1911] Aldrich — North American Species of Hydrophorus 47 



generally are of little diagnostic value. They are always wholly 

 black, except in some cases where there may be a whitish tip 

 to the arista, which I do not find worth mentioning; in a few 

 species the first joint of the antenna is elongated, and in cerutias, 

 especially the male, there is a remarkable elongation of the whole 

 organ (fig. 5). The width of the face and the color of the palpi 

 are occasionally unusual and useful. 



The thorax presents many opportunities for distinguishing 

 species, especially in its chsetotaxy. The scutellar bristles are 

 uniformly four except in gratiosus (2), intentus (6), plumbeus 

 (10), and cerutias (2). The numbers above four may be expected 

 to be a little variable. The notopleurals are generally two, 

 but are only one in gratiosus, intentus, plumbeus, cerutias and 

 agalma. Not to prolong the list too much, the dorsocentrals 

 vary i^ their size, some species having them large and others 

 small; one species, canescens, has them white, a striking feature. 

 The propleural (just above the front coxa on the side) shows 

 some interesting changes; in plumbeus there are several, in all 

 the rest one except cerutias, canescens and agalma, which have none; 

 in phoca, the single propleural is yellow instead of black. The 

 pleura often has fine, pale hairs in noticeable clusters in very 

 definite spots, which I have mentioned in many cases. The 

 halteres may be wholly yellow, or the knob may be more or less 

 infuscated, and these two alternatives divide the genus more 

 naturally than any other I can find, hence I have made primary 

 use of them in my table. The wings offer few valuable characters. 

 One of the first class is found in cerutias, but was overlooked 

 by Loew — the extreme shortness of the second vein (fig 1). No 

 other North American species has this peculiarity. The two 

 spots on the wing in a few species are very constant, but other 

 species have them in a very faint degree, also constant, which 

 makes the character unavailable in a table except in one group. 

 The paleness of the veins at the base is rather variable and I 

 have found it of little use except in a few species. The abdomen 

 presents very few salient characters. The hypopygium of the 

 male varies so much in drying that it is almost impossible to 

 make any use of it, while in other Dolichopod genera it is often 

 of the greatest use and value. The legs except the front ones 

 offer few characters. I have paid considerable attention to 



