An urban myth among zoologists states that male bed bugs traumatically inseminate other males and gain a fitness benefit from doing so. The benefit arises, so the story goes, because the ‘donor's’ sperm migrate to the recipient's testes and subsequently form part of the recipient's ejaculate.
Observation of recently fed adult males and females shows they are very similar in size (M. T. Siva-Jothy 2005, unpublished data) and appearance (). When reproductively active males are given access to recently fed individuals of both genders they show no tendency to mount one gender more than the other (χ2
>0.05). The observation that fed males are rapidly and frequently mounted by other males may well have been the origin of the ‘ejaculate parasitism’ story. However, despite the apparent inability of males to distinguish fed males from fed females prior to mounting, it is clear that they do not treat the genders similarly once mounted. Mounting males remain on virgin females for longer (93±6
=50; mean±s.d.) than they do on fed virgin males (4.4±0.6
=20; mean±s.d.; U
<0.0001). Moreover, examination of the mounted individuals after this process revealed all the mounted females (n
=50) contained sperm in the mesospermalege, while none of the mounted males (n
=20) showed mating scars or any sign of sperm in the haemocoel. It, therefore, appears that male C. lectularius
assess the gender of potential mates after they have mounted, but do not impose collateral costs other than sequestering relatively short periods of time on the fed males they mount.
(a) Adult female C. lecularius immediately after feeding to satiation. (b) Adult male C. lecularius immediately after feeding to satiation.
In contrast, males of the cimicid Afrocimex constrictus have a clearly defined ectospermalege (c), the external manifestation of the female's paragenitalia. In this species selection appears to have been sufficiently strong on males to select for the development of functional external female paragenitalia. Comparisons of mated and virgin adults collected from the wild show that females bear prominent melanized mating scars (). Dissection of the female in b revealed the scar tissue in the posterior ectospermalege (this species has two ectospermaleges) to be a very hard plug of melanized tissue that would have been difficult to penetrate. Randomly captured adult males also showed 6.3±2.9 (mean±s.d.; n=31) visually prominent ‘mating’ scars in their ectospermalege (d). Although males show fewer scars than females (female scars were so numerous that, it was impossible to distinguish individual scars and so score the damage accurately) there is clear evidence that males are wounded. Exactly why males insert their paramere into other males is unclear at present, but field observations suggest pre-mating behaviour is similar to C. lectularius, and fed males are very similar in appearance to fed females. Whatever the causal basis of ‘intrasexual traumatic intromission’, it is clear that male A. constrictus are at least paying wound healing costs and are likely to be paying immune system activation costs as well. This species represents the first example of a male conflict trait that causes collateral damage in a gonochorist species.
Figure 6 (a) Virgin adult female Afrocimex constrictus. The box encloses the region of the paired ectospermalege, which bears no mating scars in this unmated individual. (b) An adult female A. constrictus collected from the field. The box encloses the ectospermalege, (more ...)