8 E. B. Williamson 



Laidlaw) in hind wing, triangle of front wing followed by three cells (rarely 

 two in credxda) followed by three rows (usually but not always by two rows 

 in part of the field in credula), increasing to more at or distal to the level of 

 the distal cell between M4 and Mspl, hind wing with two postriangular rows 

 of cells increasing to three at or distal to the level of forking of M^.^, and M3. 



The striking difference in the form of the abdomen of haeniatogastra or 

 pleheja, for example, on one hand and mithroides or peruznana on the other, 

 more than any other one character, led authors, prior to Calvert in 1906, to 

 distribute the species of Erythemis in three genera. But between the extremes 

 named above the remaining five species may be arranged from more slender to 

 less slender in approximately the following order: carmelita, credula, sim- 

 plicicollis, attcda, and collocata. 



Another striking difference within the genus is the color of adults, espe- 

 cially of males, some having more or less of the body, but especially the abdo- 

 men, brilliant red, while in others no red or reddish appears in the colora- 

 tion. This latter group includes some which are largely greenish or yel- 

 lowish as tenerals and become dark bluish pruinose with age, while other 

 darker species become deep black with age. This not-red group includes 

 both slender and stout species, and also among the reds we have the extreme 

 forms of abdomen. In the same way, among tenerals there are species with 

 a distinctly patterned thorax and species which lack a definite pattern, the 

 thorax being practically concolorous. And in each group are slender and 

 stout species, and red and not-red species. 



Thus, the species within the genus are well marked and easily recognized, 

 but efforts to define more than one generic group seem futile. 



In addition to the characters mentioned above, the accessory genitalia of 

 the male and the vulvar lamina of the female are specifically distinct. In the 

 case of the males, however, those individuals in which the penis is extruded 

 usually, but not invariably, have the internal branch of the hamule bent out- 

 ward and backward, so that the normal relation between the internal and 

 external branches is lost. 



Key to the Species of Erythemis 



I. Lateral and ventral carinas on abdominal segment 3 at the apex, measured 

 along the apical carina, separated by .8 mm. or less ; the distance between the 

 same carinas, opposite the meeting point of the lateral and median transverse 

 carinas, equal to more than one and one-half times the distance between them 

 at apex; lateral and ventral carinas on segment 4 separated by a distance less 

 than one-sixth the length of the lateral carina of segment 4 (possibly not 

 always true of credula). Ventral teeth on the superior appendages of the 

 male extending posteriorly beyond the level of the apex of the inferior appen- 

 dage 2. 



1'. Lateral and ventral carinas on abdominal segment 3 at the apex, measured 

 along the apical carina, separated by I mm. or more; the distance between the 

 same carinas, opposite the meeting point of the lateral and median transverse 

 carinas, equal to one and one-half times or less the distance between them at 

 apex; lateral and ventral carinas on segment 4 separated by a distance greater 

 than one-sixth the length of the lateral carina of segment 4 (possibly not always 

 true of simpUcicolUs). Legs of adults largely black or dark brown 5. 



