Human bedbugs, Cimex lectularius, are “true bugs,” that is, insects in the order Hemiptera. They are an infernal pest, sucking the blood out of people and leaving a nasty, itchy rash. (I was bitten only once, but it was in a fleabag hotel in Peru, and there were many bites all over me, with the rash persisting for about three weeks.) Fortunately, bedbugs aren’t known to carry any diseases.
Still, they’re annoying, as you’ll know if you’ve followed the news over the past couple of years. Having been nearly eradicated by 1940 following applications of DDT, bedbugs started making a comeback when we declared a DDT moratorium, and the bugs are now common in American cities and a devil to eliminate.
Here’s a human bedbug sucking blood from the arm of a volunteer (photo from Wikipedia)
But where did bedbugs come from? Well, it’s long been known that their closest relative seems to be the bat bug, a similar insect that lives on bats, sucking their blood in the caves. The batbug also happens to be classified as the same species as the human bedbug, Cimex lectularius. The morphological differences between the two forms are trivial, but you can still tell them apart with a microscope. Below is a diagram and some text from Bad Bed Bugs highlighting the diagnostic differences:
The trick to identifying a bat bug is by looking at the length of hairs on the upper covering of the thorax. The picture above is the joining of one half bat bug (left side) and one half bed bug (right side). You’ll notice that the length of the bat bugs hairs is longer than the width of its eye. The bed bug however, has hairs that are smaller than the width of its eye.
There are other differences, too: as Carl Zimmer notes in a new piece in the New York Times, the human variety has longer and thinner legs than the bat variety, perhaps because the bat variety needs a firm grip on their cave-hanging hosts.
There also appear to be physiological differences. As a new paper in Molecular Ecology by Warren Booth and colleagues (reference and free link below) notes, each type does better in terms of longevity and reproduction when it feeds on its own host. A batbug forced to ingest human blood does okay, but not as well as on a bat, and vice versa. Finally, the daily rhythm (“diurnal cycle”) differs between the two forms: batbugs feed during the day, when bats are asleep in their caves, while human bedbugs feed at night, when humans are asleep in their beds.
One problem with these data, which are used by both Zimmer and Booth et al. to imply genetic differentiation, is that we don’t know whether these differences are evolved genetic differences between the forms, or are only developmental/physiological responses to feeding on different species. It’s possible that if you transferred a batbug to humans, it will develop longer legs, change its feeding cycle, and get physiologically acclimated to human blood in a generation or so, and that this is not due to evolutionary (genetic) change, but could be a purely developmental (“plastic”) response.
That’s not a far-fetched interpretation. Human head and body lice, which are not different species, also transform their physiology and morphology as a result of acclimation and not genetics, and even Anolis lizards change the shape of their legs if they’re forced to climb on thin branches rather than clamber on tree trunks or the ground. The only way to determine if the morphological differences between bedbugs and batbugs are due to genetic/evolutionary change is to rear them over several generations on a common diet, and see if the differences persist. If they do, they’re genetic.
The reason Zimmer and Booth et al. dwell on this is because bats have been suggested to be the vector that gave us human bedbugs. Bats, so the theory goes, were originally afflicted with batbugs, and early humans lived in caves alongside the infested bats. Batbugs then found a juicy new source of food nearby, a few individuals colonized humans, and the rest is history: the human bedbug.
Booth et al. wanted to see how much genetic differentiation there really is between human bedbugs and batbugs, and so their paper reports an extensive genetic analysis of several hundred of individuals from both forms of the bug. The researchers looked at mitochondrial DNA, nuclear DNA (in the form of microsatellites), and at genes that had evolved in human bedbugs to resist DDT.
What they found was that batbugs and human bedbugs do indeed show significant genetic differentiation—in all three types of genes investigated. Bedbugs and batbugs clearly form two distinct genetic lineages. This is shown by statistical analysis of bugs taken from the two hosts; the figure below shows the genetic differentiation for nuclear DNA among samples of both forms taken in Europe. Brown dots are individual human bedbugs, blue are batbugs, and you can see how well separated they are (see the caption below the figure).
