1918] Thoracic Sclerites of Dissosteira 351 



The Pro pie ur on. 



Contrary to the statement that the pronotum has "crowded 

 out" the pleural region or the prothorax (Snodgrass, 1909, 

 pp. 534, 555 and 556), the lateral portions of the pronotum "11" 

 of Fig. 1 have merely grown down over the pleural region which 

 lies beneath the overlapping pronotum, the surfaces of the two 

 regions being closely applied to each other. If one of the larger 

 grasshoppers such as Rhomalea be boiled in caustic potash to 

 soften the sclerites, it will be found very easy to separate the 

 pleural sclerites from the overlapping pronotum by inserting a 

 knife blade between the two, and gently forcing them apart. It 

 will then be seen that the episternum and epimerum (Fig. 2, 

 "esi" and "emi") are the principal sclerites of the pleural 

 region, as in the other segments, and. that both extend far 

 upward under the overlying pronotum. The lower portion of 

 the episternum and epimerum project below the lower margin 

 of the pronotum in Dissosteira 'Fig. 2, "esi" and "emi"), and 

 if a specimen be boiled in caustic potash, the pleuron can be 

 separated from the overlying pronotum, in this insect also, but 

 this is much more easily accomplished in the larger forms such 

 as Rhomalea. The middle one of the three sulci labeled "su" 

 in Fig. 1, is superimposed upon the pleural suture (separating 

 the episternal from the epimeral region) of the prothoracic 

 pleural region w^hich is overlapped by the sides of the pro- 

 notum, and, since the surfaces of the pleural region and the 

 pronotum are so closely applied together, it is possible that 

 the presence of the pleural suture is responsible for the forma- 

 tion of the middle sulcus "su" which is directly over it and 

 coincides with it exactly. The posterior suclus "su" is closely 

 associated with the margin of the infiexed portion of the pos- 

 terior region of the pronotum which is folded in under the 

 remainder of the pronotum, and the surface of the fold is closely 

 applied to the surface of the pronotum. It is possible that 

 the attachment of this margin on the inner surface of the 

 pronotum has given rise to the posterior sulcus "su," and it is 

 thus clearly evident that the sulci "su" of Fig. 1 owe their 

 origin to mechanical causes, rather than to the assumed fact 

 of their representing the four tergal subdivisions of the notum 

 of a wing-bearing segment. Since the pleural region is present, 

 although closely applied to the inner surface of the pronotum 



