The “Alien” movie franchise spawns another installment on Friday, “Alien: Covenant,” a sequel to the 2012 prequel, “Prometheus.” Once again, a drooling extraterrestrial hunts plucky space travelers. The new film, according to Washington Post critic Ann Hornaday, is “a largely turgid, often thoroughly unpleasant affair.” Savvy moviegoers certainly expect some amount of unpleasantness — that sensation being par for the course whenever egg-laying parasites are involved.

But for all the cinematic aliens' gravid grotesquerie, there exists a world where they would simply be chumps. It is a place crawling with more deceptive, more horrible things. Welcome to Earth.

The inhabitants of our planet directly inspired “Alien” screenwriter Dan O'Bannon and director Ridley Scott. O'Bannon in part looked to horror writer H.P. Lovecraft and previous works of science fiction. But he also plumbed the depths of nature, patterning “the Alien’s life cycle on real-life parasites,” as he said in his 2003 essay “Something Perfectly Disgusting.”

“Parasitic wasps treat caterpillars in an altogether revolting manner,” O'Bannon said, “the study of which I commend to anyone who is tired of having good dreams.” (In brief, female wasps lay their eggs inside still-living grubs. When the young parasites hatch, they eat their way out.) Also occasionally cited as inspiration is a saltwater crustacean named Phromina. This parasite hollows out giant plankton, then floats around in the zombie barrel while raising its offspring.

“Alien” wasn't the first time that parasites played muse for writers. Monsters that go baby bump in the night are members of an old trope. Long before Scott kicked off his series in 1979, parasitic aliens popped up in sci-fi pulps. In his 1939 short story “Discord in Scarlet,” A.E. Van Vogt described an alien race that deposits eggs in people's stomachs. (Van Vogt sued Scott over similarities between “Alien” and the short story.) Parasites also appeared in Robert Heinlein's “The Puppet Masters,” a 1951 novel about mind-controlling space slugs who conquer Iowa.

Yet the “Alien” monster will be remembered as the mother of all speculative parasites, thanks to the intimate detail with which Scott and company depicted a parasitic life cycle on the silver screen.

Kane (John Hurt) explores a vast underground chamber in the derelict ship in 1979's “Alien.” (Twentieth Century Fox)

The alien, as described in the 1979 move by the android Ash (played by Ian Holm), is a “perfect organism.” Its life cycle is certainly dramatic. The crew of the space tug Nostromo land on planet LV-426, where they stumble across a clutch of giant eggs. A “face-hugger” hatches from one of the eggs and affixes itself to a man's mouth. Between his victim's lips the creature inserts an embryo, which incubates in the man's guts. A short while later, it explodes out of his chest in a cloud of viscera. That creature grows into a giant, insect-like organism that terrorizes the survivors to critical acclaim. (Critic Roger Ebert, at least, gave the film four out of four stars: “a great original.")

But how does that organism stack up with nature? The life cycle is mostly sound, experts said, but its behavior actually falls short of what real parasites can do.

“Parasites go through massive, massive changes,” said Michael J. Smout, a parasitologist at the Australia Institute of Tropical Health and James Cook University. “That part is feasible.” Parasitic flatworms, he said, go from an egg stage to something “almost like a hairy bacteria” to a swimming creature “like a tadpole with two pronged tails” to a worm that infects humans.

Human botfly at maturity (Jonathan M. Eib/USDA)

In his nonfiction book “The Science of Monsters,” journalist Matt Kaplan praised the “Alien” life cycle as a frightening and “remarkably well thought out element of the story.” He pointed to a type of botfly that will lay its eggs on a mosquito. When the bloodsucker lands on a person to feed, the botfly eggs detach and start to grow; the larvae later wriggle out of the human's skin.

The idea that a parasite could become a hybrid of host and original organism, as seen in “Alien” sequels, has some scientific merit, too, said Tommy Leung, a parasite ecologist at the University of New England in Australia. “There are real-life parasites, particularly parasitic plants, which have been known to pick up some genes from their hosts,” he said. “I think the plants are actually better at how they use their stolen genes.”

Last October, researchers at Pennsylvania State University reported that a broomrape plant had absorbed its host's genes 52 times. “We think this is because of their very intimate connection with their host,” Penn State biologist Claude dePamphilis explained at the time. The stolen genes allowed the broomrape to undermine the host plant's defensive efforts, the scientists said.

