RESUMO
For orb-web spiders, the decision to remain at a particular site or to relocate elsewhere will ultimately depend on the quality of the site. In the past, the quality of the site has been measured in terms of prey availability; spiders experiencing large numbers of prey items are less likely to relocate. However, regular web damage caused by larger nonprey animals may also contribute to the quality of a particular site. Laboratory experiments revealed that the frequency and extent of web relocation by the orb-web spider Argiope keyserlingi was influenced by the feeding regime and the rate of nonprey web damage. Daily movement patterns were influenced by web damage, and these movements were in the direction away from the source of damage. However, the cumulative distance moved during the 7 days of the experiment was influenced by the frequency with which spiders were fed. Spiders that were not given prey moved further than spiders that obtained prey. These data indicate that spiders respond to web damage on a daily basis, but the cumulative movement of spiders over a longer period is influenced mostly by the history of prey ingestion rate. Copyright 2000 The Association for the Study of Animal Behaviour.
RESUMO
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. Copyright 1999 The Association for the Study of Animal Behaviour.