NO. 1310. NORTH AMERICAN TIIYSANOPTERA—lIINm. 97 



usually much elongated and narrow in Terebrantia, while in Tubulifera 

 it is more rounded. In front of the spiracle a narrow plate extends 

 up over the shoulder and meets the mesoscutum. This plate in some 

 cases is oul_y an upturned portion of the broad mesosternum, but in 

 others is distinctly separated therefrom. This plate may be called the 

 episternum, either separate or fused with the mesosternum. Behind 

 the spiracle and below the attachment of the fore wings, there are one 

 or two quite broad skeletal pieces which are rather triangular in shape. 

 The mesosternum usually covers the whole ventral surface of the seg- 

 ment and its edges bend upward at the sides (e. g., Ileliot/irtps, see 

 Plate XI, tig. 119). In some species, however, it is an hexagonal plate 

 similar to the mesoscutum and but little larger, while the episternal 

 and epimcral plates are elongated and meet the sternum upon the 

 ventral surface. Upon the median line of the sternum there is in all 

 species, though very weak in the wingless ones, a quite deep chitinous 

 invagination more or less forked and serving for the attachment of 

 strong muscles (Plate XI, figs. 117, 119, 127). These endothoracic 

 structures are plainly visible in most species. The middle legs are 

 inserted far apart at the very hind angles of the mesosternum. 



Metatkorax.— This segment is usually slightly narrower than the 

 preceding and generally tapers slightly to the base of the abdomen. 

 Its dorsal plates are two, usually distinctly separated: a scutum and a 

 scutellum. On each side of these a membranous strip continuing that 

 from the mesothorax, extends backward to the base of the abdomen. 

 The hind wings are attached quite close to the fore wings and in a sim- 

 ilar manner. Near the bases of the hind wings lies in Tubulifera a 

 very distinct rounded or oval spiracle. This spiracle is present and 

 visible in many (Uzel says " all ") Terebrantia, but I have been unable 

 to find it in some species; in others it is extremely small and appar- 

 ently functionless, while in still others it can be distinctly seen. The 

 metasternum is broad and its edges curve upward around the sides of 

 the body. At the front edge of this side lies a narrow triangular 

 plate, the meta-episternum, while the meta-epimeron is here a narrow 

 elongated plate lying above and close to the upturned edge of the 

 sternum. The metasternum bears also a prominent endothoracic 

 structure in the middle and the edges of the plates are often bent 

 inward and thickened. The attachment of the abdomen is so oblique 

 that the hind (^oxt\i lie beneath the first abdominal segment. The hind 

 coxje are well separated and the steriuun usually projects back between 

 them as a distinct lobe or conical protuberance. 



Variation in the structure of the pte^'othorax in wingless species. — The 

 pterothorax is similar in ])oth short and long winged individuals and 

 we ma}^ expect to find at times long winged specimens of usuall}^ short 

 winged species. In species which are entirely wingless, however, or 

 in those one sex of which is always wingless, a marked variation in 

 Proc. N. M. vol. xxvi— 02 7 



