306 ERNEST A. BACK 



from midway between the antennae and the vertex to nearer 

 the vertex. Mystax usually consists of a single row of bristles 

 on the oval margin which is about one-half the distance be- 

 tween the antennas and the proboscis. Thoracic dorsum with 

 bristles on the humeri and posterior margin; pleurae bare, 

 trichostical bristles present. Abdomen in both male and fe- 

 male long, slender, narrowed toward the base ; segment 1 with 

 bristles on its lateral margins; genitalia but little prominent. 

 Legs long, slender; front tibiae without a terminal claw-like 

 spur. All the posterior cells of the wings open: the fourth 

 narrowly so; anal cell closed at the margin. 



According to Macquart, the founder of this genus, the ocelli 

 are situated midway between the antennae and the vertex in 

 his type species, testacea (patria inconnue). This is true of 

 the North American forms only in leptogaster, but in indecora, 

 funesta and lineata the ocelli are higher up on the front, and 

 in unicolor much nearer to the vertex than to the antennae. 

 Their position varies with the degree of contiguity of the 

 eyes. The nearer the eyes approached each other at the ver- 

 tex, the farther down upon the front are the ocelli found. 



In North America, Plesiomma is a strictly southern genus; 

 being well represented in the West Indies, especially in Cuba; 

 our only United States form thus far known is unicolor from 

 New Mexico and Texas. 



This genus differs from Deromyia, which it somewhat re- 

 sembles, in that the eyes are nearly contiguous above, the 

 nearly central position of the styled antennas and the relative 

 position of the oral margin and the ocelli. Also in the open 

 fourth posterior cell, the more slender abdomen and legs and 

 the absence of the terminal claw-like spur from the front 

 tibia. 



Type. — Plesiomma testacea. 



SYNOPSIS OF SPECIES. 

 I have seen the types of funesta, indecora and leptogaster , 

 and specimens of lineata at the M. C. Z. and although unicolor 

 is the only Plesiomma now known from the United States, I 

 will include them in the following table. 



