Bedbugs, of the family Cimicidae, are small bloodsucking true bugs, found in warm and tropical areas worldwide, where they feed on birds, bats, and people. There aren't many of them, just 9 species, of which only two are known to go after humans. They're pretty easy to identify, too - the gigantic CSIRO books Insects of Australia basically boils it down to "Does it have a U-shaped 'shoulder blade'?" Yes: It's a bedbug. No: It's something else.
Before the introduction of DDT, infestation in UK homes by Cimex lectularius, the cosmopolitan bedbug, was universal. The species has only just started to recover - one of my bosses here in Australia has been in pest control for 15 years and has never seen one - and many of the rest of the staff were surprised by how big they were.
DDT worked brilliantly against bedbugs - it was cheap to make, had strong residual effects, and was easy to apply to all the nooks and crannies bedbugs spend the day in. Alas, there was that unfortunate tendency for it to accumulate in the food chain. Since then, pest controllers have used other pesticides, forgotten about bed bugs, and then switched to using more targeted and species-specific insecticides against cockroaches and fleas - which is bad news for the cockroaches and fleas, but suddenly the bedbugs can prick up their antennae and say "hey, we came come out again!"
And indeed, they've made their comeback. Apparently they've spent the last 40 years hiding out on chicken farms, building up their pesticide resistance to thousands of times what it used to be. They've re-infested homes, hospitals, hotels and jails worldwide, by hitching a lift on second-hand furniture & mattresses, and in clothing & luggage.
They breed fast - a nymph can go from egg to adult in five weeks, as long as it gets one meal a week. And then go over a year between meals. The bites are frequently strung out in a line, as the bedbug works their way along trying to find a vein (I'm sure any phlebotomists reading this will sympathise), leading to an old joke about 'breakfast, lunch, and dinner'. Oddly enough, despite being seemingly perfectly well suited to act as insect vectors for up to 41 different diseases including Hepatitis B and Chaga's disease, they've never been known to actually do so. Damage is limited to the inflammation, itching, and possible secondary infection if you scratch the lumps too much, which makes the US$382,000 awarded after guests sued a motel chain somewhat ridiculous. Bloated with their meal, they'll then return to the seams of the mattress, joints in the bed frame, edge of the carpet, wall sockets etc for the day, to digest their meal and indulge in violent and indiscriminate rape.
Dave Littler puts it like this, in his World's Most Terrifying Penises series :
Imagine. if you have the courage, if you are at home with your siblings, who you have lived with since you hatched from your common clutch of eggs (go with me here), each of you enjoying yourselves in whatever manner best suits you. All of the sudden, one of your brothers stands up and, without warning, whips out his tool. No mere shaft of soft and pliant flesh and blood, though, this phallus is a wicked hook of chitin with a curved, scimitar-like blade of a tip.
Without any evident regard for your desire, family relationship or the particulars of your anatomy, he thrusts it brutally into your belly, piercing your skin and organs alike before depositing his DNA directly into the bloody wound in your abdomen. He then climbs off of you and immediately repeats the process with your brother. And then the family dog. If you can imagine this, you can in some small way imagine what it is to be a bedbug.
And that's pretty accurate. Bedbugs, and as has since been discovered various of their relatives, reproduce by what is euphemistically described as traumatic insemination. They're hardly the only animals that add a bit of bloodshed to their foreplay - witness the corrosive sperm of the penis-fencing flatworm, the hormone-drenched harpoons of the common garden snail, and the assorted axes, blades, and spikes on the head of Australian velvet worms - but bedbugs do indeed take it all to horrifying lengths.
Naturally, the female bedbugs aren't exactly overjoyed when approached by a sex maniac wielding a semen-encrusted steak knife, but are too bloated after a meal to run away. Instead, they've developed a set of pseudogenital structures or spermaleges, which in one genus is elaborate enough to form a pair of entire reproductive tracts! With these the lady bedbug can hopefully limit the damage to mere crippling injury and shortened lifespan, at the same time as robbing the male of some 50% of his reproductive success and protecting herself from bacterial infection. To add insult to injury, the bedbug penis comes equipped with sensors to determine whether or not she's fed recently, and whether or not she's been mated recently, and adjusts the amount of sperm injected accordingly.
However.... male bedbugs also have false vaginas, and homosexual rape is frequent (and as Dave points out, they'll also assault any other insect roughly the right size). But despite popular mythology to the contrary, there is no evidence that the rapist's sperm supplant that of his victim, so we don't know why. Possibly the injury alone is enough to limit the victim's reproductive fitness. Possibly bedbugs really, really don't discriminate well.
To anybody that has nightmares after this : you're welcome.
EDIT: bother, just discovered an older post about them http://community.livejournal.com/wtf_nature/287626.html - now why didn't that turn up in google?