2.2 Violent mating: traumatic insemination in bed bugs and other  (Page 2/6)

Although its evolutionary origins are not clear, traumatic insemination most likely evolved in order to bypass the mating plug and overcome female resistance during mating (Arnqvist&Nilsson 2000). Keeping in line with the evolutionary arms race theory, female bed bugs have evolved a spermalege, a special sperm-receptacle organ in the abdomen that helps absorb trauma and reduce the damaging effects of traumatic insemination (Reinhardt et al. 2003).

Even though traumatic insemination is rare among vertebrates, there are a handful of insect species that reproduce by this mating practice. Traumatic insemination has also been observed in fruit flies, plant bugs, spiders, bat bugs, and bean weevils, to name a few (Crudgington&Siva-Jothy 2000; Wigby&Chapman 2004; Tatarnic&Hochuli 2006; Kamimura 2007; Ronn et al . 2007; Hotzy&Arnqvist 2009; Rezac 2009; Polak&Rashed 2010). While traumatic insemination occurs in these organisms, this form of mating behavior is most highly adapted and thoroughly studied in bed bugs. For this reason, this chapter will focus mostly on sexual conflict in C. lectularius . The following sections will detail the mechanisms of traumatic insemination, female adaptations, paternity of offspring, and evolutionary significance of this mating practice in bed bugs.

How does traumatic insemination work?

Before this chapter delves into specifics about the evolutionary significance of traumatic insemination in cimicids, it is necessary to understand the mechanics of this violent mating behavior. As mentioned earlier, traumatic insemination is rare, and occurs in only a few species of bugs. Insects and other various bugs have open circulatory systems, in which blood and lymph circulate together, and mix to form a substance called hemolymph. This means that in organisms with open circulatory systems, all organs, including reproductive organs, are surrounded by hemolymph, which carries oxygen and nutrients.

The fact that invertebrates have open circulatory systems is important in the reproductive function of traumatic insemination. When the male copulates with a female, he punctures the female’s abdominal region with a paramere , and injects sperm into the abdominal cavity through the wound. Because the sperm comes into direct contact with hemolymph, the sperm can travel through the hemolymph to the female’s ovaries. However, the insemination is only successful if the sperm fertilizes an ovum (Carayon 1966 as cited by Reinhardt&Siva-Jothy 2007).