Few humans have been as successful in Hollywood as parasitoids. Parasitoids are a particularly gruesome kind of parasite that invariably kills its host by the time it becomes an adult and is ready to leave the host's body. A parasitoid female wasp, to give one example, will fly along until it finds a caterpillar of some particular species. It lands on top of the caterpillar, jabs an egg-laying stinger into the caterpillar's body, and injects some eggs. The eggs hatch, the wasp larvae feed on the living caterpillar from within, and then, when they're ready to metamorphose into adults, they crawl out of their hapless host, leaving it to die.

The screenwriter Dan O'Bannon reportedly had nightmares about parasitoids, and out of those dreams emerged a script about an alien that laid its eggs inside human hosts, which then burst out of their hosts' chests. In 1979 that script became the movie Alien. Fifteen years later, with three sequels out and Alien 5 reportedly in the works, this oversized parasitoid is a genuine star. If Mary Kate and Ashley Olson have their own star on the Hollywood Walk of Fame, then surely there is room for the imprints of this creature's claws.

Alien 5 is rumored to be about the arrival of parasitoid aliens on Earth. Actually, they're already here, and they've won the battle. There are at least 200,000 species of parasitoid wasps alone, along with many more species of flies and other insects. They're not limited to some island off of Madagascar--they're found around the world, in deserts, jungles, and the tomato plants in your garden. They consume the minds of a large army of entomologists, who document their awesome, chilling success. I had a great time writing about parasitoids as part of my 2000 book Parasite Rex, but the science of parasitoids has continued to march forward. The 2004 issue of Annual Reviews of Entomology has just come out, and it has three reviews that sum up what scientists know about them so far.

Hundreds of different insect lineages have evolved into parasitoids, and so they've acquired a head-spinning diversity of ways of taking advantage of their hosts. Some parasitoids lay their eggs in the neighborhood of their particular host, and when the egg hatches, the larva may crawl, skitter, or squirm around for weeks until it finds one. Parasitoids can be very picky in the hosts they choose. They can measure how big a potential host is, and will reject ones that are too small. They can also tell if there are already parasitoids living inside. (Remember that alien in Alien Resurrection sniffing Sigourney Weaver and then sparing her?) If parasitoids determine they'll have good odds looking for a fresh host, they'll skip a parasitized host, but if options are few, they'll move in as well. In one wasp species, the larvae will normally drill their way into a caterpillar's back. If they sense that there's another wasp inside, they'll drill into its underside so as to delay a confrontation. And these confrontations do get ugly. Some species even produce special castes of killers that prowl the interior of a host, destroying any parasitoids that are not their own siblings. And once the invaders are gone, the killers go after their brothers since only a few males can keep their lineage alive.

Meanwhile, the parasitoids begin to manipulate their hosts. They need to fend off the immune system or die; in many cases, the host undergoes a kind of insect AIDS in which they can no longer fight against parasites. In other cases, the parasitoid manages to camouflage itself from attack. Some parasitoids immediately paralyze a caterpillar, chew up its insides quickly, and crawl out of the cadaver. Others play it slow. They develop over weeks, and allow their host to go on munching on leaves. As those leaves get turned into fluids, the parasitoids slurp them up. They hijack their host's physiology, preventing them from storing up energy as fat. In order to hold onto to this sweet living arrangement as long as possible, they have to stop their host from building a cocoon and turning into a moth or some other adult form. They do so by adjusting the flux of hormones in the host's body, so that it just keeps growing into an oversized infant.

Perhaps you remember John Hurt in the original Alien. An alien jumped on his face (see the picture above) and then, unbeknownst to the crew, it inserted itself into his body. The only clue that something was amiss was that Hurt was voracious. It makes sense for the alien to get Hurt to bring it some food. It makes sense for real parasitoids, too. Parasitoids do more than make their hosts hungry. Spiders will weave webs before they die that are especially suited for supporting a parasitoid wasp's cocoon. Some caterpillars will crawl to the ground and burrow into a hole, giving the parasitoids a safe refuge for the winter. When aphids are hosts to parasitoid wasps, they will crawl to the tops of plants, away from the other aphids and the predators that feed on them. There, the wasp kills the aphid, which becomes nothing but a hollow mummy inside which the wasp makes its cocoon. If a parasitoid is born late in the year and will have to hibernate through the coming winter, the aphid will find a well-protected spot instead.

Parasitoids achieve all this with cocktail of hormones, proteins, and genes that take over the workings of their host's body. Some of these compounds are made by the parasitoid larva itself. In the case of many parasitoid wasps, other compounds come from the venom the mother injects with her eggs. Others come from some extraordinary viruses that the mothers also inject. These viruses aren't really viruses in the conventional sense. Their genetic code is part of the parasitoid genome, existing in every cell of every wasp. When females prepare to lay eggs, these DNA sequences splice themselves out of the genome and get packaged in protein shells. And when they enter the host, they invade the host's cells. In some cases, they act like HIV, disabling the immune system. In other cases, they help stop the host from entering metamorphosis. In the process, they are committing a sort of viral suicide, because they will die with their host. But on balance, there's an advantage to the genes, because they promote the spread of more viruses by helping the wasps.

There are lots of very practical reasons to study parasitoids. They are far more sophisticated at altering the biochemistry of pests than we are, and they're now a mainstay in biological control. It might even be possible to use the genes of their viruses to create hybrid viruses that can be sprayed onto crops, or perhaps even past the genes into the genomes of the plants themselves. But there are more profound reasons to contemplate parasitoids as well. Parasitoids have had a special place in our imagination long before O'Bannon's bad dreams. In a letter, Charles Darwin wrote "I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae [a group of parasitoid wasps] with the express intention of their feeding within the living bodies of Caterpillars." The same holds true today. I have yet to hear from the Intelligent Design camp what the exquisitely complex cruelty of parasitoids tells us about the Designer who tailor-made them.

But it's the parasitoid viruses that trigger the most musings. What are they? What name can do them justice? Some biologists have proposed that they descend from ordinary viruses that got accidentally pasted into parasitoid genomes, and then began to serve their host. If that's true, how is it that the viruses contain genes that are common to many insects, producing proteins that help carry information from the surface of a cell to the DNA and back again? Others have suggested that these viruses are actually pieces of so-called "jumping DNA"--native genetic sequences with a knack for inserting copies of themselves around the parasitoid genome. Add a couple genes for making a protein shell, and they're ready for service. Are they Richard Dawkins's extended phenotype? Is it the wasp that is infecting its host's cells? Can an animal make itself into a viral disease? If not, just where does the parasitoid stop and its virus begin? It would be nice if Alien 5 could ponder mysteries like these, but somehow I doubt it will.

(Update 1/8/03 Thanks to Jeff Boettner at UMass for correcting my definition of parasitoids.)