1687. TIllJ THORAX OF IXSECT^—^XODaiiAtiti. 529 



a[)poar to belong to the abdomen. Yet in tlie nietalhorax there is 

 present the true metathoracic notum {N^), and the pseiidonotum 

 (P.V3) identifiable by its fusion with the metathoracic pleura. Hence 

 the laro:e corrujj^ated plate (IT) behind the pseudonotum has no place 

 in the metathoracic anatomy, and its abdominal origin is proved by its 

 spiracles (/ Sp). Thus a study of comparative anatomy proves con- 

 clusively that the " segment mediaire " is at least the first abdominal 

 tergum which has been transferred to the thorax, and Pepsis indi- 

 cates that the entire first abdominal segment is so transferred. If 

 this is true, then the ventral part disappears as a distinct plate in 

 the higher families. 



In the Lepidoptera the mesonotum has much the same appearance 

 as in the Diptera and Hymenoptera, but differs in details of structure. 

 In the Cossidije (149, 150) it consists of a large scutum (set) and 

 scutellum (scl) separated along the line of the ventral V-ridge, and 

 of a very narroAV prescutum (150, psc). The postscutellum (Z'.V) 

 is present, but normally (140) is almost hidden between the mesothorax 

 and the metathorax. In the Sphingidae (155, 156) the prescutum 

 depends vertically from the anterior edge of the scutum (155) and 

 carries the prephragma (Aph). The prescutum in Pharssu,s is, there- 

 fore, much more nearly the equivalent of the anterior division of the 

 notum in the diagram (fig. 1) than in either the Hymenoptera or 

 the Diptera and, hence, is more similar to the Orthoptera (38) and 

 Xeuroptera (142). In the Diptera, it will be recalled, the prescutum 

 is large and extends back to the bases of the Avings. In the Hy- 

 menoptera it is remote from the wing bases. In Fhassus (150) the 

 scutum carries both the anterior and the posterior-wing processes of 

 the notum, while in Frotoparce (156) the posterior processes arise 

 from the scutellum. This is due to the fact that here the lateral 

 parts of the scutellum are not defined by the ventral V-ridge, but 

 appear simply as depressed areas at the sides of the median elevated 

 part of the scutellum as in Ilolorusia (175) and Tabanus (180). In 

 this case the separation between the scutum and scutellum laterally 

 is simply a matter of topography. 



The metathorax of Fhassus (149, 151) is larger and more like the 

 mesothorax than is usual among the higher insects. The prescutum 

 {psc) and the scutellum {scl) almost meet on the median line, thus 

 separating the scutum into tAvo lateral plates {set). The posterior 

 wing processes {PNP) arise in the angles between the scutellum and 

 the scutum. 



The scutellum in all the forms so far described carries the axillary 

 cords of the wings {AxC) at its extremities. These cords, which 

 are distinct in nearly all insects, are, hence, diagnostic of the location 

 of the scutellum in Lepidoptera, Hymenoptera, and Diptera, defining 

 its posterior margin, and consequently, the posterior edge of the 

 Proc.X.M.vol.xxxvi— (I!) ^54 



