402 



absent; labium with the central tooth simple (PL XXIX, Figs. 

 7, 8) Cliironomus lohiferus (p. 430) . 



— Two pairs of blood gills present, situated low on sides and very long 



and noticeable; labium with the central tooth trifid, or the first 

 lateral tooth very small 3 



3. Central labial tooth either truncated at apex or but poorly defined, 



the separation between it and the first laterals very slight 4 



— Central labial tooth either acutely pointed, or rounded at apex and 



with a more or less distinct shoulder on either side 5 



4. The 3 central teeth almost fused (PL XXIX, Fig. 1), antenna with 



6 joints (PL XXX, Fig. 10) . . .Cliironomus flavicingula (p. 432). 



— The 3 central teeth distinctly divided; antenna with 5 joints 



Cliironomus plumosus (p. 447) . 



5. Large species, about 25 mm. in length 



Cliironomus tentans? (p. 444) . 



— Smaller species, not over 15 mm. in length 6 



6. Central labial tooth slightly rounded or acute at apex, without a 



distinct shoulder (PL XXIX, Fig. 10) 



Cliironomus viridicollis (p. 457) . 



— Central labial tooth generally distinctly rounded and with a distinct 



shoulder Cliironomus decorus (p. 472) . 



7. Abdominal segments with a distinct pencil of hairs on each side 



near posterior margin in addition to a few scattered hairs 



Cricoiopus trifasciatus (p. 503) . 



— Abdominal segments without distinct pencil of hairs 8 



8. Labium with the central portion pale, broadly rounded in outline, 



the lateral portions dark colored, heavily ehitinized, and digitate 



(PL XXX, Fig. 13) ( Cliironomus digitatus (p. 483). 



( Cliironomus sp. C. . . . (p. 529). 



— Labium and antenna not as above 9 



9. Very large species, averaging 45 mm. in length 



Cliironomus f errugineovittatus (p. 446). 



— Much smaller species, not more than 12 mm. in length 10 



10. Central labial tooth not divided in middle 11 



— Central labial tooth divided in middle 29 



11. First and second lateral teeth fused nearly to their apices, much 



more closely adherent than central and first lateral or second and 

 third ; or anterior outline of labial plate convex, never subtriangu- 

 lar 12 



— First and second lateral teeth not closely united ; or labial plate 



subtriangular 13 



12. First and second lateral teeth fused nearly to apices 



Cliironomus tenellus. 



— First and second lateral teeth separate for some distance from their 



apices Cliironomus dux. 



13. First lateral tooth longer than central tooth 14 



