Violent Mating: Traumatic Insemination in Bed Bugs and other Cimicids
Cimicids belong to a hematophagous taxon that feeds primarily on the blood of humans, birds and bats. Their best known member is the bed bug, Cimex lectularius, which parasitizes humans. In bed bugs and other cimicids, a bizarre form of mating behavior called traumatic insemination takes place in which males pierce females through the abdominal region and inseminate directly into the body cavity. This mating practice is thought to have evolved due to differences in the evolutionary interests of the two sexes, in which fitness does not increase with the number of matings in females, but does increase in males. A variety of hypotheses have arisen to explain the evolution of traumatic insemination, including bypassing the mating plug and overcoming female resistance during mating, which would be reproductively advantageous to males. Regardless of its evolutionary origin, traumatic insemination has been shown to propel male and female coevolution. In the case of the bed bug, females have evolved a spermalege, an organ that serves as a counter-adaptation to the damage caused by the males’ intromittent organs that deliver sperm during copulation. These observations have led scientists to regard cimicids, C. lectularius in particular, as ideal model systems for examining the causes and consequences of sexual selection.

Author: Christine Sun

Introduction

In the animal kingdom, mating interactions are frequently marked by conflict (Johnston & Keller 2000). In sexually reproducing organisms, both the male and the female have conflicting strategies in optimizing reproductive fitness (Morrow et al. 2003). Although male fitness increases with the number of matings, female fitness is not increased and is often lowered. At the same time, however, the two sexes must meet somewhere in the middle to be able to successfully produce offspring. As a result, sexual conflict often leads to an evolutionary arms race between males and females (Rice & Holland 1997; Morrow et al. 2003; Lessells 2006). As Dawkins and Krebs phrased it, “as swords get sharper, so shields get thicker, so swords get sharper still”(Dawkins & Krebs 1979). One organism that displays sexual conflict is the common bed bug, Cimex lectularius.

The bed bug, which belongs to the Cimicidae family, has been known to parasitize humans for more than four million years (Reinhardt & Siva-Jothy 2007). While the organism itself is relatively common, the bed bug’s particular mating behavior is rarely found in other species. Although bed bugs have fully functional reproductive tracts, they reproduce solely by traumatic insemination (Usinger 1966 as cited by Reinhardt & Siva-Jothy 2007).

During traumatic insemination, the male pierces the female’s abdomen with his knifelike intromittent organ and injects his sperm through the wound into her hemocoel, the cavity that contains the hemolymph. The sperm travels throughout the female’s hemolymph and eventually reaches the ovaries, resulting in fertilization (Carayon 1966 as cited by Reinhardt & Siva-Jothy 2007).

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.

Bed Bugs Outside of the Research Laboratory

The bed bug is one of the world’s most widely recognized insects in human history. Usually considered a pest, it has been associated with humans for more than 4 millennia (Panagiotakopulu & Buckland 1999).

Bed bugs are found in temperate environments and are found all over the world. Thus, they thrive in human environments, where they have easy and convenient access to food. When bed bugs feed on human blood, they release a type of anesthetic produced from their saliva. The itching and discomfort associated with bed bug bites is caused by an allergic reaction to this anesthetic in bed bug saliva. A small percentage of people have severe reactions to bed bug saliva, going through anaphylactic shock (Goddard & deShazo 2009).

Although bed bug bites cause discomfort and allergic reactions in humans, that seems to be the extent of their health repercussions. Studies have indicated that bed bugs are unlikely to transmit diseases from one person to another (Goddard & deShazo 2009).

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).

Homosexual Traumatic Insemination

Strangely enough, traumatic insemination is not just limited to female-male couplings. In the African bat bug Afrocimex constrictus, both sexes are subject to traumatic intromission from males. In this species, both males and females have ectospermaleges, but only females have mesospermaleges. Although the ectospermaleges of each sex differs, male bat bugs have shown symptoms of suffering from traumatic inseminations. Not only were there characteristic mating scars on the males, but there were also foreign sperm found throughout the bodies of the homosexually mated males. There is debate as to whether these same sex traumatic inseminations are a result of sexual competition or just carelessness.

Furthermore, some females in A. constrictus have developed these male spermalege structures. Males, as well as females that had the male genitalia form, experienced fewer traumatic inseminations than the typical female that had the female genitalia form. Scientists believe that females mimic the male condition in order to reduce the frequency of the costly traumatic inseminations. (Reinhardt et al. 2007).

Traumatic Insemination in C. lectularius: an Overview

In male bed bugs, the intromittent organ is highly specialized for traumatic insemination (see [link]). The organ is not only sclerotized, but also curved and needle-like, perfect for puncturing the exoskeleton (Usinger 1966 as cited by Reinhardt & Siva-Jothy 2007).

