ANIMAL BEHAVIOUR, 1999, 58, 1241–1246 Article No. anbe.1999.1255, available online at http://www.idealibrary.com on Asymmetry in spider orb webs: a result of physical constraints? M. E. HERBERSTEIN* & A. M. HEILING† *Department of Zoology, University of Melbourne †Institute of Zoology, University of Vienna (Received 21 May 1999; initial acceptance 17 June 1999; final acceptance 12 August 1999; MS. number: 6236) A typical feature of most vertical orb webs is that the upper web region is smaller and contains less silk than the lower web region, creating an asymmetrical web. The degree of web asymmetry changes during the spider’s development: small juveniles construct more symmetrical webs, but older and larger individuals decrease the upper web region. This implies that weight may control the extent of web asymmetry. Using two species, Argiope keyserlingi and Larinioides sclopetarius, we tested the effect of weight increase on web asymmetry by naturally increasing weight through feeding and by artificially adding lead weights to the abdomen of the spiders. Weight increase (natural or artificial) resulted in more asymmetric webs through a reduction of the upper web region. Added weight may interfere with spiral placement in the upper region, because the spider has to lift its abdomen above the carapace during the process. In the lower region, however, the position of the spider is mostly head up during spiral placement. Therefore, amongst other factors, weight and gravitational forces may be physical constraints during web construction.  1999 The Association for the Study of Animal Behaviour Orb web spiders are sit-and-wait foragers that, in contrast to actively foraging spiders such as Lycosidae or Saltici- dae, invest little energy in searching for prey (Shafir & Roughgarden 1998). Instead, the main energy investment is in silk synthesis and the construction of their webs (Higgins & Buskirk 1992). In general, the orb webs of araneid spiders are vertically oriented and consist of a combination of nonsticky frame threads and radii, which provide the bases for an overlay of adhesive capture threads (Fig. 1). The capture ability of orb webs, which directly affects the prey intake of the spider, is determined by the amount of silk invested but also by the arrange- ment of the capture threads. The process of web construc- tion and the design of the web itself are therefore key aspects of the spider’s foraging behaviour. Variation in the design of orb webs is apparent on several levels. First, there are considerable differences between species, although this level of variation is not a reliable phylogenetic tool per se (e.g. Levi 1978a, b). Second, web design varies between individuals of the same species which may, for example, be due to differ- ences in leg length (e.g. Vollrath 1987; Krink & Vollrath 1999). Third, individuals may vary their webs from day to day, based on various abiotic factors (e.g. Vollrath et al. 1997) and biotic factors such as satiation level, prey capture rates, prey type or egg production (e.g. Sandoval 1994; Sherman 1994; Pasquet et al. 1999; Herberstein et al., in press). A fourth level of variation exists within the web itself. The majority of vertical orb webs are not perfectly symmetric circles. The area above the web hub is generally smaller and contains less silk than the area below the hub (e.g. Vollrath & Mohren 1985; Foelix 1992; see Fig. 1). Some species such as Nephila or Phonognatha even reduce the upper web region to only a few support threads and enlarge the lower web region (e.g. Robinson & Robinson 1973). The adaptive value of web asymmetry may be improved prey capture success. Spiders typically sit head down in the web and locate prey faster when they face the source of vibration in the web (Kla ¨rner & Barth 1982). If the prey is not entangled directly below the hub, the spider changes its position to face the source of vibration, presumably to locate and approach the prey accurately (Landolfa & Barth 1996). In addition, spiders travel to prey in the lower web region more quickly than to prey in the upper web region because of the effect of gravity (Masters & Moffat 1983; ap Rhisiart & Vollrath 1994). Consequently, the lower web region may be of greater prey capture value and the spider therefore increases this section of the web and decreases the extent of the less successful upper web region. The degree of web asymmetry may reflect a compro- mise between prey capture efficiency and structural Correspondence: M. E. Herberstein, Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia (email: m.herberstein@zoology.unimelb.edu.au). A. M. Heiling is at the Insti- tute of Zoology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria. 0003–3472/99/121241+06 $30.00/0  1999 The Association for the Study of Animal Behaviour 1241