There is, however, still some evidence of gene flow between the two forms, perhaps occurring when a batbug finds itself on a human and mates with bedbugs, or vice versa. Although most human bedbugs show the DDT-resistant form of “pesticide genes”, a few don’t, and those “susceptible” genes may have come from the batbugs, which never experienced DDT. Still, what we have here are two closely-related but genetically distinct lineages, and that is the big lesson from the paper of Booth et al. But they want to say more, and that is what Carl Zimmer highlighted in his NYT piece (see question #2 below).
Two questions remain:
1. Were batbugs the ancestors of the human bedbug? It seems likely, although neither Zimmer nor Booth et al. explicitly give the information that is be crucial for ansering this question: Are the batbug and bedbug more genetically similar to each other than either is to any other species in the genus? If the batbug is the ancestor of the bedbug, then the two forms have to be “sister taxa,” that is, each other’s closest relatives. Now this may indeed be the case, and may be cited in one of Booth et al.’s references, but I didn’t look them all up. I’ll take it for granted that both Zimmer and Booth et al. know that these are in fact sister taxa.
But one problem remains: do they only look like sister taxa because there has been gene flow between batbugs and human bedbugs, making them look as if they evolved recently when in fact they didn’t? This is a problem with trying to suss out the evolutionary history for any pair of species that live in the same place and occasionally hybridize. Fortunately, it can be taken care of. For example, if bedbugs and batbugs had distinct forms of genes (as they do), but those forms are still more similar to each other than to the gene forms of other species or populations in the genus, then that would imply that they are indeed sister taxa. Neither the authors nor Zimmer discuss this, but it may be such a well-known result that neither thought it necessary to mention it explicitly.
Also, the human bedbug is genetically depauperate compared to the batbug, and that’s what one would expect if only a few individual batbugs originally colonized humans, going through what we call a “population bottleneck.” The genetically depauperate nature of the human bedbug compared to the batbug also implies that if there was a colonization from bats to humans, it happened only once or a very, very few times. If colonization was frequent, human bedbugs would be much more genetically variable among populations than we see. If the bat transfer theory is correct, the colonization of humans by batbugs must have occurred in the distant past when humans lived in caves along with bats, and that would probably be about 50,000 years ago in Eurasia. (No molecular dating of the divergence was reported.)
But what the authors and Zimmer find most exciting about the study is encapsulated in the second question:
2. Are these forms on the road to becoming different species? Are we seeing, in the form of batbugs and human bedbugs, two groups that descended from a common ancestor (on bats), and are now in the process of becoming different biological species? Indeed, Zimmer calls his piece, “In bedbugs, scientists see a model of evolution.” What he means by that is “a model of how new species form.”
We evolutionists, by and large, conceive of species as being different groups that cannot exchange genes because of biologically-produced “isolating barriers” that prevent the formation of fertile hybrids. Bedbugs and batbugs do appear to have such barriers: they don’t do well on each other’s hosts, they are active at different times of day, they seem to maintain differences in appearance, and, of course, the DNA data show a lack of genetic exchange. Now, as I said, we don’t know whether the differences in activity period, ability to thrive on the host, or morphology are based on differences in genes (we can’t assume blithely that they are), but the DNA data clearly show that these lineages don’t exchange genes very often. Could it be that we have a case of speciation in action due to host shift by the batbugs?
The answer is that we don’t know for sure. What we see are two diverged lineages, but we can’t know whether they will continue their evolutionary divergence and go on to form two “full species”, totally incapable of exchanging genes, that deserve different Latin names. It’s possible that they will maintain their status as somewhat distinct lineages, but that gene flow will be enough to keep them from achieving full reproductive isolation.
There are in fact many known cases of groups that are similar to these bedbugs in having achieved partial but not full reproductive isolation, so to imply that these bugs are unique, or that we have here a rare model of speciation in statu nascendi, is incorrect. In the book Speciation that I co-wrote with Allen Orr, we discuss many cases of “host races” in insects that show significant genetic divergence of forms living on different plants (aphids and the”true” fruit flies [tephritids] are two examples), but in which there is still gene flow between the forms. They are not considered “full species” since reproductive isolation is incomplete. In all of these cases we simply have no idea about whether they’ll go on to evolve into fully isolated species. We’d have to wait around for a couple of hundred thousand years to find that out.
