AQUATIC INSECTS IN THE ADIRONDACKS 521 



c Tibiae and tarsi yellow externally ; the black of the abdominal segments 



tending to form apical rings albifrons 



cc Tibiae and tarsi wholly black; the black of the abdominal tending to 

 form apical lateral triangles 

 d Wings with the basal half (or somewhat less) flavescent ; branches of 

 the genital hamule of the male inclosing an angular notch; vulvar 

 lamina of the female with its lobes short and sharply recurved up- 

 ward, their apices meeting the venter of the ninth segment vertically 



assimilatum 



dd Wings flavescent only at the extreme base ; branches of the genital 



hamule of the male inclosing an oval or a rounded notch ; vulvar 



lamina of the female with appressed lobes which meet the venter of 



the ninth segment more obliquely 



e Branches of the genital hamule of the male inclosing an oval notch, 



the outer about twice as stout as the inner, about equally curved ; 



the vulvar lamina of the female with its sides regularly sloping 



rubi cundulum 

 ee Branches of the genital hamule of the male inclosing a short rounded 

 notch, the inner branch more sharply incurved, the outer about four 

 times as thick as the inner ; the vulvar lamina of the female some- 

 what contracted at about midway its length, the sides more con- 

 vergent in the basal half obtrusum 



hb Superior abdominal appendage of the male without a prominent inferior 

 median tooth, bnt only with small inferior denticles of about equal size ; 

 vulvar lamina of the female not cleft 



c Wings with the basal half flavescent semicinctum 



CO Wings flavescent only at the extreme base 



d Femora and tibiae entirely yellow vicinum 



dd Femora and tibiae marked with black on the sides c o s t i f e r u m 



Nyjnphs ^ 



a Dorsal hooks of abdominal segments 6-8 long and sharp, about as long as 

 their respective segments 

 & Lateral spines straight on both outer and inner margins costiferum 



1 The nymphs of aibifrons and corruptum are unknown; that of the former species Is 

 likely to be of the type of the nymph of rubicundulum. I give a figure (pl. 25, fig. 1) of a 

 nymph from southern California of S. illotum, the nearest ally of corruptum. The nymph 

 of corruptum will probably be of this type. 



I have nymphs of rubicundulum raised at Ithaca, of o b t r u s u m, raised at Lake Forest 

 111., and of assimilatum raised at Saranac Inn. Between the nymphs of rubicundulum 

 and obtrusum I find only a scarcely perceptible difference In size, that of obtrusum being a 

 little smaller, in the bred specimens. Both these are a very little smaller than asslmilaium; and 

 I note that In the bred specimens the dorsal hooks on the fourth and fifth abdominal segments 

 (hidden between the wing cases) are larger and more nearly equal In size in assimilatum, 

 smaller and more unequal in size and paler In the other two. These differences are so slight and 

 have been studied in so few specimens that I have not thought best to introduce them as yet Into 

 the table. 



As to the imagos of these three nominal species, I know of no absolutely constant differences 

 either in size, coloration, structure, distribution or habits that will In every case distinguish be- 

 tween them. The typical r ub icund ulum Is of course, Intermediate between the other two. I 

 have examined hundreds of specimens of each, and say unhesitatingly that they intergrade com- 

 pletely; nevertheless, it is convenient to recognize the three forms, and practically, there is little 

 difficulty generally in distinguishing between them. I have therefore listed them separately. 