The process of stealing genes is called horizontal gene transfer. Researchers have uncovered examples of this transfer with increasing frequency, Smout said. “It's going literally across species,” he said. “It's amped up for sci-fi, but crossing between species isn't as crazy as it sounds.”

Aliens could learn a trick or two from these gene-stealing plants. (Despite their fangs and acid blood, the aliens keep losing to the human defenses of ingenuity and flamethrowers.) That the aliens don't undermine or change our behavior is their biggest failing, in Leung's view.

A parasitic wasp (AP Photo/U.S. Dept of Agriculture Forest Service, File)

“The major issue I have is that the movies don’t go far enough” he said. “Oh, burst out of the host’s chest — pfff, is that all they do? They don’t make the host become its surrogate parent and care for it like its own brood? They don’t multiply within the host’s body and turn it into some kind of flesh marionette?”

“Lame,” he concluded.

When barnacles of the group Rhizocephala infect crabs, the parasites force the crabs to become doting parents. Some barnacles tweak the crab's body shape, castrating male crabs to act like mothers. Others grow their roots into crabs' guts and steal nutrients. Or they grow into brains so the crabs will tend to a barnacle brood. “That's the kind of thing you don't seen on screen,” Leung said.

As for “flesh marionettes,” Leung described another type of parasitic wasp: the emerald jewel wasp, which turns cockroaches into bug puppets. The female wasp hunts down a roach three times her size. She paralyzes it but doesn't kill it. Instead she inserts the end of her tail into the cockroach brain.

She begins to feel her way around. “The tip has all these tactile and chemoreceptors, the equivalent of taste buds, to find the sweet spot on the cockroach brain,” Leung said. The wasp tastes and feels its way to just the right nerves, where it delivers a mind-controlling venom.

The roach “becomes a subservient lap dog,” said Leung. The enslaved bug crawls into the wasp's burrow, where the roach will spend the rest of its life being eaten. (Leung wonders if the roach is aware of its situation or just treats its existence like the coffee-drinking dog that remarks, “This is fine,” while the kitchen burns down.)

The “Alien” monster isn't simply outclassed by real-life brain-bending Earthling parasites. It has a few imperfections in which parasite logic was sacrificed for horror-action narrative.

Actors Sigourney Weaver and Bill Paxton attend the “Aliens: 30th Anniversary” panel at last summer's Comic-Con International in San Diego. (Photo by Chris Pizzello/Invision/AP)

In “The Science of Monsters,” Kaplan critiqued the creatures depicted in the 1986 sequel “Aliens.” The monsters slaughter us too easily, he said. It is an “evolutionary quandary,” Kaplan wrote, that “borders on being a story flaw … It makes no sense for the aliens to be such capable human-killing machines. Instead they should be masterful human kidnappers that are adept at feeding on some other species when they reach adulthood.” Each dismembered space marine, he noted, is one fewer warm body for incubating a chest-burster.

Leung argued that the chest-buster cycle was inefficient, as well. Some parasites grow to huge proportions inside their hosts. But they stay put, turning the host into a “parasite factory.” One fish-dwelling crustacean gets so big it would be like having a rabbit in your chest, he said. The crustaceans poke a tiny hole in the side of the fish to spew, over time, thousands of baby parasites into the ocean.

Smout offered a gentle critique of the critiques. The monsters in “Alien” have only been infecting humans for a few generations, he noted, whereas on Earth parasites have evolved alongside hosts for far longer. Of course extraterrestrials wouldn't be expert human parasites yet.

There is one final place where the animal logic of “Alien” holds up: the human approach to parasites. It's revealed, toward the end of the movie, that a shadowy space conglomerate wishes to capture the parasite for entrepreneurial purposes.

Parasitologists like Smout are doing the same thing — though Smout's designs for his parasites are more humanitarian. He is working on turning the spit from the parasitic liver fluke into wound-healing medicine, having discovered that it encourages cells to multiply faster. “It's ultra-potent and causes cells to hyper-proliferate,” he said. He envisions using the substance to close wounds suffered by diabetics or other people who heal more slowly than average.

“The worm crawls along your liver, munching away,” he said, “but it wants to keep you healthy.”

Parasitic aliens, take note.

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