172 Mr. W. S. MacLeay on the Anatomy of the 



developed, and preserves much the same form that it has in many Cokop- 

 tera. In Polistes the form somewhat resembles two quadrants, the radii 

 of which are joined together at their respective curves. The external 

 margin of this piece may possibly be what Mr. Kirby cdXhihe postfrcenum 

 of Hymenoptera,* and its internal developement may be perhaps his 

 mesophragma ; although, to judge from his mesophragma as it exists in 

 Hymenoptera, he does not seem to have viewed it internally, where he 

 would have found the scutum to be a very essential piece. 



3. The scutellum of the metathorax comes next after the scutum, to 

 the anterior margin of which it is joined, so as to present the external ap- 

 pearance of immediately following the praescutum, while the scutum takes 

 its vertical direction as a septum. In Hymenoptera this in general is ob- 

 liquely striated, and a very large and conspicuous piece.f As in Coleop- 

 tera, it often consists of two large convex pannels, J joined together by a 

 channel, which however in this order is more or less evanescent. This 

 channel of connexion Mr. Kirby does not notice in his figures of Hymen- 

 optera, although, according to his nomenclature, and taking a Coleopte- 

 rous insect for type, it ought to be his postscutellum. His postscutellum 

 in Hymenoptera, however, is little more than the central posterior point 

 of the praescutum of the metathorax, that is, o{ his postdor solum. ^ 



4. The postscutellum of the metathorax in our insect is elevated, sub- 

 triangular with the corners rounded off, and having in the middle an ele- 

 vation of a horse-shoe form, in which are three apertures, the central one 

 being a longitudinal slit, called by Kirby the trochlea,\\ and through 



• What this author calls the postfraenum in Coleoptera belongs to an entirely 

 different piece, namely, the scutellum of the metathorax. 



+ In Polistes the metathoracic stigmata (tt) are situated at the anterior ex- 

 ternal angles of this piece, -which is represented fig. 5, K. 



X The two pannels of this piece in Coleoptera are called by Mr. Kirby post- 

 fraena, and its connecting channel in the same order is his postscutellum. 



§ See Int. to Ent., Vol. III., p. 572. 



II I have adopted this nomenclature, although my readers must feel that this 

 story of the pulley depends more on Mr. Kirby's imagination than on any thing 

 in nature. Mr, Kirby seems to think that he is the first who has noticed this 

 curious structure of the metathorax of a wasp. If he refers, however, to M. 

 Chabrier's excellent Memoir, Mem. du Mvs., Vol. III., />. 53, he will find the 



