104 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



Alaska: St. George Island, August 2 (Bureau of Biological Survey). 

 Greenland: Umanak, July 22, W. E. Ekblaw (American Museum of Natural 

 History; University of Illinois). 



ISOTOMA (ISOTOMA) VIOLACEA Tullberg variety CAERULEATRA Gntbrie 



Plate 34, Figures 397-401; Plate 35, Figures 402, 403 

 Isotoma caeruleatra Guthrie, 1903, p. 70. 



Description. — This form agrees exactly with typical violacea except 

 in the following respects: 



Postantennal organ (pi. 34, fig. 397) narrowly elliptical, almost 

 parallel-sided, three to four times as long as broad, and one and two 

 thirds to three times as long as the diameter of an adjacent eye. 

 Second and third antennal segments subeqiial. Apical tooth of 

 mucro (pi. 34, figs. 398, 399) a little smaller than, or subequal to, the 

 anteapical; hooked. Teeth of miguis strong. 



The types of caeruleatra are clear dark blue, almost black; the head 

 marked dorsally with yellow; antennae blue, the first segment paler 

 than the others; legs blue basally, otherwise yellow; manubrium blue 

 dorsally, paler ventrally; dentes yellow except basally. 



Canadian specimens that I have studied are less heavily pigmented 

 than the types, being clear blue or purplish with numerous small 

 unpigmented spots; with the body segments narrowly bordered pos- 

 teriorly with dark blue; with white in place of yellow on head, legs, 

 and f urcula ; antennae purplish, the segments paler basally. 



The lateral teeth and inner tooth of the unguis are strongl}^ de- 

 veloped (pi. 34, fig. 400). The subapical ventral seta of the dens 

 (pi. 34, fig. 398) is short, extending only as far as the base of the mucro. 

 All the setae of the body are simple (pi. 34, fig. 401). Length, 2 mm. 



Remarks. — This variety varies into the typical form of violacea. 

 Thus, in one individual the postantennal organ was only one-fourth 

 longer than the diameter of an adjacent eye; and in several individuals 

 the wall of the postantennal organ was exceptionallj^ thick. 



As a variation, the apical tooth of the mucro may be considerably 

 smaller than the anteapical (pi. 35, fig. 402). 



This form approaches closely the European variety divergens 

 Axelson. 



I received, for study, four cotypes of caeruleatra, through the cour- 

 tesy of Professor H. F. Nachtrieb, of the University of Minnesota. 



Distribution. — Recorded as follows: 

 Wisconsin: Beloit, October, V. G. Davidson. 



Minnesota: Near Minneapolis, April 16, O. W. Oestlund (University of Minne- 

 sota. 

 Idaho: Craters of the Moon, August 21, V. G. Davidson. 

 Canada: Arnprior, Ontario, in siftings, C. Macnamara. 



