10 Body-Snatching Parasites

Hans Hillewaert, Wikimedia Commons // CC BY-SA 4.03
Hans Hillewaert, Wikimedia Commons // CC BY-SA 4.03

There are a lot of parasites out there. Some estimates suggest that as many as half of all the species on earth live inside—and feed off—other species. One new study published online (which hasn't been peer reviewed yet) argues that the parasitoid wasps might be the largest single group of animals—a title generally thought to be held by beetles.

Practically every species has its own set of parasites, and even parasites have parasites. In many cases, a parasite's host is little more than a habitat where it can eat and breed. But some parasites have gone a step further, evolving ways to manipulate their hosts in ways that give the parasite a better shot at growing up and spreading its young far and wide. Their methods can be as deliciously gross as the worst imaginings of horror movie screenwriters. Here are 10 examples to inspire new terrors of the silver screen.

1. JEWEL WASP // AUSTRALIA, PACIFIC ISLANDS


The jewel wasp Ampulex compressa is iridescently beautiful, but it's a nightmare for the American cockroach. When a pregnant female wasp gets hold of a roach, she temporarily paralyzes its muscles with a sting, then threads her stinger up into the roach's brain, injecting a cocktail of chemicals that turn the roach into a zombie. The roach could move when the paralysis wears off, but now it doesn't want to. Instead, it allows the wasp to gently lead it by one antenna to her burrow, where she walls it in with one of her eggs. That egg will soon become a larvae that spends its first week on earth eating the living roach bit by bit before pupating and emerging as a wasp to continue the cycle.

2. NEMATOMORPH HAIRWORM // EUROPE

Everything seems normal for weeks after a long-horned grasshopper has drunk water containing the microscopic larvae of the hairworm Spinochordodes tellinii, but that changes as soon as the worm grows big enough to start yearning for a mate. That's when it secretes chemicals that change its host's brain chemistry, making deep water seem enticing to the insect. The grasshopper suddenly has a suicidal urge to take a long hop off a short pier, and as it drowns, the worm—now as much as three times as long as the insect it lived in—squeezes out of its host and swims off to find a mate. Other hairworm species prefer praying mantises or spiders as hosts, but it's the same endgame for them all.

3. PARASITIC BARNACLE // MARINE COASTS

Sacculina carcini
John Aplessed, Wikimedia Commons // Public Domain

A female Sacculina carcini starts its life like any other barnacle—as a tiny planktonic baby floating free in the ocean. But unlike your average barnacle, when she drifts onto a crab she doesn't just settle down and become a warty bump riding on its shell. Instead, she burrows into the crab and grows until she infiltrates every crevice of the crab's body. This can take years, but eventually she's big enough to inflate her bulbous reproductive structures through the crab's abdomen so microscopic males of her species can fertilize her eggs. Once that happens, her crabby host stops molting and growing; all it does is eat and take care of its parasite. Her babies are incubated inside the crab's abdomen, and since part of her is inside the crab's brain by now, she also hijacks its egg-caring behaviors—even male crabs nurture them—to aerate and disperse thousands of her own future mind-controlling brood.

4. ICHNEUMOID WASP // NORTH AMERICA

ichneumoid wasp
MirandaKate, Flickr // CC BY-NC 2.0

A female ichneumoid wasp Campoletis sonorensis sneaking up on a grazing caterpillar isn't looking for a meal for herself—she's shopping for a nose-to-tail larder for her young. The wasp injects one or two fertilized eggs under the caterpillar's skin, and just for good measure, squirts in a virus that will keep the caterpillar's immune system from attacking the invaders. When she flies away, the caterpillar goes right back to eating, but it's a dead grub walking: In a few days, the wormlike wasp larvae hatch inside the caterpillar. They'll spend a couple of weeks munching away at its guts until they grow large enough to burst through its body wall. Then, they spin cocoons—often beside or on the dead body of their host—and pupate into another generation of chest-busting parasitoids (which, unlike most parasites, always kill their hosts).

