2 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. O.'). 



The species of Hydrophorus always frequent the edges of watei- 

 and are often seen running over its surface. Scellus is never seen at 

 water, but is found on the bark of trees, on the ground, or in grass. 

 Liancalus is found on rocks in cold wet places. 



The males of Scellus can be divided into two groups. The male 

 appendages are ribbonlike. In one group the appendage has a spoon- 

 like enlargement at the apex, while the other group is without this 

 enlargement at the apex. 



KEY TO MAlrBS. 



1. Tblrd joint of antenna very long pointed No. 1, virago Aldricii. 



Third .ioint of antenna very sliort 2 



2. Caudal appendage ribbonlike, with a spoonlike enlargement at the apex 3 

 Caudal appendage without an enlargement at the apex 6 



3. Spoonlike tip of appendage infuscated 5 



Spoonlike tip of appendage not infuscated 4 



4. Spoonlike tip of appendage pale; hind femur very thick, strongly curved at 



base with a cluster of short, thick spines on the underside, 



No. 2, exustus (Walker). 

 Spoonlike tip of appendage light ocher yellow ; hind femur straight, slender, 



No. 3, splnimanus (Zetterstedt). 



5. Hind tibia with two large, spinelike bristles at the apex, the larger one bent 



like a corkscrew No. 4, monstrosus Osten Sacken. 



Hind tibia normal; front tibia notched near the apex, terminating into an 

 oblique point No. S, avidus Loew. 



6. Caudal appendage with an enlargement near the middle, 



No. 6, amplus Curran. 

 Caudal appendage without enlargement 7 



7. Appendage short, whitish on apical half, infuscated on basal half; hind femur 



with pale, villous hairs on inner surface near middle, 



No. 5, vigil Osten Sacken. 



Appendage nearly as long as the abdomen, pale, infuscated at extreme base : 



hind femur without villous hairs No. 7, filiferus Loew. 



KEY TO FEMALES. 



1. Third joint of antenna very long and narrow virago Aldrich. 



Third joint of antenna short 2 



2. Shining, metallic, large species 3 



Dull metallic surfaces heavily dusted, smaller species 5 



3. Halteres brownish-black, pale at base ; legs bronze-black, sub-shining, 



exustus (Walker). 

 Halteres pale ; legs not as above 4 



4. Legs shining, dark metallic green; abdomen shining with metallic green 



reflection monstrosus Osten Sacken. 



Legs sub-shining with a deep reddish-bronze reflection; abdomen with deep 

 reddish-bronze reflection avidus Loew. 



5. Front tibia with four long, macrochaetae on under side, 



spinimanus (Zetterstedt). 

 Front tibia with two long macrochaetae on under side G 



6. Front tibia with two macrochaetae of equal length on the front side near the 



middle amplus Curran. 



Front tibia with two macrochaetae of unequal length 7 



7. Femora metallic green filiferus Loew. 



Femora bronze, with reddish tinge vigil Osten Sacken. 



