Tribe 15. Arhytodini 



This is a monogeneric tribe of eleven species confined to South America. 

 Structurally it is of great importance as the following points will demonstrate. 



The body is elongate, ovoidal, with the abdomen broad, and the body dorso- 

 ventrally flattened. The pubescence is of two types: the general body setae are 

 moderately elongate, arcuate, and more or less sparse and semierect; in addition 

 foveae, sulci, many sutures and other areas, as described later, are filled with 

 very short, appressed, matted scales which have a surgary appearance and make 

 a direct approach to the Ctenistini. 



Head elongate, flattened, with clearly marked antennal tubercles which are 

 separated by a short frontal sulcus; the vertex bears a Y or V-shaped sulcus; 

 postmedian eyes of large facets. The antennae are eleven-segmented, with the 

 last three antennomeres forming a poorly marked club. The shape and size of 

 the antennomeres offers an excellent separation of the species. 



The mouth-parts are peculiar. The large submentum-mentum is nearly 

 vertical, almost wholly covering the oral aperture. The maxillary palpi are 

 minute, apparently of one segment which is thin, pedunculate, basally arcuate, 

 apically inflated to present a rounded apex holding a terminal seta. This latter 

 may be the homologue of the palpal cone of other tribes. The mandibles are 

 well-developed, elongate, with four or five teeth. The labrum is excessively 

 transverse and short as a rule. Thus the rudimentary maxillary palpi and the 

 occluding mentum form a direct approach to the rudimentary mouth-parts of 

 Clavigerinae, while the long and narrow, but well-toothed mandibles, suggest 

 the primitive predaceous habit of the family. Since at least one species is known 

 to be myrmecophilous we may have here a group of species well on the road to 

 social parasitism, in which case the obviously primitive (mandibles) , and highly 

 specialized (mentum, labium, maxillae, and associated palpi) mouth-parts are 

 nicely balanced. 



Pronotum usually wider than long, with the disc usually longitudinally 

 gibbous and subcarinoid as in some Euplectini and Batrisini; antebasal, strongly 

 biarcuate to V-shaped sulcus present. 



Elytra with two basal foveae, and several striae some of which vary in 

 length among the species. 



Abdomen with wide, flattened margins; five visible tergites and six stemites. 

 Tergites often gibbous or diversely tuberculate. 



Metastemum more or less longitudinally sulcate. Anterior femora with the 

 antero-ventral face more or less carinoid and produced, especially in the male 

 sex. The long, slender, typically macrosceline legs have tarsi which are unique 

 in the Pselaphidae. The three-segmented tarsi have a short first tarsomere ; sec- 

 ond much longer, thick, arcuate-obconical, with the apical face very oblique, 

 as is the first segment, and ventrally bilobed; third tarsomere short, conical, 



(343) 



