861 



INSECT 



Spence), are both distinct, but not conspicuous parts 

 iti the Coleoptera, in which order the third piece, 

 (c 2), is very conspicuous *, being the triangular part 

 which is found at the base of the elytra, where they 

 unite together, and is ordinarily termed the scutel- 

 lum ; but more properly the scutellum of the meso- 

 thorax or mesoscutellum. Some beetles, indeed, are 

 said to be exscutellated, or destitute of scutellum ; 

 and so far as its occupying the ordinary exposed 

 situation is concerned, they are exscutellated ; but the 

 same part is to be found in a diminished form in all 

 beetles. 



In some few insects, as the scutellata(7V(?yns), the 

 scutellum is so much enlarged, that it completely 

 covers the abdomen. The postscutellum of the meso- 

 thorax, (d 2), or the frccnum of ,Kirby and Spence is 

 also distinct, but not very conspicuous, except by 

 dissection. 



In insects having the upper wings transformed into 

 horny or leathery wing-covers, the mesoscutellum is 

 very distinct and elevated, being evidently serviceable 

 in giving some kind of support to these wing covers. 

 In insects with membranaceous wings the mesoscu- 

 tellum is not so conspicuous an organ. 



The upper surface of the mesothorax is very va- 

 riable in its consistence, according to the degree of 

 exposure to which it is subject. Thus in the beetles, 

 where it is almost entirely covered by the prothoracic 

 shield, or by the base of the wing-covers, it is lea- 

 thery, with the exception of the exposed mesoscu- 

 tellum. If, on (he contrary, it is naked, as in the 

 Hymenoptcra, Diptera, &c., it is more solid in its con- 

 struction. In the Diptera the prcescutum and scutum 

 of the mesothorax are soldered together, see fig. 126, 

 (a 2, b 2), forming the larger part of the upper surface 

 of the thorax; the scutellum (c 2) is also distinct. In the 

 Lepidoptera the prsscutum is small, but the scutum 

 (dorsulum, Kirby and Spence) very large and dis- 

 tinct. In the Hemiptera, as in the Coleoptera, the 

 praescutum and scutum are covered by the pronotum. 

 In the Hymenoptcra the scutum and scutellum are 

 similar to those of the Diptera. 



On the under surface the mesosternum is generally 

 horny, and extends between the middle legs, some- 

 times as in some Coleoptera (Flatt-ricke), having an 

 impression to receive the spine of the prosternum ; 

 at other times, as in the Cetonia, the prosternum 

 itself is produced into a spine extending between the 

 anterior legs. 



On each side of mesosternum extend the epimera and 

 cpisterna of the mesothorax, the former being termed 

 scapularia by Kirby and Spence ; they are generally 

 of small size, except in a few instances, such, for in- 

 stance, as the Cetonud<E, in which the former are very 

 conspicuous, forming the horny plate, intervening O n 

 each side between the shoulder of the elytra and 

 the posterior angles of the pronotum. 



In addition to the above pieces, the paraptera, or 

 tegulse, being the small scales at the base of the upper 

 wings (see fig. 124, o o), which sometimes, as in 

 some of the Proctotrupida: (especially Galesus\ are of 

 a large size, must be noticed. They also acquire a 

 large size in the Lepidoptera. 





The mesothoracic appendages are the anterior 

 pair of wings, the middle pair of legs, and the meso- 

 thoracic spiracles. In dipterous insects the base of 

 the wings are often furnished with a membranous 

 scale, termed the alula or winglet, which Kirby and 

 Spence regard as the true analogue of the second 

 pair of wings, and consequently as attached to the 

 metathorax (see fig. 126 y). 



3. The Metathorax, T 3 and (T) 3 (Protrunk, Kirby 

 and Spence). 



This is the last of the three thoracic segments (or 

 the second of the thorax of Strauss, or Alitrurikof 

 Kirby and Spence), and bears the posterior pair of 

 wings, or their representatives, and the third pair of 

 legs. Its development and consistence are variable, 

 dependent upon the same principles, already stated to 

 regulate the development of the mesothorax. It con- 

 sists of the same number of pieces as the preceding 

 segment,the dorsal pieces (composing the metanotum) 

 being the praescutum (mesophragrna of Kirby and 

 Spence in Coleoptera ; postdorsulum of the same 

 authors in Hymenoptera), scutum (postdorsulum of 

 Kirby in Coleoptera), scutellum (postscutellum and 

 postfraenum of Kirby), and postscutellum (meta- 

 phragma, Kirby and Spence). The scutum and scu- 

 tellum are the most conspicuous parts in Coleoptera, 

 being channelled longitudinally. In Hymenoptera, see 

 fig. 124, T3, the mesothorax is reduced in size, and 

 this is more especially the case in Diptera, see fig. 

 126, T 3 ; but in the parasitic Strepsiptera. the fore- 

 wings are very minute and twisted appendages, and 

 the second pair of wings of very large size, the meta- 

 thorax being so greatly increased in its dimensions that 

 it may be almost said to constitute the entire thorax. 



The under surface of the mesothorax, or the meta- 

 sternum, is generally a horny covering, divisible, like 

 the metasternum, into various pieces ; of these the 

 central metasternum is generally the most enlarged 

 portion ; sometimes being produced into a point ex- 

 tending beneath the abdomen, as in Hydrous; some- 

 times it is flat, and occupies nearly the entire under 

 surface of the metathorax ; at other times, as in 

 Dyticus, the posterior coxae or basal parts of the hind 

 legs (which are of a large size), are soldered to it, 

 and hence the points of these parts of the hind legs 

 have been often termed the bifid mucroof the meta- 

 sternum. The sides of this metasternum in the Cole- 

 optera are laterally margined by a pair of longitudinal 

 pieces, which are the episterna, or the parapleura, 

 of Kirby and Spence. The construction of the various 

 parts of which the metasternum is composed would 

 require too much minute description to be available in 

 a work like the present ; we shall therefore only further 

 notice, that in winged insects having a pedunculated 

 abdomen, Messrs. Audouin and Latreille consider 

 that the piece which terminates the thorax behind is 

 not a portion of the metathorax, but, on the contrary, 

 is the basal segment of the abdomen. This opinion 

 appears to us to have been successfully refuted by 

 MacLeay and Burmeister. 



The appendages of the metathorax are the pos- 

 terior pair of wings, or their analogous organs, the 

 third pair of legs, and the metathoracic spiracle. 



It still remains to notice a curious apparatus, con- 

 sisting of a ligament, which passes through a slit at 

 the extremity of the postscutellum in the pedunculated 

 Hymenoptera, (see fig. 124, 125, xx,) in the middle of 

 which is an elevation of a horseshoe form, having two 

 apparent and one real central apertures. This ligament 



