16 NEOTROPICAL PSELAPHIDAE 



gives rise to what I have termed a ventral lobe. The ventral lobe may or may 

 not bear the lateral lobes, and its presence is regarded as a specialized con- 

 dition. The smaller apical diaphragm noted above is usually associated with 

 the median orifice. In a few cases the median lobe gives off a movable acces- 

 sory piece. This is a single, asymmetrically placed part and appears to be 

 moved by asymmetrically inserted bulbar muscles. These are the chief parts 

 of the pselaphid aedeagus to be observed at normal binocular magnification of 

 prepared specimens on points.^ Slide-mounts bring out a wealth of detail on 

 internal structure. 



The median lobe is invariably present; all other parts noted above may be 

 absent or variously modified, as is brought out by a study of the illustrations. 

 Although the aedeagus gives evidence for (a) structural modification at differ- 

 ent rates within the same tribe, (b) convergence or parallelism between genera 

 of different tribes, and (c) divergence between species within the same genus, 

 there are two definite trends of specialization to be observed in its compara- 

 tive anatomy. These are (1) simplification through loss of parts, and (2) 

 progressive asymmetry. 



The primitive pselaphid aedeagus has perfect bilateral symmetrj^ a well- 

 developed median lobe in which the diaphragms are invisible from above, a 

 right and left lateral lobe present as sclerotized parts, and no ventral lobe or 

 accessory piece. 



The specialized pselaphid aedeagus has the median lobe bilaterally asym- 

 metrical, diaphragms well-formed and visible from above, lateral lobes absent, 

 ventral lobe present or absent, accessory piece present or absent. 



Associated with these trends is the penial plate. Since it is considered 

 here as a remnant of the seventh visible sternite, and since we have shown 

 that the Pselaphidae tend towards a consolidated staphylinoid body plan, its 

 presence is primitive, and its absence specialized. Thus it is present as a twice- 

 divided plate (Sonoma tolulae) ; divided plate (Euplectus) ; a single plate 

 which must be asymmetrically moved during extrusion of the aedeagus, either 

 to the right (dextral penial plate) or to the left (sinistral penial plate) — 

 many genera previously noted belong here, and are either dextral or sinestral 

 while at least one genus (Cedius) has both dextral and sinestral species — and 

 finally this plate drops out in numerous genera. 



The aedeagus of Reichenbachia is primitive among Brachyglutini, and is 

 very similar to the Oxytelinae; Pilopius among Ctenistini, and Cylindrarctus 

 among Tychini, are primitive. These three genera have lateral lobes but each 

 presents certain specializations, thus Reichenbachia has a poorly developed 

 basal diaphragm, Pilopius has a slight apical asymmetry, and Cylindrarctus 

 a ventral lobe. The Batrisini are highly specialized, and the dorsal surface of 

 the Pilopius aedeagus is strangely batrisine. Within the large genus Batrisodes 

 a reliable key can be made to species, based on the aedeagus. In the first place, 



^ It will be remembered that when a specimen with exserted aedeagus is examined 

 under the binoculars, it is ventral surface uppermost and therefore the aedeagus, which 

 is exserted, lies with its apical end directed anteriorly and its dorsal surface uppermost. 