5. GREEN-BANDED BROODSAC FLATWORM // EUROPE AND NORTH AMERICA

A land snail's eyestalks are normally a pretty drab affair, but that all changes if the snail licks up bird droppings infected with larvae from the flatworm Leucochloridium paradoxum. The baby worms move into the snail's digestive gland, forming an asexual colony that can eventually make up a quarter of the snail's mass. As the colony matures, it starts packing members into bright green, squirming brood sacs that writhe up into the snail's eyestalks, swelling them into fat approximations of wriggling caterpillars. If that's not enough to grab a hungry bird's attention, those pulsing, writhing brood sacs can also break through the snail's body wall and crawl off to mimic a juicy grub on their own.

6. PHRONIMA AMPHIPOD // DEEP OCEAN WORLDWIDE


Freshwater and Marine Image Bank, Wikimedia Commons // Public Domain

The deep-sea amphipod genus Phronima is a literal body-snatcher. This parasitoid captures gelatinous salps—jet-propelled, filter-feeding planktonic animals that are closely related to vertebrates—and hollows them out with jaws and claws, consuming the salp's brain, gills, stomach, and muscles, and scraping its inner walls smooth. The salp body—technically still living—becomes a barrel-shaped, ocean-going home that the amphipod can maneuver like a miniature submarine. It might eventually be a full house, too—female Phronima keep their young in the barrel and care for them until they've grown.

7. RIBEIROIAN TREMATODE FLATWORM // NORTH AMERICA

deformed pacific chorus frog infected with ribeiroia ondatrae parasite
Brett A. Goodman, Pieter T. J. Johnson, Wikimedia Commons // CC BY 2.5

The horror starts when larvae of the parasitic flatworm Ribeiroia ondatrae leave the snail they used as a nursery and burrow into the tail of a bullfrog tadpole. When the tadpole metamorphoses into an adult frog—a period of time that varies between species—the flatworms form cysts around its developing legs, disrupting their growth in ways that damage or double them. The crippled, flatworm-infested frog can't jump away from predatory birds like herons, which gobble them up. The flatworm then spreads to new waterways wherever the bird poops.

8. GALL WASP // WORLDWIDE

gall wasp eggs
Justin 0 of 0, Flickr // CC BY-NC 2.0

Not even plants are safe from parasitism. Females of Cynipidae, the family of gall wasps, lay their eggs inside leaves or under bark, and their larvae make the plant cells surrounding them grow faster than they would normally, effectively forcing the plant to grow them a house. Weird, nonleafy shapes rise up out of the plant, filled with juicy nutritious tissues that feed the wasp larvae and surrounded by tough woody walls that protect it until it becomes an adult (more than a year in some species) and chews its way out of its safe space.

9. ENTOMOPATHOGENIC FUNGUS // NORTH AMERICA

goldenrod beetle infected by mind-controlling E. lampyridarum fungus
Steinkraus et al. in Journal of Invertebrate Pathology, 2017

Goldenrod soldier beetles depend on the family of flowering plants commonly known as asters, which includes goldenrods and daisies. The beetles eat the plants' pollen and mate in their shade. But if a beetle gets infected with the fungus Eryniopsis lampyridarum, it climbs up an aster's stem, clenches the base of a flower with its mandibles, and dies. Within a day, the fungus forces the dead beetle's wings open to expose its spores, which rain down on the hapless beetles below.

10. OPECOELID TREMATODE FLATWORM // PACIFIC MARINE REEFS

Coral colony infected by trematode Podocotyloides stenometra
Alamy

The tiny polyps that build stony corals are usually an inconspicuous brown. But that changes whenever a polyp inadvertently grabs a young Podocotyloides stenometra flatworm for a meal. Somehow, the trematode worm doesn't get digested—instead, it invades the polyp's tentacles, swelling them and turning them bright pink. The color is a bright billboard advertising deliciousness to butterflyfish on the reef, who eat the flashy polyps and spread the worm to other corals across the reef.

