1 2005. The Journal of Arachnology 33:1–6 BEHAVIOR OF WEB-INVADING SPIDERS ARGYRODES ARGENTATUS (THERIDIIDAE) IN ARGIOPE APPENSA (ARANEIDAE) HOST WEBS IN GUAM Alexander M. Kerr 1 : Marine Laboratory, University of Guam, Mangilao GU 96923 USA ABSTRACT. Most Argyrodes live in the webs of other spiders, stealing food from the host, scavenging small prey from the web or killing and eating the host. I observed the behavior of A. argentatus from Guam, where it is a frequent inhabitant of the large orb webs of Argiope appensa. I examined the pro- portion of time spent in different activities, whether behavior differed between the sexes and if population density of Argyrodes on a host web affects Argyrodes behavior. Argyrodes spent 55% of the time hanging immobile and inverted in the support strands at the webs’ margin. This was significantly more time than that spent in stationary activity, forward movement at the web’s margin, feeding, foraging on the sticky spiral or in aggressive interaction. Females foraged significantly more often than did males, though the sexes spent about the same amount of time feeding and in other activities. Females also engaged in more bouts of feeding and 21% of these bouts were at prey bundles prepared by the host. In contrast, males invariably foraged for small insects unnoticed by the host. Keywords: Kleptoparasite, Mariana Islands, Micronesia, Araneae Most members of the large, cosmopolitan genus of Argyrodes Simon 1864 live in the webs of other spiders. They feed on small in- sects that have gone unnoticed by the host (Whitehouse 1986), prey stolen from the host (Robinson & Olazarri 1971), the host itself (Trail 1980; Tanaka 1984; Larcher & Wise 1985) or host-web silk (Shinkai 1988; Tso & Severinghaus 1998). Argyrodes may also cap- ture prey themselves using an abandoned host web (Larcher & Wise 1985) or use their own small web (Whitehouse 1986). Despite a growing literature on the ecology of this interesting spider genus (e.g., Henaut 2000; Miyashita 2001, 2002) and the prospect of powerful comparative phylogenetic ap- proaches (Agnarsson 2002; Whitehouse et al. 2002), the behavior of most species is still poorly known. One little studied species is Ar- gyrodes argentatus O.P. Cambridge 1880, a small (adult female body length c. 5 mm) spi- der with a tall, conical, silvered abdomen that is reported from Madagascar eastward through southeast Asia to South America (Cambridge 1 Current address: Department of Ecology, Evolu- tion and Marine Biology, University of California, Santa Barbara CA 93601 USA. E-mail: alexander. kerr@aya.yale.edu 1880; Exline & Levi 1962; photo in Koh 2000). General observations of this species have been made on host webs of Argiope ar- gentata (Fabricius 1775) in Panama (Robin- son & Olazarri 1971) and Nephila maculata (Fabricius 1793) from New Guinea (Robinson & Robinson 1973). Argyrodes argentatus on the island of Guam in western Micronesia is a frequent inhabitant of the large orb webs of several species. Kerr and Quenga (2003) re- port on population variation in different host species and habitats for Guamanian Argyro- des, including A. argentatus. The most com- mon orb-weaving spider on Guam hosting A. argentatus in their webs is Argiope appensa (Walckenaer 1841) (Araneidae) (25 mm), which occurs from New Caledonia and across the tropical western Pacific to Hawaii (Levi 1983). It builds a nearly vertical planar orb web with sticky spiral strands, occasionally with cruciate or diagonal strips of white silk near the center (Kerr 1993). In this paper, I record further aspects of the behavior of Ar- gyrodes argentatus from Guam. Specifically, I asked: (1) What is the proportion of time spent in different activities? (2) How does be- havior differ between the sexes? (3) Does population density on a host web affect Ar- gyrodes behavior?