10 BULLETIN 85, UNITED STATES NATIONAL MUSEUM. 



epimeron of the prothorax, having overlooked or disregarded the 

 pleural suture in what he calls the episternum (really both pleurites). 



The third of the sclerites is not so easily accounted for. Judging 

 from its position behind the peritreme it can hardly be prothoracic. 

 It is quite evident that it is an accessory sclerite, but its homology 

 in other insects is not certain at present. Stough does not show this 

 in his figures, although it is present in all the species that I examined, 

 including Pachypsylla, with which he worked. 



Mesothorax. — The mesothorax is the largest part of the thorax, 

 being both long and broad and well adapted for muscle attachment. 

 The prsescutum (often called the dorsulum) is always as long as the 

 pronotum and very often much longer. The anterior edge is some- 

 what flexed upward to hook into the posterior margin of the prono- 

 tum. The lateral margin is not so low as that of the pronotum, 

 but a long, nan-ow process extends from this margin down and a 

 little backward to the mesepisternum, articulating with the latter 

 somewhat below the wing base. In many species, notably Apsylla 

 cistellala (fig. 2), there seems to be a suture dividing this process 

 at its base from the praescutum. Whether this is a true suture or 

 only an internal ridge is difficult to state. It seems more reasonable 

 that it is only a ridge and that the part in question is a real process 

 and not a distinct sclerite. 



The scutum, sometimes spoken of as the mesonotum, is a large and 

 heavy sclerite. It is separated from the praescutum and scuteUum 

 by prominent furrows externally and ridges internally. There is a 

 strong articulation between this and the episternum near the wing 

 base and epimeron behind the wing base. 



Between the lateral process of the praescutum and the anterior 

 margin of the scutum and the wing base are two tuberculous sclerites, 

 the anterior one being often large and very conspicuous. These are 

 evidently the paraptera, which in most other insects are simple 

 sclerites and not knob-hke or tuberculous. Behind the wing base 

 and near the end of the axillary cord is a third parapteron, but this 

 is not swollen as the others are. 



The scuteUum is very much smaller than the scutum and is hemi- 

 spherical in shape with a long slender axillary cord extending down 

 laterad from it to the wing base and forming part of the latter. 



The postscutellum, or pseudonotum of Verhoeff and Snodgrass,* 

 is a very short sclerite behind the scuteUum and extends down laterad 

 beneath the axillary cord in the basal portion and then behind it 

 from there on, articulating at its apex with the epimeron. 



The mesopleurites are very large and extensive, forming a large 

 part of the entire pleural region. The wing base is a large bifurcate 



'Snodgrass's The Thorax of Insects and the Articulation of the Wings(Proc. U. S. Nat. Mus., '36: 511-595, 

 1909), has been used continually in these interpretations of the thoracic sclerites. 



