Reproductive anatomy and fecundity estimation of the haematophagous ectoparasite Carnus hemapterus

The dynamics of host–parasite interactions depends to a large extent on the effect of host responses on parasite fitness. An increased research effort is currently being invested in the study of host influence on parasite fitness both at a population and at an individual level even though basic information (e.g. the reproductive anatomy of parasites) is frequently missing. Here, we study for the first time the reproductive system of the diptera Carnus hemapterus, a 2-mm long, highly mobile haematophagous fly parasitizing nestlings of a broad variety of bird species. Although this species is poorly known, it is being increasingly used for the study of host–parasite interactions. We also assess the reliability of a method to estimate fecundity based on the number of laid eggs per gravid female and analyse the effect of body size on fecundity estimates. Our results show that carnid flies are synovigenic, so that both the number of laid eggs at a given moment and the egg load represent only a fraction of the true potential fecundity. Moreover, laid eggs are probably a fraction of the number of mature eggs ready to be laid since females withheld seemingly mature eggs at oviposition. A high proportion of pregnant females did not lay eggs, and the number of eggs laid per pregnant female varied remarkably. The latter is explained partly by a positive relationship with body size (thorax length and abdomen width). Caution is needed when using the number of laid eggs as a shortcut estimation of fecundity in C. hemapterus. We propose some improvements to the method for assessing Carnus fertility.