NO. 1687. 



THE THORAX OF JN>iECTS—S\ODGRASS. 



537 



the wing process (WP) above and the coxal process (CxP) below, 

 and bearing a plcirral arm {PA) projecting inward and downward. 



Anterior and posterior to the pleural suture, or ventral arid dorsal 

 to it when the suture is horizontal, are the episternum (/i'/^s) and 

 the epimerum (Epm), respectivel3\ These are the two principal 

 plates of the pleurum, and, by their contiguous and infolded edges, 

 they form the pleural suture externally and the pleural ridge inter- 

 nally. The epinierum is nearly always connected, either by an 

 articulation or l)y fusion, with the lateral 

 part of the pseudonotum (text fig. 3, PN). 



A study of nymphal pleura {Jlelwioplus, 

 55, 5G) shows clearly that the episternum and 

 epimerum are merely subdivisions of one 

 original plate to which the leg is articulated. 

 Before the wings are developed, the pleural 

 suture does not extend to the dorsal edge of 

 the plate. On the inner surface (55) the 

 pleural ridge (PR) is well developed ven- 

 trally to strengthen the plate in its function 

 as a supporter of the ]eg, and the pleural 

 suture is merely the external mark of the 

 formation of the ridge. All the upper pleu- 

 ral structures, the Aving process, and the 

 parapteral plates are developed only when 

 the wing becomes functional. In forms with 

 rudimentary wings in the adult stage, such 

 as An ah PUS simplex (43, 44), these parts are 

 present, but reduced in size. A study of the 

 Chilopoda also appears to indicate that the 

 episternum and epimerum originate as sub- 

 divisions of one plate. The lower chilopods, 

 such as Jlecistocephalus (^0), present a num- 

 ber of plates on the side of the segment, one 

 of which (PI) lies immediately over the 

 coxa. The series running tlii"Ough Scolopo- 

 cryptops (21), Litlwhkis (22), and Cermafia 

 (23) shows a disa})pearance of all the other 



plates, while this one in Cermatia (23, PI) becomes divided by a 

 median thickening into two parts resembling the episternum and 

 epimerum of a nymphal orthopteran (5G). 



Associated with the base of the wing are several small plates lying 

 before and behind the Aving process {WP), These are the paraptera 

 {P). There are never more than two hi front of the wing process 

 and they may be called the episternal paraptera or the preparapteru. 

 {IP, 2 P) . There is generally only one epimeral parapterwm or post- 

 parapterum {3 P), though some Perlidae present a second. Voss 



TnC 



Fig. 4. — Diagrammatic 

 view of inner surface 

 of the pleurum of any 

 complete wing-bearing 



SEGMENT ; CxP, COXAL 

 PROCESS OF PLEURUM ; 

 Epm, EPIMERUM ; Eps, 

 EPISTEKNUM ; IP, ?.P , EPI- 

 STERNAL PARAPTERA OR 

 PlfEPAKAPTERA ; SP, EPI- 

 MERAL PARAPTERUM OR 

 POSTPARAPTERUM ; PA, 



PLEURAL ARM ; Pcps, PRE- 

 EPISTERNUM ; PR, PLEU- 

 RAL RIDGE (ENTOPLEU- 

 RUM) ; Tn, TROCHANTIN ; 

 TnC, TROCIIANTINAL 



COXAL CONDYLE ; IfP, 



WING PROCESS OF PLEU- 

 RUM. 



