518 



dd. Dorsal marginal setae of the median gill more than twenty in 

 full-grown nymphs ; only the proximal third of the third anten- 

 nal segment dark brown ; gills commonly 5.5-6 mm. in length. . 

 civile. 



cc. Lateral keel of the first abdominal segment with two to four 

 heavy setae ; axis of the gills opaque or slightly smoky, 

 d. Dorsal setae of the median gill extending beyond the middle ; 

 mental setae four, the meso-caudal seta in each row one-half 

 as long or nearly as long as the remaining setae . . < cyatJiigerum. 



\ calverti. 



dd. Dorsal setae of the median gill not extending to the middle 



of the gill ; mental setae three, the meso-caudal seta in each 



row representing a small fourth, but minute and always less 



than one-half the length of the three larger setae, carunculatum. 



aa. Gills with pigment other than in the tracheae. 



b. Tracheal branches of the gills in alga-like patches ; distance from 

 the caudal margins of the compound eyes to the caudo-lateral angles 

 of the head greater than half the distance between the antenna! 

 fossae. Abdomen without a median, ventral, black line. 

 c. Dark portion of the base of the gills extending less than half 



their length (Fig. 55) traviatum. 



cc. Dark portion of the base of the gills extending more than half 

 their length. 



d. Length of the gills less than eight times the greatest Mddth ; 

 gills of mature nymphs with a prominent hinge just caudad 



of the middle (Fig. 53) exsulans. 



dd. Length of the gills eight times the greatest w^idth ; gills of 

 mature nymphs without a prominent hinge caudad of the 



middle (Fig. 54) antennatum. 



bb. Tracheal branches of the gills not in alga-like patches ; distance from 



the caudal margins of the compound eyes to the caudo-lateral angles 



of the head less than half the distance between the antennal fossae. 



Abdomen with a median, ventral, black line. 



T^ c. Median gill with a bunch of setae proximad of the first dark 



transverse cross-band; distal cross-bands of the same degree of 



blackness as the proximal ones (Fig. 56) ; median gill not greatly 



expanded distad of the first cross-band ; dorsal setae of the apical 



margins of the abdominal terga not prominent signatum. 



cc. Median gill without a bunch of setae proximad of the first dark 

 transverse cross-band; distal cross-bands faint, and lighter in 

 color than the proximal ones (Fig. 57) ; median gill much ex- 

 panded distad of the first cross-band; dorsal setae of the apical 

 margins of the abdominal terga very prominent pollutum. 