The fact is that most populations of a species showing some reproductive incompatibility probably do not go on to form full species. Either they fuse back into one species, or one form goes extinct, or they maintain their status as incompletely isolated forms. To ask the question, “Are these going to become full species?” is to ask a question that can’t be answered.
Nevertheless, there is of course a lot of interesting information about batbugs and bedbugs in the paper of Booth et al., regardless of their unknown evolutionary fate. At least we know (probably) where an annoying human parasite came from, and something about the evolutionary differences between them. That might not help us eradicate bedbugs, but isn’t it fascinating to contemplate that our affliction with that creature is a remnant of our evolutionary history as cave-dwellers?
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Booth, W. et al. 2015. Host association drives genetic divergence in the bed bug, Cimex lectuarius. Molecular Ecology, online.
Amazing post, I can tell — unfortunately, am in a huge emergency work crunch at the moment, so have to suck the blood out of this one later this evening. Will devour then, and maybe lay a few eggs on my return… 😉
Very interesting. Now… how to send them the way of the smallpox virus!
Is DDT really the only insecticide that is effective against these guys? I can only imagine the good people at chemical companies are working on this – there’s a big market, as it is affecting the hospitality industry; there are websites devoted to mapping hotels with bedbugs.
And DDT is completely banned? Wouldn’t there be a way to control its use to keep it out if the environment where it can harm wildlife?
What I read was that DDT was rapidly becoming ineffective anyway, since populations were building up resistance.
And that is too bad. DDT, for all its faults, was considered one of the less toxic insecticides to humans.
Yep. Very, very innocuous to us. To eggshells, not so much.
A little over 10 years ago now, I was doing a project in southern Mexico. Going through some pretty barren & impoverished lands. We (both of us) managed to stop at a few roadside impromptu eateries.
Our “chef” would quickly get out the spray bottle and mist & wipe down the plastic checkered cover of the card table, then urge us to sit down and eat. This was DDT being sprayed around to keep the flies at bay; this was a common practice out in the boonies. Yum, yum.
Holy Crap.
In my childhood, some US cities used to routinely spray entire neighborhoods with DDT to control mosquitoes.
Seems mechanical methods works fine. There is a plant with hairs that trap bedbug legs at the joints,* and there is a swedish company that develops a paper trap filled with silica that suffocates them.**
* I have forgotten its name, but IIRC it is locally used in Germany.
** The problem with silica being that if you just spread it around it gets everywhere. The traps are supposed to take care of that.
Silica is chemically innocuous but you really don’t want to breathe it in.
It’s really good at getting rid of insects, Diatomaceous earth is a good ant barrier. Thanks to the little diatoms building tough little skeletons that get into the insects joints.
Talcum powder works well on ants too, no idea about bed bugs as I’ve never had those. I assume it gets into their spiracles and they can’t breathe. I had talc dusted across my kitchen shelves for years.
Thank you for writing an excellent summary of an important paper and study. Hair and body lice (dead of course) have been found on Egyptian mummies. Hopefully we can examine the lice DNA from some three to four thousand years ago and compare to lice DNA of today. Who knows, perhaps we can found dead bedbugs from thousands of years ago as well for a proper DNA study.
IIRC people have done “molecular clock” work (yes, there are a lot of caveats about “molecular clocks”) on human lice and their closest known relatives in ape body lice.
Ach, why beat my braincells about something I barely remember? Wikipedia says :
If that work has been validated (there are fair grounds to be careful with molecular horology, but that’s because it’s difficult, not worthless), then that window for the development of clothing doesn’t seem wildly unrealistic.
Anyone else feel itchy reading this?
YES!!!!!
Just reading/hearing about lice/fleas/begbugs makes me feel itchy.
Yes.
*scratch scratch scratch*
And does anyone else hear that bell?
I spent some time investigating head lice when I had young children; I eventually got over the need to itch when thinking of them, but it took several days. Head lice are another pest that has made a comeback since my childhood. I don’t ever remembering that anyone got head lice, now the damn things are everywhere.
