Trapped indoors? Long-distance dispersal in mygalomorph spiders and its effect on species ranges.

The Mygalomorphae includes tarantulas, trapdoor, funnel-web, purse-web and sheet-web spiders, species known for poor dispersal abilities. Here, we attempted to compile all the information available on their long-distance dispersal mechanisms from observations that are often spread throughout the taxonomic literature. Mygalomorphs can disperse terrestrially, and in some tarantulas, for example, spiderlings walk together in single files away from their maternal burrow, a mechanism limited in distance covered. Conversely, at least eight species disperse aerially, via dropping on drag lines from elevated positions and being passively blown off ('suspended ballooning'). The first record of this behaviour is 135 years old, but we still know very little about it. Phylogeographic studies suggest several occurrences of transcontinental dispersal in the evolutionary history of mygalomorphs, but these might result from contingent rafting events, rather than regular dispersal mechanisms. We use occurrence data to show that suspended ballooning increases the species ranges in Australian mygalomorph families where this behaviour has been recorded. We also identified Anamidae, Idiopidae, and especially Atracidae, as families that might employ suspended ballooning or another efficient but undiscovered dispersal mechanism. Finally, we suggest that molecular studies with mitochondrial genes will help disentangle behavioural limitations of dispersal from ecological or physical ones.