THORNY-HEADED WORM Pseudocorynosoma constrictum AMPHIPOD Hyalella azteca A tiny amphipod, Hyalella azteca, lives in obscurity at the murky bottom of lakes and ponds—unless it’s invaded by the larva of a thorny-headed worm. When the larva matures, the amphipod abandons its safe dark home and swims toward the light of the surface. For the host, it’s a fatal mistake. Waiting above are ducks and other waterfowl keen to eat the amphipods as they surface. But for the parasite—turned orange by pigments pilfered from its victim’s tissue—it’s just part of the plan. Thorny-headed worms can grow to maturity only in the guts of waterfowl.
