Biology of the Membra cidae of the Cayuga Lake Basin 319 



represented by Plate xxxiii, 5, 6. The episternum is a single irregular 

 sclerite, closely fused with the lateral notum in the mature insect but 

 separated by the anteriorly extended wing cavity in the nymph. The 

 distal (ventral) extremity is produced into the sternal region. The caudo- 

 ventral margin forms the upper edge of the mesocoxal cavity. In certain 

 forms of the subfamily IVIembracinae the episternum seems to be divided 

 by a transverse suture across its lower third (Plate xxxiii, 7). In this 

 subfamily, also, the entire episternum is elevated so that it forms part of 

 the articulatory surface of the wing. It will be noted that in the more 

 usual arrangement (Plate xxxiii, 6) the episternum is crowded down- 

 ward, and the produced notum serves as both the dorsal and the ventral 

 margin of the wing cavity at its anterior end and only braces the wing at 

 the posterior extremity of this cavity. Just cephalad of the episternum 

 is a well-developed spiracle situated in the intersegmental membrane. 



The epimeron consists of two distinct sclerites. The larger is roughly 

 subquadrangular and joins tlu^ notum cephalo-dorsad and the episternum 

 cephalo-ventrad. The second is a small triangular piece attached to the 

 dorso-caudal margin of the first and no doubt originating as part of that 

 sclerite. In the nymphal exoskeleton (Plate xxxm, 5) the suture between 

 these two sclerites is indicated but is not pronounced. The dorsal margins 

 of the two epimeral sclerites form the larger part of the lower margin of 

 the wing cavity, while the ventral margin of the anterior sclerite forms 

 part of the dorso-caudal boundary of the coxal cavity. 



In general it would appear that both the pleural sclerites of the meso- 

 thorax of the IVIembracidae tend toward subdivision. This would agree 

 with the anepimeron and katepimeron and the anepisternum and katepi- 

 sternum of C'rampton (1909 : 21-24), but the homologies are not clear if that 

 author's terminology limits the division to "upper" and "lower" regions. 



No paraptera of any description have been found. A much- wrinkled 

 connecting membrane at the anterior base of the wing may represent 

 an episternal paraptcrum or preparapterum, but there seems to be no 

 indication of epimeral paraptera or postparaptera. The basal wing 

 membranes are not thickened and certainly not chitinized. 



Directly ventrad of the epistcrnmn is a small but well-defined trochantin. 

 This sclerite is roughly triangular in shape, with the base against the 

 episternum and the apex extending cephalo-ventrad to form pai't of the 

 ventral margin of the coxal cavity. 