I read somewhere they are proliferating because kids tend to be cleaner, as in more frequently bathed, now than they were in my (maybe our) day. Apparently the little f**kers like a nice, clean head of hair in which to abide.
Head lice do prefer cleanliness (godliness doesn’t interest them ;-)) but I find it hard to believe that today’s children are any cleaner than the ones that grew up in the middle class suburb I lived in. I think it’s partly because mothers like me had no idea that head lice were a thing that could happen. I was flabbergasted when my son got them, I really thought head lice belonged in the Dickensian past and third world slums. I had never heard of anyone I knew actually getting the little f*ckers and I was shocked! All it takes is one mother who is oblivious (like me) or careless like some others, and the whole class is in trouble. It would be interesting to know more however.
Batbugcave – I will never view the comic in the same light again.
So given batbug adoption was hard (likely happened only once), why did our ancestors lug around the same bed material from cave into tents and then houses? It had been so simple to break the parasite chain at the time… :-/
Perhaps without artificial light it was hard to know where bedbugs were hiding. You move from the winter cave to the summer camps, you take your bedrolls with you, there are almost no bedbugs for a while, then they appear again. Happens all over in the winter.
And if they did figure it out, the main insecticides they would be using would be phenols like thymol and cedar oil. Not the most effective stuff.
By “bedroll”, are you implying that this was after the development of woven fabrics?
Using predated animal furs is likely to have pre-dated (sorry!) the development of woven fabrics (or tailored garments for that matter) by a considerable margin.
Most likely the notion of bedbugs as a transmissible infection was simply foreign to them. They would have regarded bedbugs as simply a natural (if unfortunate) consequence of piling up bedding material, just as maggots appear spontaneously on rotten meat.
How to get rid of them, that requires – dare I say it? – a miracle.
Patience and simple furniture + no wall covering help a lot. I was infested once in a flat I rented. It took me a long time until I realized what is happening, since I did not know them. I exterminated them without outside help with a consumer bug-killer spray from the local mall. I had to take apart my bed twice and brush down every element. Also I encircled the bed with poison every evening for weeks.
A colleague of mine who had a similar problem earlier had to burn the bed and the wallpapers. (Wallpapers are hopeless when you are against these bugs.)
Buy a fabric steamer or steam cleaner, and keep it handy, and wrap your mattress and pillows in special covers. If there’s an infestation, you have to steam everywhere and everything!
Very interesting article. I don’t really mind sharing a bit of blood, but the rash is just asking for too much. How churlish of them.
A while back in a comment on another science oriented site I characterized current genetic studies comparing Neanderthals to Modern Humans as a snap shot from the midst of an ongoing speciation event. This may be an example of a speciation event that never achieved complete speciation because one of the sub species went extinct.
Instead of merely the traditional “sleep tight, don’t let the bedbugs bite,” bed time salutation, I always told my kids “but if they do just bite them back.”
We finished it with: “if they do, take your shoe; hit them ’till they’re black and blue”.
“Having been nearly eradicated by 1940 following applications of DDT, bedbugs started making a comeback when we declared a DDT moratorium”
Naomi Oreskes in “Merchants of Doubt” says that DDT was rapidly becoming ineffective due to resistance in the populations, so the prospect of actually destroying any insect forever was an illusion.
I read Bat Man vs. Man-Bat to my son just last night. It seems a well meaning bat researcher drank a serum made up of bat DNA to help with his hearing loss. Unfortunately, after consuming the serum the scientists was terribly transformed into a half man, half bat, monstrosity.
The important question here is which lineage of bedbug would do best on Man-Bat? Maybe they would do equally well? Seemed like a very plausible story line. Stuff like that is always happening in comic books.
You would have to breed a specific bug from a cross of the two. Of course some supervillain will use this against Bat Man now, he’s doomed to sleepless nights!
Robin is well known for deploying pearl necklaces as a remedy against sleepless nights.
Very interesting. This goes into my collection of evolution articles. Another great example of incipient speciation that might one day go all the way.
Of course the process might now be collapsing because of the decline in bat populations.
Fascinating. I enjoy reading about speciation – examples like this and cases such as the population of gulls around the northern pole. It brings out subtle details of the process. Almost, but not quite different species – yet.
