NO. I INSECT THORAX — SNODGRASS 57 



(PIS). It is braced internally by the upper end of the pleural ridge. 

 The tergum does not always rest upon the pleura of its segment, but, 

 when it does, the supports consist of the anterior lateral processes 

 of the tergum that form the prealar bridges (Aiv) to the episterna, 

 and of the lateral extensions of the postnotum (PN) to the epimera 

 that constitute the postalar bridges (Pw). These props give effective 

 resistance to the downward pull of the tergo-sternal and the tergo- 

 coxal muscles. 



The epi sternum and the epimeron (fig. 20, Eps, Epm) undergo 

 numerous variations in form, and various subdivisions into secondary 

 sclerites in the mesothorax and metathorax of the different winged 

 orders, but their modifications are in general easy to follow, and are 

 so well understood that they need not be reviewed here. The precoxal 

 and postcoxal elements (Acx, Per) are usually well developed, 

 though the second is generally the smaller, and may be absent. The 

 trochantinal plate (Tti) is best developed in the more generalized 

 Pterygota (figs. 13, 14, Tn), but shows always a tendency toward 

 reduction, and is lost in the higher orders. 



The small sclerites lying in the pleural membrane immediately 

 beneath the base of the wing (fig. 20, Ba, Sa) serve in the adult as 

 insertion points for two important muscles of the wings (figs. 28, 30 A, 

 E, F). There are at most two of these sclerites above the episternum, 

 and two above the epimeron, but more usually there is only a single 

 plate in each position, one before the pleural wing process, the other 

 behind it. Many entomologists, supposedly following Audouin(i824), 

 have called the episternal sclerite the " parapteron," and Audouin 

 says of the plate that he defines by this name, " elle a des rapports avec 

 I'episternum et avec I'aile, toujours elle s'appuie sur Tepisternum, 

 se prolonge quelquefois inferieurment le long de son bord anterieure, 

 ou bien, devenant libre, passe au devant de I'aile, et se place meme 

 accidentellment au-dessus." It is possible, therefore, that Audouin 

 in some cases confused the plate ordinarily beneath the wing with 

 the tegula, though of his " paraptere " he says, " toujours elle s'appuie 

 sur I'episternum." Crampton (1914), however, claiming that Audouin 

 first applied the term '' parapteron " to the tegula, designates the epi- 

 sternal plates the bosalarcs {Ba), and the epimeral plates the suhalares 

 {So). These terms commemd themselves because they carry specific 

 distinction, though all the ])lates are subalar in position. Voss ( 1905) 

 and other German writers call the sclerites the " pleural hinge plates " 

 (Pleuralgelenkplatten), but they are not true articular elements of the 

 wing base. Collectively, we may call the plates the epipleurites. 



