Love Hurts

People generally know distressingly little about insects. The gossip section of the weekly telly guide with the paper on Saturday mentioned a comment by Jerry Seinfeld on an upcoming movie of his featuring bees: "Other insects are just kind of crawling around. They don't have the sophistication of the bee. They have no crime, they have no drugs, they have no rape. A little rape, but it's not that bad." Leaving aside the question of the propriety of these comments, I have one word to say about their accuracy: bedbugs. Welcome to the world of Traumatic Insemination.

Male bedbugs (Cimicidae) have a sharpened intromittent organ (or if you prefer, a great spike on their knob). The usual means of entry is ignored in mating - rather, the female genital tract is only used in egg-laying (Stutt & Siva-Jothy, 2001). Instead, the male uses his sharpened organ to pierce through the female's body wall and inject semen directly into the body cavity.

Males and females do not always have the same aims in sexual reproduction. Because the limits on reproduction rates for males are relatively minimal, it is generally in the interest of males to maximise their insemination rate, in order to maximise the number of offspring they produce. Females, on the other hand, are more likely to have a maximum reproductive rate limited by the number of offspring they can safely produce in a given time period. Therefore, it is more advantageous for them to limit fertilisation to the best males to maximise the health of their offspring. As well as the obvious restrictions a female can place on fertilisation by limiting the ability of males to mate with her, the genital tract of females often has adaptations to 'test' sperm. The genital tract itself may be hostile to sperm survival (by being highly acidic, for instance). There may be structures such as a bursa copulatrix that store and/or digest sperm, limiting their access to further parts of the tract.

It is suspected that traumatic insemination evolved in males to bypass these restrictions on the part of the females. A good demonstration of this is seen in bugs of the family Nabidae, where entry into the female is still by the genital tract, but the sharpened intromittent organ pierces the wall of the bursa copulatrix (Tatarnic et al., 2006). Experimental evidence has shown that in traumatic inseminations the fertilisation advantage is held by the last males to mate with a female (Stutt & Siva-Jothy, 2001).

Nevertheless, females are not without defenses. While female bedbugs show a surprising lack of behavioural defenses against mating, they have developed an entirely novel paragenital system called the spermalege. In the common bedbug (Cimex lectularius), the external part of the spermalege is a special notch and a thickened part of the cuticle. Internally, there is a pocket filled with cells that receives the male ejaculate. Morrow & Arnqvist (2003) demonstrated that traumatic insemination through the spermalege had relatively little effect on the health of the recipient females. However, if the body wall was pierced anywhere else the female's survival rate was severely compromised. The internal part of the spermalege probably fulfils the sperm-killing role of the usual genital tract, as well as reducing the direct exposure of the female body cavity to potentially harmful ejaculate (Morrow & Arnqvist, 2003).

Different genera of bedbugs show a wide range of variation in the complexity of the paragenital system, from species entirely lacking one to species in which the system is exceedingly complex. In at least one genus, Afrocimex, a spermalege is present in both males and females (Tatarnic et al., 2006). It has been suggested that the presence of a spermalege in males defends against damage from homosexual matings - male bedbugs apparently tending to stab first and check suitability afterwards.

REFERENCES

Morrow, E. H., & G. Arnqvist. 2003. Costly traumatic insemination and a female counter-measure in bed bugs. Proceedings of the Royal Society of London Series B - Biological Sciences 270: 2377-2381.

Stutt, A. D., & M. T. Siva-Jothy. 2001. Traumatic insemination and sexual conflict in the bed bug Cimex lectularius. Proceedings of the National Academy of Sciences of the USA 98 (10): 5683-5687.

Tatarnic, N. J., G. Cassis & D. F. Hochuli. 2006. Traumatic insemination in the plant bug genus Coridromius Signoret (Heteroptera: Miridae). Biology Letters 2: 58-61.