It seems odd that bed bugs or bat bugs do not seem to carry or transmit any diseases like the flea or mosquito are know to do.
Don’t give ’em any ideas. 🙂
Great article!! I was told by my parents that in Poland many old houses had boards for ceilings and the bedbugs hiding between them would drop onto the bed and crawl under the covers. People kept a dish with water and two pieces of soap in it so that one side of the soap was sticky. When going to bed at night, two people would do the following: One person would very quickly pull the cover off and the other would very quickly slap the exposed sheet all over while holding the two pieces of soap sticky side down. This method was supposedly efficient enough to be worth the
trouble.
I’d heard of exactly the same technique from ex-military of various continents, assorted “oil field trash” describing barracks all over the world (I’ve only suffered noticeable insect activity at night in Abu Dhabi and Azerbaijan, but maybe I’ve been lucky), and seen it reported by mountaineers from Himalayas and Andes. It seems a venerable approach. Or it’s an oft-copied tale. It’s always “a friend of a friend” who did this, and I can’t recall anyone ever claiming to have done it themselves.
I would think that under the definition of “full species” used here, a great number, perhaps the majority, of described species of vertebrates are not full species. At least there are plenty of described species of birds aren’t. Almost all ducks, for example. And that’s just discrimination.
I was thinking the same thing. It’s been awhile since I’ve read Speciation so I may be misunderstanding Dr Coyne, but if “full species” is only in cases when there is no gene flow, were Neanderthals/Sapiens “full species”?
I am sure I am missing something.
No, neandertals and sapiens weren’t full species: they hybridized and the hybrids were at least partly fertile, as evidenced by the fact that some Neandertal genes persist in modern humans. Although they are classified as different species by some,they don’t fit the classification of distinct biological species. Of course there is big disagreement as to how to classify or demarcate species, and we argue in Speciation that one chooses a species concept based on the question one wants to answer. If your question is “why is nature composed of discrete groups of plants and animals rather than forming a complete continuum?”, then the reproductive-isolation concept–the BSC–is the best one
I really don’t want to have an argument about species concepts on this website, though. I’m explicitly using the biological species concept in the discussion above.
Thank you!
A counter example would be from my territory of examining fossils. All we have got (most of the time) is the hard parts in various degrees of preservation, so we have to stick to a “morphological species” concept. A situation such as I heard of recently with two populations of salmon breeding in the same rivers on alternating years of a two year cycle would be invisible to the fossil record. There’s another example which I think is better known in America of the 13- and 17- year cicadas which would only be able to experience gene flow every (counts ; removes socks ; counts more) 221 years.
A current, related and unresolved, controversy is the proposal that the horned dinosaur Torosaurus is the adult or senior form of the better-known Triceratops. (I think the hypothesis is looking pretty weak at the moment, but it’s not dead yet.)
Wow, you have a lot of toes!
I use my metatarsals as … [wait for it] carrying digits.
Kaboom-tish!
Thank you, I’ll be here all night!
Good one! 😀
I hope that by “good” you mean “awesomely bad!” Or is “sick” still the word to aspire to?
Oh, you’ve reminded me of one we’ve forgotten (unless I’ve missed it):
Badass!
Just because ducks species hybridize with each other doesn’t mean they’re not full species. The hybrids have to be viable and fertile, and also find mates among the parental species. People think that if hybrids are formed, that’s evidence that things are not full species. But there are quite a few Drosophila species that hybridize in nature, but the hybrids are completely sterile.
We discuss our species concept, and how to distinguish between “full” species and “species” in our book.
Reblogged this on Shashank Patel.
Batbug is the next entomological Superhero! He rides bats to catch insect criminals–a literal bat-mobile!!! Bedbug is his lazy distant cousin who never really accomplishes anything except irritating people.
Yes, fascinating. Reminds me of some sedges.
[I’ve had the opportunity to practice this on my Dad. He got botanically “into” sedges when he got bored with grasses as being too easily identified. Personally, if it ain’t fossilised, I say “don’t ask me, ask Dad”.]
I couldn’t Carex less about your sedges!
