• encapsulation rate;
  • immunocompetence;
  • lytic activity;
  • sexual ornament;
  • sexual selection

Females of many species select their mates on the basis of the size or intensity of sexual ornaments, and it has been suggested that these provide reliable signals of a male’s ability to resist parasites and pathogens. European earwigs, Forficula auricularia, are sexually dimorphic in forceps shape and length. Male forceps are used as weapons in male contests for access to females, but recent findings suggest that females also choose males on the basis of their forceps length. In the present study, we tested the hypotheses that in the European earwig, F. auricularia, the size of forceps is correlated with immune function and that immune function differs between the sexes. We found that encapsulation rate was not correlated with the length of forceps in either sex, but was negatively correlated with body size. By contrast, lytic activity of the haemolymph increased with overall body size in both sexes but, in females, lytic activity increased with relative forceps length whereas, in males, it decreased with relative forceps length. After accounting for effects of body size, there was no remaining significant correlation in females but the negative correlation in males remained. Furthermore, we found that males had higher encapsulation rate and higher lytic activity than females, suggesting that males have stronger immune defence. The results of the study indicate that the size of forceps in male earwigs does not reliably reflect male immune defence. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 509–516.