Discrete dragline attachment induces aggregation in spiderlings of a solitary species
Section snippets
Study species
Larinioides cornutus (Clerck) is a common orbitel ecribellate araneomorphae spider (male: 5–8 mm without legs; female: 6–12 mm) living in humid lands (Ysnel 1992). In the field, the spiderlings undergo a transient gregarious phase lasting about 10 days (F. Ysnel, personal communication) and during which individuals cluster in a silk network close to the cocoon before dispersing. Inseminated females were collected during the winters of 1999–2000 and 2000–2001 near Nailloux and Soumensac, France
Dragline attachment
There was no correlation between the order of the interval and its length (Spearman rank correlation: for 27 spiders: −0.51<rS<0.47, 10<N<22, NS; for one spider: rS=−0.48, N=23, P=0.02; for one spider: rS=0.58, N=23, P=0.003; for one spider: rS=0.47, N=18, P=0.048). There was no significant difference in the distribution of lengths between the six cocoons (median test: χ52=9.7, P=0.09). Therefore, we pooled the data recorded for the 30 spiders (N=468 length intervals); the resulting
Model description
We used a spatially explicit agent-based numerical model, in the form of Monte Carlo simulations, to test the hypothesis that the discrete pattern of silk attachment and the resulting creation of additional escape routes were the key mechanisms leading to an asymmetrical distribution of spiderlings between the branches. In the model, as in the experiments, individuals following each other in a sequential order were set in motion at the basis of the vertical thread, and only one spider had
Discussion
Spiderlings of L. cornutus were able to maintain group cohesion when they migrated to a new site of aggregation. Our experiments, confirmed by a theoretical study, show that the collective choice of groups of juveniles of this solitary spider resulted mainly from their pattern of dragline attachment and the resulting creation of additional escape routes without active silk-following behaviour. When a spiderling encountered a silk shortcut, it chose with a nearly equal probability either the
Acknowledgements
We thank Bertrand Krafft and Frédéric Ysnel for fruitful discussions and Christian Jost and two anonymous referees for their comments on the manuscript. R.J. was supported by a doctoral grant from the French Ministry of Scientific Research. This work was supported in part by the Programme Cognitique from the French Ministry of Scientific Research.
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J.-L. Deneubourg is at the Center for Nonlinear Phenomena and Complex Systems, Université Libre de Bruxelles, C.P. 231, Campus Plaine, B-1050, Brussels, Belgium.