The frequent wounding of female bedbugs (Cimex lectularius: Cimicidae) during copulation has been shown to decrease their fitness, but how females have responded to this cost in evolutionary terms is unclear. The evolution of a unique anatomical structure found in female bedbugs, the spermalege, into which the male's intromittent organ passes during traumatic insemination, is a possible counteradaptation to harmful male traits. Several functions have been proposed for this organ, and we test two hypotheses related to its role in sexual conflict. We examine the hypotheses that the spermalege functions to (i) defend against pathogens introduced during traumatic insemination; and (ii) reduce the costs of wound healing during traumatic insemination. Our results support the 'defence against pathogens' hypothesis, suggesting that the evolution of this unique cimicid organ resulted, at least partly, from selection to reduce the costs of mating-associated infection. We found no evidence that the spermalege reduces the costs of wound healing.

Optimal male and female mating rates rarely coincide. Males often shift the rate in their favor by either increased signaling and by overcoming female resistance to copulation. The concept of sensory exploitation posits that males produce signals that mimic naturally selected benefits and so deceitfully attract females. However, males also have to overcome female resistance to actual copulation. Males may do so by copulating during situations when the female's ability to resist is decreased because of competing naturally selected demands. Males of the common bedbug, Cimex lectularius, an obligate blood feeder, mate at a rate, and in a manner that is harmful to females. Females have to feed regularly to produce eggs, and during feeding female body volume increases by 300%. Choice trials using unfed and either fed or experimentally enlarged but unfed females showed that the increased postfeeding body volume of females attracted more male mating attempts, strongly reduced female resistance to male mating attempts and resulted in a net increase in female mating rate. Our results, therefore, suggest that males have increased mating success in a situation that females cannot avoid because it is naturally selected. Such "situation exploitation" of low resistance may be a common phenomenon. K E Y W O R D S :Body size, mating rate, sensory bias, sensory exploitation, sexual conflict.

Abstract.  1. Very little attention has been paid to the importance of environmental microbes and how they gain access to potential hosts during host wounding or sexual interactions. Male members of the family Cimicidae (including the haematophagous human pest, the bed bug, Cimex lectularius) show traumatic insemination: in order to transfer sperm the male punctures the female's cuticle using a needle‐like intromittent organ. The microbes from the local environment that gain entry into females during mating have previously been shown to be an important source of mortality. This study aimed to identify these microbes and their natural environmental substrates. 2. Nine different microbe taxa from the surface of the cuticle were identified, five of them (two Penicillium spp., Stenotrophomonas, Enterobacter, Bacillus) were found in the local environment as well as on the intromittent organ of male bed bugs: these are strong candidates for the causal agents of mating‐induced female mortality. Local differences in the microbial communities and their hosts' response to them are discussed as a source of reproductive isolation. 3. Four microbe genera (Scopulariopsis, Staphylococcus, Arthrobacter, Micrococcus) were found exclusively on blood agar. One of them, and four grown on regular agar, are classified as human pathogens. However, because no microbes were isolated from the piercing and sucking mouthparts the epidemiological significance of bed bugs carrying externally attached microbes is probably low.

Sexual selection, differences in reproductive success between individuals, continues beyond acquiring a mating partner and affects ejaculate size and composition (sperm competition). Sperm and seminal fluid have very different roles in sperm competition but both components encompass production costs for the male. Theoretical models predict that males should spend ejaculate components prudently and differently for sperm and seminal fluid but empirical evidence for independent variation of sperm number and seminal fluid volume is scarce. It is also largely unknown how sperm and seminal fluid variation affect future mating rate. In bedbugs we developed a protocol to examine the role of seminal fluids in ejaculate allocation and its effect on future male mating rate. Using age-related changes in sperm and seminal fluid volume we estimated the lowest capacity at which mating activity started. We then showed that sexually active males allocate 12% of their sperm and 19% of their seminal fluid volume per mating and predicted that males would be depleted of seminal fluid but not of sperm. We tested (and confirmed) this prediction empirically. Finally, the slightly faster replenishment of seminal fluid compared to sperm did not outweigh the faster decrease during mating. Our results suggest that male mating rate can be constrained by the availability of seminal fluids. Our protocol might be applicable to a range of other organisms. We discuss the idea that economic considerations in sexual conflict research might benefit from distinguishing between costs and benefits that are ejaculate dose-dependent and those that are frequency-dependent on the mating rate per se.

The rapid evolution of ejaculate components is considered to be largely driven by sexual selection. Less attention has been paid to the fact that sperm and microorganisms frequently meet; we consequently predict selection for substances that protect a male's ejaculate. We report, for the first time, bacteriolytic activity (lysozyme-like immune activity [LLA]) in the ejaculate of an animal, the common bedbug Cimex lectularius. We also show that in almost half the males LLA in the seminal fluid exceeded LLA in the hemolymph. We detected no antimicrobial peptide activity in seminal fluid. Because lysozymes degrade only bacteria, our results suggest that sperm-microbe interactions are probably important in the evolution of ejaculate components and thereby provide a route for natural selection to account for some of the diversity of seminal components.