Overview
The Kew Bug (Bipalium kewense), commonly designated as the hammerhead flatworm or broadhead planarian, is an extraordinary terrestrial predatory flatworm native to Southeast Asia that has successfully colonized gardens, greenhouses, and humid forests worldwide. First documented in 1878 inside the Royal Botanic Gardens at Kew near London, this bizarre creature resembles a miniature gliding serpent with a distinctive crescent-shaped head and a voracious appetite for earthworms.
Biology and Physical Characteristics
Bipalium kewense features a dramatically flattened, ribbon-like body reaching up to 30 centimeters in length, colored pale ochre to golden-buff with five prominent, dark brown longitudinal dorsal stripes. Its anterior end terminates in a broad, lunate hammerhead plate lined along its margins with dozens of microscopic sensory pits and light-sensitive eye-spots (ocelli). Remarkably, Bipalium kewense is one of the only known terrestrial invertebrates to produce tetrodotoxin—the same potent neurotoxin found in blue-ringed octopuses and pufferfish—which it uses to paralyze prey and deter predators.
Behavior and Ecology
Operating as a nocturnal ambush predator, the Kew bug tracks earthworms across damp soil using chemosensory receptors on its expanded head. Upon detecting a victim, it pins the worm with its muscular body, coats it in sticky mucus, and secretes neurotoxic secretions to subdue it. It then extends an eversible muscular pharynx from the midpoint of its ventral body, releasing proteolytic digestive enzymes that liquefy the prey’s tissues externally before sucking up the nutrient fluid. In introduced ranges, it reproduces primarily asexually by pinching off a fragment of its tail, which regenerates a new head within two to three weeks.
Conservation and Human Impact
Not evaluated by the IUCN, Bipalium kewense is considered an invasive ecological concern in many parts of North America and Europe due to its heavy predation on beneficial native earthworms. Because of its tetrodotoxin content and potential to cause skin irritation upon handling, gardening organizations advise avoiding bare-skin contact and managing garden populations through physical removal rather than chemical pesticides.
Interesting Factoid
The Kew Bug possesses miraculous regenerative powers: if sliced into several pieces, every single severed segment can fully regrow its own hammer-shaped head, brain, and organs to become a completely new living worm!
Last updated: October 2026