EUPLECTINI 83 



All members of Rhexius have a common habitus (PI. VIII, 6-9) which af- 

 fects the antennae and articulation of the head to the pronotum. This is so 

 striking that once seen it is seldom forgotten. The eleven-segmented antennae 

 have the basal segment very elongate, varying from one-half of the total anten- 

 nal length to as long as segments II to VI inclusive. The antennae are typically 

 geniculate, the arc being between the first two segments. 



The articulation of head and pronotum is especially diagnostic. The very 

 transverse head narrows abruptly to a small subcylindrical neck. This peduncle 

 is closely invested by the exceptionally small anterior lobe of the pronotum, 

 in the manner of a bearing, and the large posterior lobe of the pronotum seems 

 to constitute the pronotum as a whole, the anterior lobe appearing as a part 

 of the head. Sharp (1887, pp. 40-41) describes the condition succinctly, "the 

 structure of the prothorax is most remarkable, its anterior part being con- 

 stricted so as to form a short tube in which the head is inserted by a very 

 short neck; this thoracic tube has the appearance of being only a development, 

 rather greater than usual, of the neck of the head. . . ." 



Six stemites in both sexes. The sixth stemite is usually medianly prominent 

 and sinuate in the male; rounded in the female. The males usually have the 

 anterior femora carinated on the internal face. Tarsi with two very unequal 

 claws, of which the first is very large, arcuate and acute, and the accessory 

 claw very thin, and not more than half as long. 



Key to the Species 



Antennae with first segment as long as II to X inclusive umied. elegans 

 (1.4 mm.; Sao Paulo, Brazil. Male metas- 

 ternum with a minute median impression) 



Antennae with first segment not this long 2 



2. First visible tergite very long, longer than second and third tergites 



united brasiliensis 



(Amazon basin, Brazil) 

 First visible tergite relatively short, about as long as the second 



tergite optatus 



(1.5 mm.; 4000-6000 ft., Volcan de Chiriqui, Panama) 

 The species of Rhexius inhabiting the neotropics are: 



brasiliensis Schaufuss. 1872. Brazil. 

 elegans Raffray. 1909. Brazil. 

 optatus Sharp. 1887. Panama. 



MITRACEPHALA (Raffray, 1890) 



Raffray (1890, 1904) 



This and the next genus are so specialized that they show little close af- 

 finity with others of the tribe. The present genus, founded on a single peculiar 

 species, may be generally described as follows: 