However, males do not just intromit their parameres anywhere on their partners (Reinhardt et al. 2003). In C. lectularius, females have a unique organ called the spermalege, which is thought to have evolved as a counter adaptation to the antagonistic traits of the male (see [link]). The spermalege consists of both the ectospermalege and the mesospermalege. During traumatic insemination, a male bed bug inserts his intromittent organ into the ectospermalege, a groove in the right-hand posterior margin of the fifth sclerite, and pierces the pleural membrane (Stutt & Siva-Jothy 2001). The sperm is then injected into the mesospermalege, which contains hemocytes. Once the wound at the injection site heals, a melanized scar forms (Usinger 1966 as cited by Reinhardt & Siva-Jothy 2007).

An interesting point to note is that bed bugs copulate exclusively by traumatic insemination. This may seem odd because female cimicids possess a fully functional female reproductive tract. In the past few decades, not once has a male bed bug ever been observed copulating by placing his paramere into a female’s vagina (Carayon 1966 as cited by Reinhardt & Siva-Jothy 2007, Reinhardt et al. 2003) In fact, females only use the genital tract for oviposition (Carayon 1966 as cited by Reinhard & Siva-Jothy 2007).

Scanning electron micrograph of the male intromittent organ (paramere) of Cimex lectularius. Males bed bugs have evolved a needlelike penis that they insert directly into the abdomen of their mates during traumatic insemination. Photo courtesy of A. Syred (Siva-Jothy 2006).
A male intromittent organ of bed bugs.

Reducing the Cost of Traumatic Insemination in Females

While advantageous to the reproductive success of the individual male, traumatic insemination imposes a cost on the females. When the female is ready to mate, as demonstrated by her large post-feeding body volume, she receives an average of five traumatic inseminations, not necessarily from the same male (Reinhardt et al. 2009a). The frequent wounding of female bed bugs during copulation has been shown to result in reduced lifespan and decreased reproductive output.

The evolution of the spermalege in female bed bugs is a possible counteradaptation to the harmful male traits (see [link], [link]). It has been suggested that the ectospermalege serves to alleviate the female’s costs associated with the physical piercing trauma of the male’s paramere, and that the mesospermalege restricts diffusion of the sperm within the female, thereby minimizing the female’s costs of receiving male ejaculates (Stutt & Siva-Jothy 2001).

Inter-species Traumatic Insemination

Traumatic insemination between different species has also been documented. For example, researchers have observed male Cimex hemipterus traumatically inseminate Cimex lectularius, a different cimicid species. Traumatic insemination between these two different cimids prompts an immune response in the female; the female will swell up at the site of the wound in response to the ejaculates. This swelling further reduces the female’s lifespan, and there are even some cases where the swelling reaction causes immediate death. In addition to reduced longevity, fertile egg production in a female C. lectularius is also reduced when she mates with a male C. hemipterus. Among a population consisting of both C. lectularius and C. hemipterus, most of the female C. lectularius lay only sterile eggs when C. hemipterus make up more than 75% of the group. Scientists are unsure as to the exact reason why inter-species traumatic insemination occurs, but some have hypothesized they may occur out of carelessness or inter-species competition. (Newberry 2008).

Furthermore, there is support for the evolution of the spermalege resulting at least partly from selection to reduce the costs of mating-associated infection (Morrow & Arnqvist 2003; Reinhardt et al. 2003). Bed bugs spend the majority of their lifetime in dark and cramped crevices in the walls. These small spaces not only house many bed bugs, but they also contain their feces and dead bed bugs, making the bed bug’s living area very unsanitary. Therefore, when a male bed bug pierces the female with his paramere to deliver his sperm, he also introduces pathogens, which have potential to cause infections in females (Reinhardt et al. 2005).

Researchers have demonstrated that the spermalege has adaptive value in relation to traumatic insemination because female bed bugs that have been stabbed in the spermalege with a needle contaminated with bacteria have greater egg production and longevity than those stabbed elsewhere on the abdomen (see [link]). Because egg production and longevity are indicators of an individual’s fitness, the greater the egg production and longevity in a female, the higher the fitness is for that same female.

These findings demonstrate that the spermalege serves as an organ with immune function. The source of this protection comes from the abundance of spermalege dwelling phagocytic hemocytes, which can kill the pathogens that enter the female’s body during traumatic insemination (Reinhardt et al. 2003; Siva-Jothy 2006).

In C. lectularius, competition among males for mates is thought to have led to the evolution of traumatic insemination by males, followed by the evolution of the spermalege in females, which in turn selected for cooperation by the male with regards to pierce the spermalege, where it would do the least amount of harm. It is not in the interest of the male bed bug to do unnecessary harm to his partner. Given that male fitness also depends on female survival, male bed bugs want the female to live long enough to lay at least some eggs fertilized with his sperm (Reinhardt et al. 2003).

