COLLEMBOLA OP THE FAMILY ISOTOMIDAE 17 



seated in a deep pit almost surrounded by a thick chitinous border. 

 Fourth antennal segment with several olfactory setae. Postantennal 

 organ large, elongate, elliptical to parallel-sided, with relatively weak 

 chitinous wall and with several guard setae. Unguiculus present. 

 Furcula present. Manubrium conspicuously broad basally; dentes 

 not crenulate; between the dentes two pairs of remarkably strong 

 basal hooks. Ventral tube short. Anal spines absent. Eyes absent 

 in the known species. 



The genus Isotomodes was founded on a single species, /. produdus 

 Axelson, found in Finland, Russia, and England under stones on damp 

 humus and in Morocco under stones as a myrmecophile. 



In the species tenuis, described here from Massachusetts, some of 

 the generic characters are less pronounced than in the genotype. The 

 pro thorax, for example, is not so long as in produdus: the anus is not 

 quite ventral in position; the sensory pits of the third antennal seg- 

 ment are more open ; the olfactory setae of the fourth antennal seg- 

 ment are not so strongly differentiated from ordinary setae; and the 

 ventral tube is not especially short. Like produdus, tenuis has bi- 

 dentate mucrones, elongate postantennal organs, and no eyes. /. 

 produdus is unpigmented, but tenuis, though white to the naked eye, 

 shows scattered specks of black pigment under the microscope. In 

 specific characters, the two species differ considerably, 



Linnaniemi notes that in certain respects Isotomodes occupies a place 

 between Folsomia and Cryptopygus. 



In his figures of produdus, Ijinnaniemi (1912) show^s a trace of a 

 dorsal suture between the anal and genital segments; but in tenuis 

 I have found no trace of such a suture. The terminal structure of the 

 abdomen is much like that of Cryptopygus WUlem (1902c), which is 

 described as having the anal segment completely concealed in a de- 

 pression of the genital segment. I believe that in Isotomodes, how- 

 ever, the anal segment is not concealed, but is simply ankylosed with 

 the genital segment, this opinion being supported by the existence of 

 the suture shown by Ijinnaniemi. 



Isotomodes agrees with certain species of Folsomia in respect to its 

 weak pigmentation, eye reduction, ankylosis of the last two abdominal 

 segments (last three in Folsomia), reduced furcula, bidentate mu- 

 crones, and the presence of manubrial hooks. As in some species of 

 Proisotoma, the body setae are short and simple, the furcula is short, 

 and the dentes are dorsally without crenulations or tubercles. 



ISOTOMODES TENUIS, new species 



Plate 3, Figures 27-34 



Description. — Greatly elongate (slenderer than in pi. 3, fig. 27, 

 which is from a flattened specimen); length about nine times the 

 height of the body. White, with a few scattered black specks, 

 largest and most numerous on the posterior part of the abdomen. 



