“There is a strange ecto on this vesper”, said Jen, a sentence that only recently would have been difficult for me to comprehend. But now, after a few years of rubbing shoulders with mammalogists in Gorongosa I osmotically absorbed enough jargon to understand that she had noticed an interesting parasitic insect on a bat of the family Vespertilionidae. My ears perked up. Jen skillfully disentangled the screeching animal from the mist net and gently stretched the leg of the bat to reveal a small, fuzzy insect snuggly nestled between its fur and the naked tail membrane. Although the circumstances were unusual, considering that we were in the middle of a montane rainforest in Mozambique, and the insect was sitting on a flying mammal, a neural circuit that develops very early on during every entomologist’s training immediately fired a signal – it’s a bed bug!
To be precise the insect sitting on the bat’s body was a bat bug (Cacodmus villosus), a species common in sub-Saharan Africa and associated mostly with bats of the genus Neoromicia. These insects are indeed close relatives of the infamous human bed bugs (Cimex lectularius and C. hemipterus) and share a nearly identical morphology. Until recently entomologists thought that bat bugs spend all of their time in caves and other bat roosting sites, and only briefly visit their hosts’ bodies to feed when the bats are resting. But recent observations, supported by our find, indicate that members of at least this species of bat bugs live permanently on their host. And this is surprisingly interesting.
As it turns out, repeated feeding on the same host and in the same spot on the body can be deadly. Not only because the host is more likely to find and kill the annoying parasite, but also because the immune response from the host gets cumulatively stronger over time and greatly increases the mortality of the blood suckers. A few groups of arthropods have successfully managed to adapt (ticks, ceratopogonid and nycteribiid flies, lice, to name a few) but the initial stage of the transformation from a visiting to resident parasite must surely be difficult. This change also requires a great deal of morphological adaptation to become harder to locate and remove by the host. And the bat bug that we saw, despite being very similar in its overall form to the human bed bug, was already displaying some indication of this transition. Its body was harder and smaller than that of the bed bug, which only visits its human hosts for a few minutes every few days. The animal was also covered with long hair, which probably makes it more difficult to be grasped by a bat grooming itself; similar long setae covering the body are the characteristic of another group of ectoparasites, the bat flies (Nycteribiidae).
Bed, bat, and bird bugs, members of the family Cimicidae, are obligate hematophages – they must drink animal blood to live. It does not matter much to them whose blood they are drinking. Bat bugs will happily drink human blood, and bed bugs love to feed on chickens. Blood, regardless of its origin, appears to be uniformly nutritious. The reason these insects specialize on particular hosts has to do with the morphology of the red blood cells (erythrocytes) as their sizes very among animals. For example, chicken erythrocytes are 11.2 µm in diameter, whereas human ones are only 6-7 µm. Since bat and bed bugs drink blood through a needle-like stylet, its diameter has to match that of the erythrocytes of their host and the viscosity of the blood. If you ever had a really good, thick strawberry frap then you know what I am talking about – the pieces of fruit clog the straw and you end up scooping them out of the cup with your fingers (everybody does it, right?) The point is that human blood is easier to drink than that of birds, which might have been the reason why these insects switched hosts sometime during the early stages of human social evolution, from birds or bats that inhabited the same dwellings (swallows are highly probable original hosts). Blood morphology also explains why some bats have and others do not have bat bugs. Bats of the family Vespertilionidae, like the one we caught in Gorongosa, have really low hematocrit (the percentage of red cells in blood) compared to other bats, which makes their blood “thinner” and easier to drink. Not surprisingly they are the most common hosts of bat bugs.
The recent upsurge in bed bug infestations across the world, caused in all likelihood by the sudden availability of cheap airfare and thus a dramatic increase in mixing up of the human population (damn you, JetBlue!), has put these insects into the spotlight. But bed bugs have always been the darlings of behavioral biologists, primarily because of their unusual reproductive behavior. Bat and bed bugs are practitioners of traumatic insemination – males in these insects don’t bother finding the proper opening in the female’s body, but simply jab their sharp copulatory organ into the side of her abdomen and ejaculate directly into the body cavity. This cannot possibly be pleasant. In fact, females who were inseminated in this way show 20-30% decrease in their lifespan due to injuries, and some die immediately after the mating. For this reason female bed bugs had to evolve separate paragenital structures that channel sperm injected into their body cavity into the true reproductive organs. Unfortunately, male bed bugs are particularly horny creatures that will attack anything that moves, including other males, and mate with it. In most bed bug species such intrasexual rape results in the death of the victim male due to ruptured intestines. So severe is the risk of dying from misplaced mating attempts that in the African bat bug Afrocimex constrictus males have developed paragenital structures similar to those of females, just to protect themselves from other lusty males.
Why such bizarre mating strategy has evolved in bed bugs (and a few other invertebrate groups) is still a mystery. Most explanations center around sperm competition – by injecting sperm directly into the body of the female the males bypass mating plugs that females of many animals develop to stop future matings. It may also give males a chance to send sperm closer to the ovaries, or simply avoid having to perform some ridiculous dance or other display in order to be accepted by the female as a mating partner. There is also a theory that by injecting sperm directly into the gut the male bed bug feeds the female (his sperm is indeed partially digested), a form of a nuptial gift. Thanks, but no thanks!