54 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39, 



define ''pseudonotum" as a general term, the name '^postnotum" is 

 used as such in the present paper, while "postscutellum" is used as an 

 alternative in the higher orders where the scutum and scutellum are 

 well differentiated. 



The simplest thoracic terga occur amongst nymphal and larval 

 forms. The nymph of a stonefly (fig. 11) has each segment protected 

 above by an undivided notal plate, those of the mesothorax and 

 metathorax carrying the rudiments of the wings. Between these 

 dorsal plates are wide white membranous areas, which, as shown by 

 sections (fig. 14 Mb^, Mh^, Mh^), belong to the posterior parts of the 

 segments hecause they lie in front of the intersegmental constrictions. 

 The dorsum of each thoracic segment of this nymph consists, there- 

 fore, of a chitinous notum (iV^) and of a non- 

 chitinous postnotal membrane (Mb). 



Amongst winged adults the simplest terga 

 are probably to be found in some of the smaller 

 cockroaches. A good example is afforded by 

 the mesothorax of Blatella germanica in the 

 dorsum of which there is but one plate present, 

 and this one is unquestionably the true notum 

 (fig. 2), since it carries the wings and has the 

 axillary cords {AxC) arising from the outer 

 ends of its posterior reduplication {Rd). On 

 the sides are the two wing processes {A NP and 

 PNP) separated by a deep emargination. On 

 its anterior part is a thin flap (a) which is at- 

 tached to the pronotum, being reflected upon 

 the posterior overlapping part of the latter 

 from the anterior phragma. The surface is 

 gently convex and there are no divisions into 

 subsclerites corresponding with those of the 

 higher orders, though there are several lines on the surface due to the 

 internal ridges shown in figure 3. There is no postnotal plate pres- 

 ent. The notum of the metathorax is almost identical with that 

 of the mesothorax, and, if there is a postnotum present, it is fused 

 with the first abdominal segment. In the Isoptera, likewise the 

 terga of the wing-bearing segments consist each of a single notal 

 plate, which, however, is often greatly constricted in the middle by 

 the deep lateral emarginations. 



In almost all other adult winged insects the tergum consists of two 

 plates in those segments that have the wings well developed as organs 

 of flight. The anterior plate is the true notum, being identical with 

 the entire nymphal tergum, since it alone carries the wings. The 

 posterior plate is the yostnotum and is not represented in the nymphal 

 tergum. The Ephemerida, Odonata, Plecoptera, Neuroptera, and 



Fig. 11.— Nymph of a stonefly, 

 doesal view, showing wide 

 postnotal membranous 

 aeeas, which, in the meso- 

 thorax and metathorax of 

 the adult, are occupied by 

 the postnotal plates. 



