Kensuke Nakata

Spider orientation and hub position in orb webs

Orb-web building spiders (Araneae: Araneoidea, Uloboridae) can be considered as territorial central place foragers. In territorial central place foragers, the optimal foraging arena is circular, with the forager sitting in its centre. In orb webs, the spider’s orientation (head up or head down) whilst waiting for prey on the hub of its web and the downwards–upwards asymmetry of its running speeds are the probable causes for the observed deviation of the hub from the web’s centre. Here, we present an analytical model and a more refined simulation model to analyse the relationships amongst the spider’s running speeds, its orientation whilst waiting for prey and the vertical asymmetry of orb webs. The results of our models suggest that (a) waiting for prey head down is generally favourable because it allows the spider to reach the prey in its web on average quicker than spiders waiting head up, (b) the downwards–upwards running speed asymmetry, together with the head-down orientation of most spiders, are likely causes for the observed vertical asymmetry of orb webs, (c) waiting head up can be advantageous for spiders whose downwards–upwards running speed asymmetry is small and who experience high prey tumbling rates and (d) spiders waiting head up should place their hub lower than similar spiders waiting head down.

Upside-down spiders build upside-down orb webs: web asymmetry, spider orientation and running speed in Cyclosa

Almost all spiders building vertical orb webs face downwards when sitting on the hubs of their webs, and their webs exhibit an up–down size asymmetry, with the lower part of the capture area being larger than the upper. However, spiders of the genus Cyclosa, which all build vertical orb webs, exhibit inter- and intraspecific variation in orientation. In particular, Cyclosa ginnaga and C. argenteoalba always face upwards, and C. octotuberculata always face downwards, whereas some C. confusa face upwards and others face downwards or even sideways. These spiders provide a unique opportunity to examine why most spiders face downwards and have asymmetrical webs. We found that upward-facing spiders had upside-down webs with larger upper parts, downward-facing spiders had normal webs with larger lower parts and sideways-facing spiders had more symmetrical webs. Downward-facing C. confusa spiders were larger than upward- and sideways-facing individuals. We also found that during prey attacks, downward-facing spiders ran significantly faster downwards than upwards, which was not the case in upward-facing spiders. These results suggest that the spiders orientation at the hub and web asymmetry enhance its foraging efficiency by minimizing the time to reach prey trapped in the web.

Prey detection without successful capture affects spider’s orb-web building behaviour

Animals obtain information from past foraging experience to adjust their foraging activity according to their environment. The ability of spiders to obtain information from unsuccessful predation experiences was investigated by examining the effects on web building, a significant foraging investment, of prey detection without successful capture in the orb-web spider Cyclosa octotuberculata. Four treatments were employed: (1) successful capture and feeding: one syrphid fly was allowed to be captured and consumed by the spider on the web; (2) single prey-item detection: a syrphid fly was placed on the web to lure the spider, but was removed before capture; (3) five prey-item detection: above prey-item detection stimulus was given five times; and, (4) control: neither prey nor feeding on the web. While control spiders decreased the total thread length and capture area of their webs, prey-item detection spiders in both conditions increased them, indicating that the spider obtained information from unsuccessful predation experience to adjust their foraging investment. The fed spiders exhibited a significantly greater increase than the prey-detection-only spiders, suggesting that prey detection alone and prey detection with consumption had different informational effects. Total thread length did not differ between single and five prey-item detection spiders, but distance between two adjacent sticky spirals increased only in the former spiders, possibly because five times unsuccessful predations prevented spiders to reduce web stickiness. It suggests that the spider changed web morphology according to the number of prey detection.

Does ontogenetic change in orb web asymmetry reflect biogenetic law

Most orb web spiders face downward on the web hub, and their webs are vertically asymmetrical, that is, the lower part of the web is larger than the upper part and the ratio of the lower part to the whole web area increases as the spider grows. This phenomenon may reflect biogenetic law such that young animals exhibit a general ancestral trait whereas adults exhibit specific and derived traits. An alternative explanation is that vertical asymmetry may arise from the difference in time required by spiders to move up or down the web to capture prey. The present study tested these two hypotheses for Eriophora sagana. Subadults of this species build their webs with reverse asymmetry in that the upper part of the web area is larger than the lower part. In both subadults and adults, the upper proportion decreased with spider weight, and adult spiders built more symmetric webs. These results support the capture time difference hypothesis.

Cost of complex behaviour and its implications in antipredator defence in orb-web spiders

Complex behaviour may incur a cost. We assumed here that web-building behaviour for two species of orb-web spider, Cyclosa argenteoalba and Eriophora sagana, was more complex when their webs were asymmetric from top to bottom than when their webs were symmetric. The rationale for this assumption was that, while spiders have to adjust their spiral building behaviour in different web sectors to build asymmetric webs, they do not have to make these adjustments for symmetric webs. To estimate the costs involved in building more asymmetric webs, we measured the time taken for spiders to build orb-webs with various up-down size asymmetries and used this as a measure of the complexity of web-building behaviour. The results showed that the spiders required more time to lay the spiral threads as their webs became more asymmetric even when the length of spiral threads was the same, suggesting a time cost of processing complex information. Furthermore, we found that spiders built more symmetric webs when they perceived a risk of predation, perhaps to reduce the web-building time during which they are more vulnerable. This suggests that the cost of behavioural complexity may mediate the outcome of interspecific interactions and thus may be ecologically important.