Abstract
Very few parasite species are directly predated but most of them inherit the predators of their host. Weexplored the behavioural response of nematomorph hairworms when their hosts are preyed upon byone of the commonest invertebrate predators in the aquatic habitat of hairworms, notonectids. The hair-wormParagordius tricuspidatuscan alter the behaviour of its terrestrial insect host (the cricketNemobius syl-vestris), causing it to jump into the water; an aquatic habitat is required for the adult free-living stage of theparasite. We predicted that hairworms whose hosts are captured by a notonectid should accelerate theiremergence to leave the host before being killed. As predicted, the emergence length of the worm was sig-nificantly shortened in cases of notonectid predation, but the exact reason of this response seems to bemore complex than expected. Indeed, experimental manipulations revealed that hairworms are remark-ably insensitive to a prolonged exposure to predator effluvia which notonectids inject into prey, so accel-erated emergence is not a protective response against digestive enzymes. We discuss other possibilities forthe accelerated exit observed, ranging from unspecific stress responses to other scenarios requiring consid-eration of the ecological context.