How balls of blackworms avoid the knotty step

MANY ANIMALS find safety in herds, colonies, schools or swarms. But few species opt for the technique of the stringy, water-dwelling blackworm , a creature that at a few centimetres in length is far longer than it is wide. In trying times, for instance when water is scarce, tens of thousands of them can swiftly wriggle together into a tangled ball, to seek a wetter environment and save from desiccation all but the poor creatures on the blob’s periphery. How they escape that situation, however, had until recently been a mystery both to biologists and to anyone who knows about knots.Saad Bhamla, a professor of bioengineering at the Georgia Institute of Technology, has made his career investigating such unusual talents. He was first intrigued by the motion of these blackworm blobs. But another behaviour was even more beguiling. If the blob is spooked in some way, for example by a bright flash of light, it dissolves back into its thousands of constituents in mere thousandths of a second. Dr Bhamla was perplexed: how do they manage not to knot?