Some species of web building spiders use different capture tactics for different prey types. The main factors influencing the attack behaviour are the ability of the insect to escape, the risks of injury to the spiders and prey size. This study evaluated the effects of size and prey type on prey capture behaviour of the social spider Anelosimus eximius as influenced by the number of spiders attracted by prey movements that did not bite until the immobilization (bystanders) and the number of spiders that contributed to prey immobilization (catchers). We carried out a two-factor (prey size and type) experiment offering prey belonging to four orders: Diptera, Lepidoptera, Hymenoptera and Orthoptera, in a size gradient within each prey type. Both factors influenced the number of spiders recruited as bystanders, but only prey body size influenced the number of catchers in the subduing process. The possible advantages of the presence of bystanders around the interception site are discussed. © 2007 The Authors.1139856861Amir, N., Whitehouse, M.E.A., Lubin, Y., Food consumption rates and competition in a communally feeding social spider, Stegodyphus dumicola (Eresidae) (2000) J. Arachnol., 28, pp. 195-200Avilés, L., Causes and consequences of cooperation and permanent-sociality in spiders (1997) The Evolution of Social Behavior in Insects and Arachnids, pp. 476-499. , In: Choe, J. Crespi, B., eds). Cambridge Univ. Press, Cambridge, ppBarnard, C.J., Sibly, R.M., Producers and scroungers: A general model and its application to captive flocks of house sparrows (1981) Anim. Behav., 29, pp. 543-555Box, G.E.P., Hunter, W.G., Hunter, J.S., (1978) Statistics for Experimenters: An Introduction to Design, Data Analysis and Model Building., , Wiley, New YorkCaraco, T., Risk sensitivity and foraging groups (1981) Ecology, 62, pp. 527-531Caraco, T., Uetz, G.W., Gillespie, R.G., Giraldeau, L., Resource consumption variance within and among individuals: On coloniality in spiders (1995) Ecology, 76, pp. 196-205Clark, C.W., Mangel, M., The evolutionary advantages of group foraging (1984) Theor. Popul. Ecol., 30, pp. 45-75Ebert, D., Behavioral asymmetry in relation to body weight and hunger in the tropical social spider Anelosimus eximius (Araneae, Theridiidae) (1998) J. Arachnol., 26, pp. 70-80Gonzaga, M.O., Vasconcellos-Neto, J., Collective prey capture and feeding behaviours of Anelosimus jabaquara Levi 1956 (Araneae: Theridiidae) (2002) Behaviour, 139, pp. 573-584McMahon, T.A., Bonner, J.T., (1986) Tamaño Y Vida., , Prensa Cientifica, BarcelonaNentwig, W., Social spiders catch larger prey: A study of Anelosimus eximius (1985) Behav. Ecol. Sociobiol., 17, pp. 79-85Pasquet, A., Krafft, B., Cooperation and prey capture in a social spider Anelosimus eximius (Araneae, Theridiidae) (1992) Ethology, 90, pp. 121-133Pulliam, H.R., Caraco, T., Living in groups: Is there an optimal group size? (1984) Behavioural Ecology: An Evolutionary Approach, pp. 122-147. , In: 2nd edn (. Krebs, J. R. Davies, N. B., eds). Sinauer Associates Inc Publishers, Sunderland, MA, ppRanta, E., Peuhkuri, N., Laurila, A., Rita, H., Metcalfe, N.B., Producers, scroungers and foraging group structure (1996) Anim. Behav., 51, pp. 171-175Rita, H., Ranta, E., Peuhkuri, N., Group foraging, patch exploitation time and the finder's advantage (1997) Behav. Ecol. Sociobiol., 40, pp. 35-39Rypstra, A.L., Foraging success of solitary and aggregated spiders: Insights into flock formation (1989) Anim. Behav., 37, pp. 274-281Rypstra, A.L., Prey capture and feeding efficiency of social and solitary spiders: A comparison (1990) Acta Zool. Fenn., 190, pp. 339-343Rypstra, A.L., Prey size, social competition, and the development of reproductive division of labor in social spider groups (1993) Am. Nat., 142, pp. 868-880Rypstra, A.L., Tirey, R.S., Prey size, prey perishability and group foraging in a social spider (1991) Oecologia, 86, pp. 25-30Snedecor, G.W., Cochran, W.G., (1989) Statistical Methods, , 8th edn. Iowa State Univ. Press, Ames, IAUetz, G.W., The ''ricochet effect'' and prey capture in colonial spiders (1989) Oecologia, 81, pp. 154-159Vollrath, F., Rohde-Arndt, D., Prey capture and feeding in the social spider Anelosimus eximius (1983) Z. Tierpsychol., 61, pp. 334-340Ward, P.I., Enders, M.M., Conflict and cooperation in the group feeding of the social spider Stegodyphus mimosarum (1985) Behaviour, 94, pp. 167-182Whitehouse, M.E.A., Lubin, Y., Competitive foraging in the social spider Stegodyphus dumicola (1999) Anim. Behav., 58, pp. 677-688Willey, M.B., Jackson, R.R., Predatory of a social spider Stegodyphus sarasinorum (Araneae: Eresidae): why attack first? (1993) Can. J. Zool., 71, pp. 2220-2223