 A 



Parts of the meta-thorax. (Audouin.) 

 A, meso-sternum ; 4 a, prae-scutum ; 4 b, scutum ; 

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

 4/, episternum ; 4g, meta-sternum ; 4 A, epimeron; 

 4 *, post-furca. 



The meta-thorax (fig. 386) is the fourth seg- 

 ment of the body, and the third of the thoracic 

 region. Its upper surface, or meta-notum, as 

 in the preceding segments, is divided into four 

 portions. The prtEscutum (4 a) is a narrow 

 transverse plate, which is bent down at its ante- 

 rior margin like the prsescutum of the meso- 

 notum, to form the meta-phragma, and is ex- 

 tended on each side as far as the paraptera, 

 bounding the insertion of the wings. In the 

 middle line it is extended backwards upon the 

 dorsal surface as far as the scutellum, thus di- 

 viding into two parts the second piece, the 

 scutum, (4 &,) \vhich, like the corresponding 

 part of the meso-notum, is connected with the 



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