COLLEMBOLA OF THE FAMILY ISOTOMIDAE 19 



The only specimens of this species that I have seen are 12 that I 

 found at Arhngton, Mass., on March 20, in damp moss on stones in 

 a bog. 



Cotypes.—U.S.'NM. no. 42991. 



Genus FOLSOMIA Willem 



Isotoma TuLLBERG, 1871, p. 150 (part). 



Folsornia Willem, 1902b, p. 280.— Borner, 1903, p. 171; 1906, p. 164.— Axelson, 

 1905b, p. 29.— LiNNANiEMi (Axelson), 1907, p. 21; 1912, p. 109. 



Genotype. — Isotoma Jimetaria (Linnaeus, Tullberg). 



Fourth, fifth, and sixth abdominal segments ankylosed, forming a 

 single mass (pi. 7, fig. 74). Anus ventrocaudal. Furcula short, not 

 attaining the ventral tube, appended to the fourth abdominal seg- 

 ment. Mucrones usually bidentate, rarely tridentate, without 

 lameUae. Manubrial hooks moderately developed. Tenaculum with 

 a single ventral seta. Eyes in most species reduced in number or 

 absent. Postantennal organ narrow, often slightly bent. Sense 

 organ of third antennal segment with two oblique rods and a pro- 

 tecting integumentary fold. Fourth antennal segment with a ter- 

 minal tubercle, a subapical papilla, and usually with olfactory setae. 

 Unguis simple, without lateral teeth, and seldom with an inner tooth. 

 Unguiculus present, simple, untoothed. Tenent hairs absent. Body 

 setae simple, rarely feebly and unilaterally feathered. Bothriotricha 

 absent. Body pigment commonly weakly developed or absent. 

 Integument smooth. 



Species of Folsornia exhibit various stages of reduction in the num- 

 ber of eyes, the length of the furcula, size of body, and amount of 

 pigment. The blind species are white, in correlation with their 

 habit of hving in darkness. 



KEY TO species OF FOLSOMIA 



1. Eyes absent. White species. Manubrium shorter than dentes. 



Dentes with many dorsal crenulations 2 



Eyes present. Pigmented species, with one exception 3 



2. Postantennal organs subelliptical or subovate. Unguis un- 



toothed in the typical form. Dentes twice as long as manu- 

 brium fimetaria (p. 28) 



Postantennal organs long and narrow, with parallel sides. 

 Unguis strongly unidentate. Dentes one and one-half times 

 as long as manubrium nivalis (p. 32) 



3. Eyes 16. Manubrium longer than dentes. Dentes with a few 



dorsal crenulations 4 



Eyes fewer than 16. Tenent hairs absent 6 



4. Mucrones bidentate 5 



Mucrones tridentate. Unguis untoothed. Tenent hairs absent. 



Grayish blue guthriei (p . 22) 



5. Unguis untoothed. Tenent hairs absent. Manubrium with 



one pair of ventral subapical setae. Grayish, bluish, or 



brownish elongata (p. 21) 



