INSECTA. 



919 



In Orthoptera the structure of the thorax is 

 similar to that of the Coleoptera, but it is un- 

 necessary to describe it more minutely at pre- 

 sent, a greater interest being attached to the 

 whole skeleton of those insects which undergo 

 metamorphoses, more particularly the Hymen- 

 optera and Lepidoptera, than to those in which 

 these interesting changes do not take place. 



The structure of the thorax in Hymenoptera 

 merits considerable attention, from the circum- 

 stance that it is scarcely yet decided whether it 

 be composed only of three distinct segments of 

 the larva, or whether a fourth one enters in part 

 into the composition of it. We have seen that 

 in the larva state in this order there are fourteen 

 distinct segments, besides an anal tubercle, and 

 that during the transformations the body is con- 

 stricted in the fifth segment, which seems to 

 form the connexion between the thorax and 

 abdomen. According to the usually received 

 opinions, the true thorax is always composed of 

 but three segments, but M. Audouin believes 

 that this is not strictly the case in Hymenop- 

 tera, and has endeavoured to shew that in this 

 orderthe posterior portion of the thoracic region 

 is part of a segment that belongs to the abdo- 

 men. Mr. Macleay, on the contrary, contends* 

 that this is not an additional segment, but is in 

 reality part of the fourth or meta-thoracic seg- 

 ment of the larva. In this opinion he is sup- 

 ported by Burmeister and Westwood, while the 

 views of Audouin are advocated by Latreille 

 and Kirby and Spence. 



The pro-thorax, which is a large segment in 

 the larva state, is greatly reduced in size in 

 the perfect insect, owing to the operation of 

 causes which take place during the metamor- 

 phoses. But it is not so much reduced as in 

 the Lepidoptera and Diptera. The boundaries 

 of this segment in Hymenoptera, like those of 

 the meta-thorax, are a subject of dispute among 

 naturalists, owing to the segment in the perfect 

 state being divided into two distinct parts, the 

 first of which is articulated with the head, and 

 freely moveable upon the other, which is at- 

 tached firmly to the meso-thorax. The piece 

 articulated with the head is believed by Kirby 

 and Spence to represent the entire pro-thorax, 

 or second segment of the larva. It bears the 

 first pair of legs, and in the winged species is 

 readily detached from the other, which is the 

 collare of those authors, who, on account of its 

 being attached to the great meso-thorax, believe 

 it forms a part of that segment. This, as Mr. 

 Macleay has shewn, is not the fact, as is proved 

 by the circumstance that in the Ants and other 

 walking Hymenoptera it is readily removed 

 from the meso-thorax, and is united to the 

 anterior piece, which bears the first pair of legs ; 

 while he suggests that the reason for its being 

 attached to the meso-thorax in the flying spe- 

 cies, is to give strength to that segment, and 

 support the wings.f We have convinced our- 

 selves of the correctness of this view of the 

 subject by an examination of the parts in Ich- 

 neumon atropos, (Jig. 389, 390,) in which the 



* Zoological Journal, vol. i. p. 145. et seq. 

 t Op. cit. p. 168. 



Fig. 389. 



Ichneumon Atropoe. 



2, pronotum j 3 b, scutum of meso-thorax ; 3 c, 

 scutellum ; 3 e, parapteron j *, wing ; 4 b, scutum 

 of meta-thorax ; 4 d, freuum j 4 e, scutellum j * % 

 spiracle j /, coxa. 



two pieces are freely separable. The pra-scutum 

 and scutum of the pro-notum are exceedingly 

 short and evanescent, as described by Mr. 

 Macleay m Polistes, the prae-scutum being 

 merely a ligamentous membrane that unites 

 this segment to the head. The scutum is a 

 short plate that forms the upper surface of the 

 anterior portion of the segment, the sides being 

 formed, as we shall see, by the epimera and 

 episterna (2 g). The posterior piece, the scutel- 

 lum, is of considerable size laterally (2 A), but 

 it is short on the upper surface (2), and is deeply 

 notched to fit it to the anterior part of the meso- 

 notum, its two sides being produced into a some- 

 what triangular shape, and wedged in between 

 the scutum of the meso-thorax and the epister- 

 num on each side. The post-scuteltum exists 

 only as a rudimentary membrane, which assists 

 to mark the proper boundary of the pro- thorax, 

 this being, as Mr. Macleay has observed, one 

 of the proofs that the scutellum now described 

 does not belong to the meso-thorax, while the 

 non-existence of a similar membrane, or 

 phragma, between the two portions of the pro- 

 thorax itself, affords an additional reason for 

 considering these but as parts only of one 

 segment. 



The mcso-notum is the most largely deve- 

 loped portion of the thorax in this order, as in 

 Diptera and Lepidoptera. It is a convex elon- 

 gated plate that covers nearly the whole of the 

 dorsal surface of the thorax. The prae-scutum 

 is a vertical piece, developed inwards to assist 

 in the formation of the pro-phragma that di- 

 vides the collar from the scutum. The scutum 

 (3 6) is broad, convex, and lozenge-shaped. 

 At its sides are developed the anterior pair of 

 wings, and at its base, which is slightly trun- 