Drawing of an abdomen of a typical female C. lectularius with an enlarged photograph of the ectospermalege, the site of traumatic insemination. Photo courtesy of M. T. Siva-Jothy (Siva-Jothy 2006).
female bed bug abdomen
Traumatic insemination in C. lectularius. The male only inseminates the female at the spermalege, which is most likely a counteradaptation that reduces the damaging effects of male sexual behavior. Photo courtesy of R. Ignell.
An image of a female being forcefully inseminated by a male bed bug.
Female bed bugs have evolved a specialized organ, the spermalege, in the abdominal region that receives sperm injected into them during traumatic insemination. Females that are pricked in the spermalege with a contaminated needle produce more eggs than those penetrated with a contaminated needle elsewhere. SS= sterile needle in spermalege, SA= sterile needle in abdomen, CS= contaminated needle in spermalege, CA= contaminated needle in abdomen. After Reinhardt et al. 2003.
A chart of the egg laying rate of female bed bugs inseminated in different ways.

Paternity of Offspring Produced by Traumatic Insemination

The males of many animals adjust their ejaculate size according to the probable mating status of females because relative sperm numbers can determine paternity outcomes in a system of polyandry, in which one female copulates with multiple partners (Simmons 2001 as cited by Siva-Jothy & Stutt 2003; Parker 2008). Male bed bugs are no exception to this phenomenon. They can detect the mating status of females through chemoreceptors located on their intromittent organs. These receptors allow the male bed bug to sense the presence of ejaculates in the female. If a male bed bug perceives that he is not the first male to mate with a female bed bug, he will copulate for a significantly shorter amount of time than if he were the first male (see [link]) (Siva-Jothy & Stutt 2003).

Protecting Sperm: Bacteriolytic Activity in Male Ejaculate

As mentioned in the chapter, the spermalege contains phagocytic hemocytes that provide immunity by killing pathogens foreign to the female bed bug. Thus, as a male bed bug ejaculates are introduced to the female’s hemocoel, many components of his semen, including sperm, are under threat of attack from the female’s immune system (Reinhardt et al. 2003; Siva-Jothy 2006). Furthermore, contact with microbes during traumatic insemination can also damage sperm and reduce male reproductive success as well (Otti et al. 2009).

Thus, in C. lectularius, males have been selected to protect their sperm. Research has shown that male med bugs have bacteriolytic activity (lysozyme-like immune activity [LLA]) in their ejaculates.

While LLA is found in the seminal fluid of male bed bugs, its antimicrobial effects may be beneficial to both the male and the female, given that the female is introduced to potentially harmful microbes during sexual transmission (Otti et al. 2009; Reinhardt et al. 2000b).

However, what is interesting to note is that while a male who mates with a virgin female bed bug has a significantly longer copulation duration and ejaculate size than a male who mates with the female afterwards, the latter actually has a higher fertilization success, with a 68% last-male sperm precedence in C. lectularius after two matings (Stutt & Siva-Jothy 2001). One explanation for the longer copulation duration between a male and a virgin female is that it is possible the first male’s ejaculate is subjected to disproportionate phagocytic attack from hemocytes in the female’s spermalege. Therefore, selection via sperm competition may favor first males who have a large ejaculate size (Siva-Jothy & Stutt 2003).

Mean copulation durations for virgin males sequentially mated to females. The female’s first copulation in a bout of copulations last significantly longer than subsequent copulations. (p< 0.0001) After Siva-Jothy & Stutt 2003.
Duration of subsequent copulation sessions.

Conclusion: Evolutionary Significance of Traumatic Insemination

Consistent to the evolutionary arms race theory, both male and female bed bugs have evolved certain adaptations and counter-adaptations against each other in order to optimize their own reproductive fitness (Morrow et al. 2003). Although the origins of traumatic insemination are unclear, many hypotheses have been proposed, including bypassing the mating plug and overcoming female resistance during copulation, which would be reproductively advantageous to males (Parker 2008). Although we may never know its origins, the studies surrounding traumatic insemination in C. lectularius have truly provided valuable insight into sexual conflict and further research should be conducted to get an even clearer picture.

Discussion Questions:

  1. Why is the spermalege considered a counteradaptation? In what ways does the spermalege benefit the female?
  2. Can you think of a scenario in which the traumatic insemination and evolutionary arms race in the bed bug would be reversed due to selective pressures? In what scenario would increased matings benefit females?

Glossary

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Biography

A portrait of the author in a lab.

Christine Sun was born in Gaithersburg, MD and currently attends Rice University in Houston, TX. There, she is a sophomore double majoring in Biochemistry and Asian Studies. She loves to travel, and recently came back from a service trip to Taiwan. It was actually because of the numerous bed bug bites she received in Taiwan that compelled her to go to Wikipedia to research bed bugs in the first place. Although she does not think bed bugs are the most pleasant creatures, she finds their reproductive behavior fascinating.