Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?


Frédéric Thomas, Centre d'Etude sur le Polymorphisme des Micro-Organismes, CEPM/UMR CNRS-IRD 9926, Equipe: `Evolution des Systèmes Symbiotiques', IRD, 911 Avenue Agropolis, B.P. 5045, 34032 Montpellier Cedex 1, France. Tel: +33 4 6741 6232; fax: +33 4 6741 6299; e-mail:


Several anecdotal reports in the literature have suggested that insects parasitized by hairworms (Nematomorpha) commit `suicide' by jumping into an aquatic environment needed by an adult worm for the continuation of its life cycle. Based on 2 years of observations at a swimming pool in open air, we saw this aberrant behaviour in nine insect species followed by the emergence of hairworms. We conducted field and laboratory experiments in order to compare the behaviour of infected and uninfected individuals of the cricket Nemobius sylvestris. The results clearly indicate that crickets infected by the nematomorph Paragordius tricuspidatus are more likely to jump into water than uninfected ones. The idea that this manipulation involved water detection from long distances by infected insects is not supported. Instead, our observations suggest that infected insects may first display an erratic behaviour which brings them sooner or later close to a stream and then a behavioural change that makes them enter the water.