THK (.ANADIAN KNTOMOI.OGIST 151 



b. The Sulci. 



Rhomalea microptera (Figs. 1.2 and 12), having four wdl developed 

 pronctal sulci gi\es gcud material for a study of these structures. The first 

 sulcus (sui) lies a very short distance behind the anterior edge of the pronotum 

 and does not quite extend either to the median carina or to the ventral edge of 

 the pronotum. The three other sulci all cut the median carina; the second 

 (su2) runs about half wa>- down the pronotum and curves forward; the third 

 extends to the ventral edge, and its lower half is coincident with the entopleural 

 ridge formed by the infolding of the contiguous edges of the episternum and 

 epimeron; the fourth runs from the median carina almost to the postero-ventral 

 angle of the pronotum. An examination of Figure 12 will show that the first 

 sulcus lies near the line where the infolded integumental membrane of the 

 anterior end of the pronotum merges into the cervical membrane, and marks 

 the furthest point to which the head may be retracted within the prothorax. 

 This sulcus may be regarded then as marking the extremity of the primitive 

 prothorax, the portion of the pronotum in front of this being probably a later 

 developed growth forming a flange to receive and protect the retracted head. 

 The anterior sulcus represents the fold which would naturally be formed when 

 this flanking piece is pushed back by the retracted head. That this is the case 

 is shown in Tettix granidatus (Fig. 5) where the prosternum has grown forward, 

 completing the flange formed by the pronotum. In this insect the anterior 

 notal sulcus is continuous with a sternal sulcus (s.su) which marks the position 

 of the anterior end of the sternum in other Orthoptera. 



A similar explanation accounts for the origin of the fourth sulcus (su4). 



As Crampton (I.e.) has pointed out, the third sulcus originated with the 

 infolding of the contiguous edges of the pleural sclerites to which it is closely 

 attached. This accounts for the origin of the lateral portion of the sulcus, and 

 it will be noted that in Dissosteira and many other insects this sulcus does not 

 extend to the dorsal portion of the pronotum. The dorsal portion of this sulcus 

 in Rhomalea arose as a result of the pull of the muscles which are attached to 

 it, viz., one head of the third pronotal muscle (20) and one of the elevator of the 

 head (4). 



The origin of the second sulcus (su2) is also due to the pull given the integu- 

 ment by the muscles attached to it. These muscles are an elevator of the head 

 (4), a rotator of the head (8), one head of the third pronotal muscle (19), and 

 an extensor of the coxa (23). 



At the time the sulci were formed the integument was probably more 

 flexible and less highly chitinized than it is at present. The method of origin 

 of the second sulcus is well illustrated in the condition which now obtains in the 

 flexible sutural membrane between the pro- and mesonotum. Here the pull 

 of the first (XIX) and third (19, 20) pronotal muscles has produced a distinct 

 fold in the integument (Fig. 12 F.). The hardening of this membrane would 

 undoubtedly produce a sulcus exactly similar to those of the pronotum. 



Summary. 

 1. The propleuron in the Orthoptera has not been forced out by the 

 downgrowing notum, but has persisted on the inner side of the pronotum which 

 has grown over it. 



