12 BULLETIN 85, UNITED STATES NATIONAL MUSEUM. 



The metapleurse have been very much changed from their original 

 simple condition and show complications which are difficult to 

 understand without a more generalized form, as Apsylla, to compare 

 with. In the more highly speciahzed species (which include nearly 

 all) the metacoxse have become enormously enlarged and have 

 pushed far up into the pleural region nearly up to the tergum and 

 have displaced the pleurites from their normal position. A careful 

 examination of Apsylla (figs. 2, 9, 12) first will aid greatly m inter- 

 preting the sclerites and sutures in the other species. 



The pleural suture is distinct in Apsylla, extending from the wing 

 process to the articulatory condyle of the metacoxa at the upper- 

 most point of the latter. The metacoxa is not much larger than 

 the mesocoxa and does not extend very far upward. The episternum 

 is much larger than the epimeron and bears the metathoracic spu'acle 

 near its anterior margin. This is not a distinct sclerite, peritreme, 

 as it is in the other species, but the chitin between the spiracle and 

 the part of the episterum posterior to the spiracle is thinner. The 

 episternum articulates on its lower margin with the small metasternum, 

 which is a small, narrow sclerite between the meta- and mesocoxae. 

 A rather large trochantin articulates with the rest of the lower mar- 

 gin of the episternum and with the sternum. 



The epimerom is broad above, articulating with the postscutellum 

 on its posterior corner. A thickened ridge passes down its posterior 

 edge to the articulatory condyle of the coxa, and at this point the 

 epimeron, still thickened, is flexed backward and passes down 

 behind the coxa. Here it turns inward and unites with the corre- 

 sponding prolongation of the epimeron of the other side, and together 

 they fuse with the end of the metasterum, which is described below. 



The metacoxa is more or less cordate and not very much larger 

 than the mesocoxa. By reflected light it appears to be uniformly 

 thick throughout, but by transmitted light, when the specimen has 

 been well cleared, a distinct thickening along the posterior edge is 

 easily visible. Tliis tliickened ridge becomes broader below. From 

 near the coxal cavity, caudad, there projects a thick spiniform 

 process, the meracantlius or coxal spur. This appears to be articulated 

 and is not a true process. 



In the more highly specialized species the metacoxa, as already 

 stated, has become very enormously enlarged and has pushed far 

 up into the pleura. A comparison of Apsylla (fig. 2) and Psylla alni 

 americana (fig. 4) shows very plainly that this pushing upward of 

 the coxa has been between the pleural suture and the posterior 

 thickened ridge of the epimeron. As a consequence, the pleural 

 suture in Psylla does not extend to the upper end of the coxa, but 

 terminates in front of and below that point. The lower extremity 

 of the episterum articulates with the sternum, which is even smaller 



