Acta Zoologica (Stockholm) 86: 33– 40 ( January 2005) © 2005 The Royal Swedish Academy of Sciences  Abstract Huber, B.A., Rheims, C.A., Brescovit, A.D. 2005. Two new species of litter- dwelling Metagonia spiders (Araneae, Pholcidae) document both rapid and slow genital evolution. — Acta Zoologica (Stockholm) 86: 33–40 The observation that genitalia, rather than other characters, are useful in distinguishing species has resulted in the statement that genitalia evolve relatively rapidly. In this paper we claim that relatively rapid evolution of genitalia may often be restricted to the shapes, numbers, and sizes of individual structures. In contrast, the more basic pattern, including the presence or absence of structures and their interrelationships (the bauplan), does not seem to evolve more rapidly than other (nongenital) aspects of morphology. We document this idea by contrasting two litter-dwelling pholcid species, Metagonia petropolis sp. n. and M. paranapiacaba sp. n., with their mostly leaf- dwelling congeners. Significant differences occur with regard to nongenital aspects of morphology as well as shapes of individual genital structures, but the bauplan of their genitalia is basically the same. Bernhard A. Huber, Zoological Research Institute and Museum Alexander Koenig, Adenauerallee 160, 53113 Bonn, Germany. E-mail: b.huber.zfmk@uni-bonn.de Blackwell Publishing, Ltd. Two new species of litter-dwelling Metagonia spiders (Araneae, Pholcidae) document both rapid and slow genital evolution Bernhard A. Huber, 1 Cristina A. Rheims 2 and Antonio D. Brescovit 3 1 Zoological Research Institute and Museum Alexander Koenig, Adenauerallee 160, 53113 Bonn, Germany; 2 Laboratório de Artropódes, Instituto Butantan, Avenida Vital Brazil, 1500, CEP 05503–900, São Paulo, SP, Brazil; 3 Departamento de Zoologia, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil Keywords: Araneae, genitalia, leaf-litter, Metagonia, Pholcidae, rapid evolution Accepted for publication: 27 January 2005 Introduction There is a wide consensus that genitalia and other sexual- contact structures commonly evolve more rapidly than other morphological characters (reviews in Eberhard 1985; Hosken and Stockley 2004). This relatively rapid evolution, coupled with a relative uniformity of genitalia within popu- lations and supposed reproductive communities (Eberhard et al. 1998; Palestrini et al. 2000; Tatsuta et al. 2001), explains the importance of genitalia in species discrimination in many animal taxa, including spiders (Huber 2004). Nongenitalia, on the other hand, have traditionally been preferred by tax- onomists for the delimitation of genera and higher categories (Platnick 1975: ‘As is usual in spiders, the genera are defined by somatic characters and the species groups by genital characters’; see also Griswold 1993; Foelix 1996). How- ever, in one important sense, genitalia may evolve relatively slowly rather than relatively rapidly compared with other characters, i.e. with regard to their bauplan, which probably reflects the mechanics of copulation. The high percentages of genital characters in recent matrices for cladistic analyses document this conservative aspect of genital evolution at species-group, generic, and higher levels (an average of 58% of characters in the eight matrices cited in Huber 2004). In pholcids, some of the most valuable characters at family and subfamily levels refer to genital or other sexual-contact structures (e.g. characters 36, 40, 47 and 52 in Huber 2000). Further support comes from individual cases of slow genital evolution coupled with prominent changes in nongenital morphology (Huber 1998, 2002). The present paper describes two new species of Metagonia that illustrate (1) species- specificity in the details, i.e. rapid evolution of genitalia with respect to shapes and sizes of individual structures, and (2) conspicuous changes in overall morphology because of changes in ecology without any effect on the genital bauplan of the genus, i.e. relatively slow evolution of genitalia with respect to the basic bauplan. The genus Metagonia Simon, 1893 is one of the most species- rich pholcid genera in the Neotropics. The present count is at about 80 nominal species, but dozens of undescribed