Genetic analysis seems to be the way to go. No matter how much it costs, it will be less than what an undergrad will require to sleep with batbugs for a year.
Lol!
Oh, I don’t know, I think undergraduates come pretty cheap. 😀
Interesting post on an interesting paper. John Hawks adds also some useful comments from a paleoantropologist’s point of view.
http://johnhawks.net/weblog/reviews/parasites/bedbugs-bats-to-people-zimmer-2015.html
Excellent post, thanks.
“To ask the question, “Are these going to become full species?” is to ask a question that can’t be answered.”
The important point is, they illustrate one of the numerous and diverse ways speciation can – or could – occur.
Yeah, but we have plenty of examples of that already in host-specific herbivores, other parasites, fig wasps, and so on.
“early humans lived in caves alongside the infested bats”
It was my understanding that few early humans actually lived in caves. Caves with paintings on the walls were thought to have been used for religious/ceremonial purposes and not lived in habitually.
Of course, under this scenario batbugs still could have found human hosts in caves.
I think that’s Hawks’s contention, and since he’s an anthropologist I’ll go with what he says. I was just reiterating what the authors (and Zimmer) pass on as one theory for how bedbugs got onto humans.
“the human bedbug is genetically depauperate compared to the batbug”
Please correct me if I’m wrong, but is it safe to interpret the displayed Figure 4 as evidence for this? I haven’t looked at the paper myself, but it seems like Fig. 4 shows the results of some kind of factor analysis on the nuclear DNA of bedbugs and batbugs. The bedbug data are far less variable than those from the batbugs, implying that bedbugs are far less genetically diverse.
Great article – thanks for the write-up!
I just wanted to say thanks for this very interesting post.
What I want to know is where do cooties fit in this taxonomy? And what accounts for their apparent superiority at late-night baseball?
Thanks for the informative and enjoyable post. I’ve never seen nor been bitten by bedbugs and I didn’t know they are becoming such a nuisance; nor did I realize they are so difficult to eradicate. I thought they transmitted Chagas disease so after reading they didn’t carry any diseases, I looked up Chagas and found out its’ vector is the “kissing bug” which is also in the order Hemiptera, but found in south/central America. I don’t know how that bit of misinformation lodged in my brain, but I’m glad to be set straight.
Chagas Disease has reached Texas, some say due to climate change, some say immigration (human). Other insect-disseminated diseases are also moving north (such as Dengue Fever).
I didn’t know it has reached the States. Yikes, that’s scary. It is well known that climate change will increase insect populations, and the diseases they can carry…not good.
You can say that again.
And the ones that have been in the south for a while will probably be coming, too–fire ants, killer bees…
Just to keep you happy, malaria-bearing species of mosquito have been reported in both southern Italy and Greece over the last few years – though no reported indigenous malaria, yet.
Fortunately, it’s not something I have to worry about for my next trip, otherwise I’d be popping the prophylactic pills down my throat already.
Chaga’s disease is suggested (amongst other suspects) as being the cause of Darwin’s post-Beagle recurrent ill-health.
It’s not unheard of to have bats roosting in attics in Ireland especially during winter. I would suggest this might be common in some places in the rest of Europe also. So it’s possible we’re not so far removed from the presence of batbugs as might be thought.
I think I’m relieved by the general conclusions (there’s no active gene transfer going on), since I currently have a bat in the basement of Schwixon.
Interesting post. Thanks.
Genuinely fascinating!
It would seem that batbugs would bother bats much less than bedbugs do humans (or the bats might become too debilitated to nourish themselves & their offspring). I suppose the reason humans are still so irritated by bedbugs is because our immune system hasn’t had enough time to adjust?
And do batbugs migrate and/or hibernate with their hosts? Or perhaps stay in northern caves in some age stage in which they can handle the cold, to reactivate when the bats return? Have bedbugs then lost the need to overwinter in a dormant stage, so they are then producing more generations per year, and evolving faster?
Hmmm, that seems a bit dubious to me. Since the main reason that DDT was banned was it’s habit of accumulating as it moved up the food chain, and most bats are insectivorous, then I’d have expected batbugs to have been exposed to appreciable DDT from their host’s blood stream. No?