Discrete dragline attachment induces aggregation in spiderlings of a solitary species

In the early stages of their life, solitary spiders undergo a transient gregarious phase. We designed a series of experiments in which collective displacements were induced in groups of spiderlings of the solitary species Larinioides cornutus (Araneae: Araneidae). Spiderlings were given access to a bifurcated escape route (Y-choice set-up) from a container. Consecutive passages in the set-up led to an asymmetrical distribution of individuals between branches. Individual behaviours of spiderlings were quantified and then implemented into a model with which we simulated collective displacements. Comparison between the outcome of the model and the experimental data shows that the discrete pattern of silk dragline attachment is the key mechanism involved in this collective phenomenon. Our results show that the collective responses in spiderlings of a solitary species and in social spiders are similar, and that a simple mechanism independent of the presence of silk attraction can lead to aggregation.