436 MISS A. M. EVANS ON THE STRUCTURE AND OCCURRENCE 



Folsom (17. p. 116) is convinced of their homology with the lingua and 

 superlinguse of Anurida, which in a certain stage of their development bear 

 a great resemblance to Heymons' figure of Ephemera. 



Chloeon dipterum. — The oval hypopharynx (text- fig. 4) measures "3 mm. in 

 length and -21 mm. in breadth at the widest part. The maxillulse (m.) 

 resemble this structure in shape and size, and in the membranous nature of 

 their walls. Each measures '2 mm. in width and extends *3 mm. beyond 

 the sclerite (p.). Each maxillula is supported ventrally by a curved chitinous 

 plate (m.p.), which is proximally merged with the chitinised posterior portion 

 of the wall of the hypopharynx. 



The inner edges of the maxillulfe are continuous with a pair of convergent 

 setose ridges (?'.), which extend backward along the upper surface of the 

 hypopharynx. Outside these occur a second pair of ridges («'.) also bearing 

 sette, whose distal portions lie on the upper surface of each maxillula. 



PEilLARIA. 



Packard (36) describes the hypopharynx of Perlidae as " an unusually 

 large tongue-like mass nearly filling the buccal cavity." I have examined 

 this organ in the adult of a species of Perlodes closely resembling P. dispar, 

 and find that it answers to the above description. No signs of maxilluhe 

 were discovered. 



The larvae of the two sub-orders Plecoptera filipalpia and Plecoptera seti- 

 palpia Klapalek (27) are distinguished by the structure of the labium. Tins 

 in the former group is apparently less specialised than in the latter. An 

 examination of the hypopharynx in the two genera Leuctra and Nemura of 

 the Filipalpia and Perlodes, Chloroperla, and Perla of the Setipalpia suggests 

 that the form of this organ offers a further distinction *. 



Nemura. — In the larva of an undetermined species of this genus the hypo- 

 pharynx (text-tig. 5) is a much-rounded structure convex dorsally. The 

 surface is thin and membranous. Tt measures •13 mm. in length and "2 mm. 

 in width in a specimen with length of abdomen 2 - 88 mm. From its dorsal 

 surface project two large lobes (hi.) measuring "09 mm. in length and '05 mm. 

 in width, which extend laterally some distance beyond the hypopharynx, and 

 whose cavities are confluent with that of the latter. Ventrally a chitinous 

 plate (p.m.) supports the free portion of each lobe. Proximally the dorsal 

 surface of the hypopharynx (or ventral wall of the pharynx) bears a pair of 

 setose ridges {■>:). Each ridge is distally continuous with the inner margin 

 of the lateral lobe. Thus the position of these ridges agrees strikingly r with 



* The only traces of maxilluke associated with the large fleshy tongue of larval Setipalpia 

 were a pair of very small setiferous lobes, but in the Filipalpia these organs were large, and 

 well developed and similar in both genera examined. 



