NO. 1687. THE THORAX OF INSECTIS—H\'ODaRA^8. 543 



verse divisions of the chitinous ventral parts and designated tliem 

 the presternum^ sternum .^ and steriieUum. These three parts are 

 shown in (lie mesothorax of a cockroach (-52, Ps, tS, and SI). The 

 presternnni, however, is more commonly present as two small plates 

 lying near the anterior angles of the sternnm, the " Vori^latten " of 

 (lerman entomologists. Snch plates are present in the prothorax of 

 the Odonata, in a fcAv species of which they unite across the median 

 line in front of the sternnm (11, /*s), but more usually form two 

 lateral plates separated from the median presternal jiart (G, 7, 10, 

 Ps). In most cases they are, furthermore, fused Avith the episterna 

 (11, 12, 13, Ps and Eps). The presternal plates are shown also in 

 the Locustida? (43), the Gryllid* (40, 47), the Perlidte (80), the 

 Corrodentia (82), and in the Forficulida3 (91, 94, 98). 



From the inner sternal surface there projects dorsally the ento- 

 sternum., consisting most commonly of two chitinous arms, the furca 

 (text fig. C, Fe). In some cases the base of the entosternum appears 

 to mark the line between the sternum and sternellum. It is shoAvn to 

 be an invagination by the pit or pair of pits which often marks its 

 location externally (10, 11, e). Sometimes the sternum bears also a 

 long median posterior apodemal arm. This is shown in the prothorax 

 of a moth (152, I). 



Y. THE WING ARTICULATION. 



The wings are articulated to the body b}^ a simple arrangement of 

 axillary sclerites, two of which hinge upon the anterior and pos- 

 terior wing processes of the notum while one rests upon the wing 

 process of the pleurum. This is true of all the orders except the 

 Ephemerida and the Odonata. These will, therefore, be omitted 

 from the present discussion and described later under the special 

 descriptions of the orders. 



Text figs. 1 and 5 sufficiently represent the axillary selerites {1 Ax, 

 2 Ax, 3 Ax, Jf Ax) in their relations to one another, to the notal wing 

 processes, and to the bases of the veins, while fig. 6 shows the articu- 

 lation with the pleurum. The fourth axillar}^ is usually absent, but 

 since it occurs in the Orthoptera and the Ilymenoptera and since, 

 when it is present, the arrangement is more symmetrical, it is in- 

 cluded in the diagrams. When it is absent the third axillary ar- 

 ticulates directly with the posterior notal wing process. Several 

 other less definite j^lates usually occur in the central part of the 

 wing base associated with the median, cubital, and first anal veins. 

 These plates, however, are too variable to be given distinctive names 

 and will be referred to in general as the median plates. 



The membrane of the wing ■ base may be named the axillary 

 memhrane {AxM). On its anterior margin opposite the base of 

 the costal vein is a small, hairy, semichitinous pad (Tg). This, in 



