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



4. The postscutellum of the mesothorax is a very remarkable 

 piece in Hymenoptera, from being in general well developed in point of 

 size. It is an internal and concealed piece, running under the tergum of 

 the metathorax and parallel to the medipectus. It is, I believe, an essen- 

 tial character of this order that the postscutellum shall be separated from 

 the scutellum except by two lateral processes. In Polistes the former 

 piece is of a triangular, concave shape, the base of the triangle facing the 

 scutellum and being connected with it at the angles. * Mr. Kirby does 

 not appear to have noticed this important piece, as his frana in Hymen- 

 optera appear to be the paraptera, so that the frsena, according to him, 

 in Coleoptera (where it is the true postscutellum,) and his fraena in Ht/- 

 menopfera, are totally distinct pieces ! Our author has been led into 

 this singular mistake apparently by never having dissected the thorax, 

 and indeed it is rather a delicate operation to separate the metathorax 

 from the mesothorax. The best mode is to make one transverse incision 

 behind the scutellum and another slanting upwards under the middle pair 

 of feet to meet the former, but so as not to communicate with it about 

 the wings. By then breaking off the two pieces we shall have the meso- 

 thorax and metathorax properly separated, that is, the upper wings with 

 the mesothorax and the under with the metathorax. 



Audouin, and charges him with confounding the scutum cfthe mesothorax with 

 the scutellum, but to what work of M. Audouin he refers I am not aware. I 

 think there must be some mistake, as the whole theory, as well as observations, 

 of M. Audouin, go to separate them. M, Chabrier, Mem. du Mus., Vol. VIII. 

 p. 61, says of this piece, " Ses bras semblent tendre sans cesse a s'^chapper 

 " en glissant des pieces entre lesquelles ils sont situes; et I'extremite de 

 " chaque bras est pourvue de languettes internes qui sont tout a fait couvertes 

 " par les integumens." 



* It is of the same shape in Xylocopa, and has the same kind of insertion. 

 See Chabrier in Mem. du Mus. d'Hist. Nat. Vol. VIII. tab. 4, fig. 9, where this 

 piece is admirably figured, and called the costal. M. Chabrier is aware that 

 it belongs to the vertebral axis, for he says, " Je crois que ces pieces superi- 

 " cures du tronc y compris le costal peuvent etre considerees comme des ver- 

 " tebres." The manner in which this piece articulates with the arms of the 

 scutellum of the mesothorax, and with the vectiform bone of the wing ought 

 to be studied in the Memoires of MM. Juriiie and Chabrier. For a figure of the 

 piece inP()?i.«/es, seefig. 4, ©. See also Bennett in Zool. Journal, Vol. I. p. 397. 



