TAXONOMIC MORPHOLOGY 21 



pubescence or squamose areas (Arhytodes) . This wealth of detail is made much 

 of in the taxonomy of the family, especially in separating species within a 

 genus. 



Staphylinidae do not present this picture, the vertexal foveae being of very 

 rare occurrence [Edaphiis). 



No comparative treatment would be complete without a discussion of the 

 pubescence. The great range of pselaphid pubescence is suggested by exami- 

 nation of the plates, where a number of setal types are illustrated. These setae 

 vary about a norm, commonly found throughout the family, which may be 

 described as an elongate, aciculate seta which is circular in cross-section. Of 

 great use in pselaphid taxonomy are the extremes developed; the length, thick- 

 ness, stiffness, amount of taper, apical contour, amount, and degree of incli- 

 nation are all of value (cf. especially Plate IV). 



One of the more striking modifications is the development of trichomes. 

 These are usually thick, tortuous bundles of golden setae which adorn the 

 apical elytral margins and the latero-basal angles of the dorsum and are not- 

 ably developed in Clavigerinae. Here the trichomes are specifically modified 

 for the conduction of a trichomal secretion which appears to be very stimu- 

 lating to their formicid hosts. Trichomes are variably developed in the Clavi- 

 gerinae, and may be absent. What appear to be homologues of clavigerid 

 trichomes, at a lower level of organization, are found in the Attapseniini, 

 also wholly myrmecophilous. 



Trichomes occur upon the abdomen of certain symphilic staphylinids 

 {Xenodusa, Lomechusa, Atemeles). Therefore the trichomes may not be con- 

 sidered as a phylogenetic character of use in the separation of clavigerines 

 from pselaphines, since (1) they are not universal in Clavigerinae, (2) they 

 occur on non-clavigerine tribes (Attapseniini), (3) occur in certain symphilic 

 staphylinids. Rather I should regard trichomes as an ecological adjustment 

 to life with social insects. 



Tufts of setae, not trichomoid in the symphilic sense but similar in ap- 

 pearance, may be found in other pselaphids, usually the male sex in some 

 Euplectus, Actium, Dalmosella or in the female sex of some Thesium. Such 

 setal tufts usually arise from a tuberculoid area and are usually abdominal. 



Some genera are wholly glabrous (Eupsenitis) ; other are nearly glabrous 

 (Eupsenina) ; others scantily pubescent {Melba, Trimiomelba) ; many are 

 normally pubescent {Euplectus, Arthmms, Reichenbaohia) ; some are heavily 

 pubescent (many Hamotv^, Hamotocellus). In the genus Pselaphus the 

 pubescence is massed upon sterna and ventral face of the head into aggregations 

 of broad, thick, membranous scales more or less agglutinated. This has been 

 described by Casey (1893), and has been aptly described as "sugary" by 

 David Sharp. 



Typically squamoid pubescence is found in the Ctenistini. Here the setae 

 are sparse, long, appressed scales. These scales are especially massed in pronotal 

 depressions, elytral apices and base of dorsum. These scales may be very 

 narrow (femora of Pilopius), or conspicuously spade-shaped (elytral apices of 



