The Mystery of the Wandering Daddy Longlegs

Carl Zimmer in his blog, The Loom:

Here is a lovely little creature from Sri Lanka, Pettalus cf. cimiciformis, a member of the same lineage that includes the daddy longlegs we're all familiar with. You could call it a daddy longlegs too, but its legs aren't particularly long (plus it's tiny--the size of a sesame seed.)

It may not seem like much, but it poses a fascinating riddle. It belongs to a family of daddy longlegs called Petallidae. Below is a map of where other species of Petallidae can be found. They seem to be scattered randomly across the world. But petallids are terrible at dispersing. Their ranges are small (usually less than fifty miles). And they live only on ancient continent crust. Petallids live on Sri Lanka and on Madagascar. But they live on none of the young volcanic islands in between--or anywhere else in the world, for that matter. So they couldn't have swum or flown to their farflung locations. Yet DNA evidence clearly shows that the petallids all descend from a common ancestor. So, how did they get there? The answer...

More here.