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Winter is Coming: Why Some People Seem to Feel Colder Than Others

Work blanket? Check. Hot tea? Check. Writing gloves? Check.
Work blanket? Check. Hot tea? Check. Writing gloves? Check.
shironosov/iStock via Getty Images

For a few weeks a year, as winter turns into spring, or summer gives way to fall, people in heavy coats coexist with those in sandals and shorts. Similarly, in an office where the thermostat is set at 74°F, some workers will be comfortable in short sleeves, while others will be wearing sweaters and scarves.

Underlying this disagreement are the different ways people perceive cold—and scientists are still trying to understand them.  

Men, Women, and Metabolism

In work settings, men and women often have different opinions about the ideal temperature. A 2019 study found that women performed better in math and verbal tasks at temperatures between 70°F and 80°F, while men did better below 70°F. The researchers proposed that gender-mixed workplaces might boost productivity by setting the thermostat higher than the current norm (which the Occupational Safety and Health Administration suggests should be between 68°F and 76°F).  

The discrepancy has a known physical basis: Women tend to have lower resting metabolic rates than men, due to having smaller bodies and higher fat-to-muscle ratio. According to a 2015 study, indoor climate regulations are based on an “empirical thermal comfort model” developed in the 1960s with the male workers in mind, which may overestimate female metabolic rates by up to 35 percent. To compound the problem, men in business settings might wear suits year-round, while women tend to have more flexibility to wear skirts or sundresses when it's warm outside.

Culture and the Cold

Cultural factors are also involved. European visitors are habitually alarmed by the chilly temperatures in American movie theaters and department stores, while American tourists are flabbergasted at the lack of air conditioning in many European hotels, shops, and offices. The preferred temperature for American workspaces, 70°F, is too cold for Europeans that grew up without the icy blast of air conditioners, Michael Sivak, a transportation researcher formerly at the University of Michigan, told The Washington Post in 2015.

The effects of cultural change on the human ability to withstand extreme temperatures can be dramatic. In the 19th century, 22 percent of women on the Korean island of Jeju were breath-hold divers (haenyeo). Wearing thin cotton bathing suits, haenyeo dove nearly 100 feet to gather shellfish from the sea floor, holding their breath for more than three minutes in each dive. In winter, they stayed in 55°F-57°F water for up to an hour at the time, and then warmed up by the fire for three of four hours before jumping back in.

In the 1970s, haenyeo starting wearing protective wet suits. Studies conducted between the 1960s and the 1980s showed that their tolerance for cold diminished [PDF].

Blame Your Brain

Beyond the effects of cultural practice and body composition, scientists have started to identify the cognitive factors that influence our temperature perception. It turns out that what feels unpleasantly cold versus comfortably chill is partly in our own minds.

One example is the phenomenon described as “cold contagion.” A 2014 study asked participants to view videos of people immersing their hands in visibly warm or cold water. Observers not only rated the hands in cold water as cooler than those in hot water, but their own hands became cooler when watching the cold-water videos. There was no comparable effect for the warm water videos, however. The findings suggest that we may feel colder when surrounded by shivering people at the office than if we're there by ourselves, even when setting the thermostat at the same temperature in both cases.

Other studies highlight the psychological aspects of temperature perception. Experimental participants at the Institute of Biomedical Investigations in Barcelona, Spain, watched their arms become blue, red, or green by means of virtual reality, while the neuroscientist Maria Victoria Sanchez-Vives and her team applied heat to their actual wrists. As the temperature increased, participants felt pain earlier when their virtual skin turned red than when it turned blue or green.

Subjectivity in temperature perception has led to some creative treatments for burn patients. In the 1990s, Hunter Hoffman, David Patterson, and Sam Sharar of the University of Washington developed a virtual-reality game called SnowWorld, which allows patients in hospital burn units to experience virtual immersion in a frozen environment. Amazingly, playing SnowWorld counteracted pain during wound care more effectively than morphine did.

“The perception of temperature is influenced by expectations,” Sanchez-Vives tells Mental Floss. “Putting one’s hand inside a virtual oven is perceived as ‘hot,’ while sticking one’s hand into a virtual bucket filled with iced water is perceived as ‘cold,’ despite being at room temperature in each scenario.”

In other words, if you expect to feel cold walking into the office or out on the street, chances are that you will.