58 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



the back plates of the nymph of a stonefly (fig. 14) shows that the dor- 

 sal muscles (DMcl) are segmentally arranged, being attached to the 

 anterior and posterior parts of the same segment, i, e., just back of 

 and just before the intersegmental constrictions. The correspond- 

 ing sections of adult stoneflies (figs. 15 and 16) indicate that the 

 phragmas {2Ph and SPh) are simply downward ingrowths from the 

 deepest parts of the intersegmental grooves to accommodate the 

 increased thickness of the dorsal muscles (DMd). Hence, the 

 phragmas appear to be truly intersegmental in their origin, and it 

 may easily be imagined that the common bilaminate structure results 



Fig. 14.— Longitudinal section THRotroH back op thoeax and base of abdomen of a stonefly nymph 



(ISOGENUS), SHOWING EACH BACK PLATE OF THE THORAX SEPARATED FROM THE ONE BEHIND IT BY A WIDE 

 POSTNOTAL membrane: DMcl, DORSAL LONGITUDINAL MUSCLES; /T, FIRST ABDOMINAL TERGUM; Mbi 



Mhi, Mhz, POSTNOTAL membranes; Ni, pronotum; N2, mesonotum; N3, metanotum; Wi, front wing; 



W3, HIND WING. 



DMcl 



Fig. 15.— Longitudinal section through back of mesothorax, metathorax and base of abdomen 



OF AN adult stonefly (ALLOPEKLA) SHOWING POSTNOTAL PLATES (P Ni, P Ni) OCCUPYING POSITION OF 

 POSTNOTAL MEMBRANES (3/6;, Jl/63) IN FIG. 14: 2Ph, SPh, SECOND AND THIRD PHRAGMAS; OTHER LETTER- 

 ING AS IN FIG. 14. 



DMcl 



2Ph 



Fig. 16.— Corresponding section through another adult stonefly (T^niopteryx frigida), show» 



ING SAME thing AS FIG. 15: LETTERING AS EN FIGS. 14 AND 15. 



from an apposition and fusion of the infolded surfaces of the adjoin- 

 ing terga, thus increasing the depth of the phragmas. 



In such forms as Alloperla (fig. 15) and Txnio'pteryx (fig. 16) it is 

 seen that there is no movable articulation between the mesotergum and 

 the metatergum, the two being united in the middle phragma {2Ph). 

 But in most insects there is more or less motion possible between 

 these two parts due to an intervening membranous area, as in the 

 Orthoptera and Coleoptera. Fig. 12, representing a longitudinal 

 section through the back of a grasshopper, shows that while the mid- 

 dle phragma {2Pli) is solidly attached to the front of the metanotum 

 (iVg), it is separated from the mesonotum {N^ by a narrow mem- 



