8 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



KEY TO GENERA OF ISOTOMIDAE 



1. Anal spines present Tetracanthella (p. 10) 



Anal spines absent 2 



2. Furcula absent Anurophorus (p. 12) 



Furcula present 3 



3. Body cylindrical, greatly elongate, in length as much as six to 



eight times its height. Prothorax unusually long. Fourth 

 abdominal segment simple, relatively unmodified. Terga not 

 imbricate. Furcula very small, appended to the fourth ab- 

 dominal segment and not extending beyond the middle of the 

 third abdominal segment. Eyes reduced in number or absent. 

 Postantennal organs long and narrow. Sense organ of third 

 antennal segment with two papillae, each in a deep circular 

 pit. Unguiculus present. Tenent hairs absent. Manubrium 

 longer than dentes. Dentes smooth dorsally, not tuberculate 

 or crenulate. Mucrones bidentate. Clothing of short simple 



setae. Body pigment absent 4 



Body not remarkably elongate. Prothorax reduced more or 

 less. Fourth abdominal segment not a simple ring, but modi- 

 fied by changes in form or by ankylosis. Furcula not very 

 small. Eyes various in number, but commonly eight on each 

 side. Postantennal organs of various forms. Sense organ of 

 third antennal segment with a pair of rods or papillae not situ- 

 ated in deep pits, but naked or covered basally by a single in- 

 tegumentary fold 5 



4. Genital and anal segments simple, or relatively unmodified, with- 



out ankylosis. Anus not ventral. Manubrial hooks not 

 strongly developed, represented by two pairs of narrow curving 



ridges. Integument smooth Folsomides (p. 13) 



Genital and anal segments modified, shortened, ankylosed. 

 Anus ventral. Manubrial hooks strongly developed. Integu- 

 ment granulate Isotomodes (p. 16) 



5. Last three abdominal segments ankylosed, forming a single mass, 



in which sutures are mostly obsolete. Anus ventrocaudal. 

 Furcula not attaining the ventral tube. Mucrones bidentate 

 (tridentate in <7tt</inei), not lamellate. Tenent hairs absent. 

 Eyes usually reduced in number or absent. Body pigment 

 often weakly developed or absent. Integument smooth. .Folsomia (p. 19) 

 Last three abdominal segments not ankylosed, though the last 

 two may be. Anus caudal. Furcula of various lengths. 

 Eyes usually 16, though fewer in some species. Pigmentation 

 usually well developed 6 



6. Fourth abdominal tergite almost always longer than the third; 



the two subequal, however, in a few species. Third urotergite 

 not ventrolaterally prolonged backward to any significant 

 degree (pi. 9, fig. 89). Paratergite of fourth abdominal seg- 

 ment distinct, not united to the tergite of the genital segment. 

 Genital and anal segments sometimes ankylosed. Furcula not 

 attaining the ventral tube in most of the species. Manubrium 

 often longer than the dentes, with ventral setae usually few in 

 number and subapical in position; or naked ventrally. Den- 

 tes usually stout, and dorsally smooth or with tubercles or 

 lobes or a relatively few coarse transverse folds. The dentes 

 when slender and tapering are longer than the manubrium and 



