Fish-induced variation in abdominal spine length of Leucorrhinia dubia (Odonata) larvae?

The lengths of dorsal and lateral spines on abdominal segments 4, 6 and 9 were measured on last-instar larvae of Leucorrhinia dubia (Odonata) from seven natural lentic systems containing fish and nine systems lacking fish. Larvae from systems with fish had significantly longer spines than larvae from systems without fish. In contrast, lake/pond area and pH had no effect on the length of spines. The length of the spines was not correlated with larval size, but there was a high correlation between the length of the three spines measured. Also, abundances of L. dubia larvae differed between systems, being significantly lower in systems with fish. Laboratory experiments showed that perch (Perca fluviatilis) handled long-spined larvae for significantly longer times than short-spined larvae. The results suggest that fish predators may induce morphological defences in insects.