Weirder Xenomorphs: Strepsipteran Insects

You know the alien from the movie Alien? The creepy, scary one with the shiny black carapace and the chest-bursting larval form and the well-regarded RPG by Free League? If you’re like me, you want to put that monster (the ‘xenomorph’) in your ongoing campaign, but you don’t want to use exactly that same monster; you want to evoke a lot of the same fears (and ‘oh wow that’s cool!’) that the xenomorph plays on while still feeling original. In that case, let me introduce you to the weirdest insects you’ll hear about all week: the strepsipterans, also called ‘stylops’ or ‘twisted-wing parasites’. They’re bizarre and gross. Their life cycle is terrifying. And your players have never heard of them, so you barely have to file the serial numbers off ‘em when you drop these nasty critters in your game.

Trigger warnings for parasites and some really awful insect sex stuff.

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An adult female stylop. The head is the dark brown part at the top-right. The stuff on the left is the head viewed from different angles.
Image credit: Zhiwei Dong, Xingyue Liu, Chuyang Mao, Jinwu He, Xueyan Li,  released under a CC BY 4.0 license.

Stylops are unusual. They’re not closely related to other kinds of insects. They’re classified in their own order (Strepsiptera), which means they’re their own thing, just like flies (order Diptera) or beetles (order Coleoptera). They spend almost their whole lives inside other insects. Only the males can exist outside a host as adults, and even then only for a few hours – and they’re tiny anyway. All of which means you never see stylops, so they’re criminally understudied. You can barely find a paper about them that doesn’t use a word like ‘enigmatic’.

A stylop begins life as a microscopic larva covered in hairs that it uses to walk or hop along vegetation until it bumps into the sort of insect that it parasitizes. If randomly bumping into your food sounds unlikely, you’d be right; stylops produce tiny offspring by the truckload in the hopes that a few will survive to reach a host. When a microscopic larva encounters a host, it waits until the host comes into contact with its own larval offspring. For example, many stylops parasitize ants and bees, so they’re hitching rides on adult ants or bees back to the nest/hive, where they encounter baby ants or bees. Then the microscopic baby stylop burrows into the flesh of the much larger baby host.

As the host develops and grows up, so too does the stylop living inside it. The stylop gets bigger as it drinks from the hemolymph (insect blood) of its host. It also does this weird thing where the larval stylop’s maggot-like body molts as it grows bigger (totally normal), but the stylop doesn’t then get rid of its old cuticle. Instead, it wears its former skin around itself like a coat. Eventually the larva forms its cocoon from these old skins. While the stylop is turning into an adult inside its cocoon, most of its body is still lodged inside the host, with only its little head sticking out of the host’s side.

This poor tiny bee has three female stylops poking their heads out of her abdomen.
Image credit: Aiwok, released under a CC BY-SA 3.0 license.

When female stylops become adults, they don’t leave. Their adult forms remain maggot-like: just an armored head sticking out the side of the host, and a hideous white legless body inside the host. The female doesn’t stop growing. In some species, a single female stylop expands to fill up to 90% of the host’s abdomen! Though her head is outside the host’s body, she can still feed because her soft white body is in direct contact with the flesh and hemolymph of her host. Nutrients thus pass directly from her host into her body without her having to do anything as plebian as using her mouth to eat.

The males look quite different and do leave their hosts. An adult male stylop looks like a tiny fly. He doesn’t eat; his mouth is adapted into a sort of sensory organ. As a consequence, he can only live five to six hours after leaving his host. He has to get a move on quickly to find a female and mate before he dies. This would be a tall order, save that the female helps him out. She emits a pheromone, a smelly chemical that males can follow to find her. It’s possible only virgin females emit this pheromone, which feels creepy for reasons I can’t quite put my finger on.

You may be wondering how mating can occur if only the female’s head lies outside her host. The female doesn’t even have genitals. Don’t worry, because the answer is awful and is known by the scientific term ‘traumatic insemination’. The female has a sort of divot behind her mouth in her heavily-armored head. The divot is still armored, but the carapace is thinner there. The male’s penis is sharp and shaped like an old-fashioned can opener. He stabs it into the divot, pierces the female’s head, and ejaculates into the interior of her body. The sperm then swims around inside the female’s body until it finds the eggs. By the way, the eggs aren’t inside a special organ or anything. They’re just strewn about higgledy-piggledy throughout her abdomen. In some cases, the male’s penis can even break off in the process and get stuck inside the divot.

The female lives for a few weeks to a few months after insemination. The eggs inside her grow and eventually hatch inside her body. She then gives birth to live, hatched young through the hole in her head left behind by the babies’ father. Scads and scads of microscopic stylop larvae pour out of this ad-hoc birth canal and the cycle begins anew.

The miracle of birth! This is the abdomen of a snow miner bee, with the head of a female stylop protruding from it. All those little silverfish-looking things are newborn stylop larvae that have just crawled out of the dark hole you can see in the stylop head.

Stylops have some other weird business going on besides their awful life cycle. Chief among these is mind control. Mind-control fungi are hip right now, thanks to The Last of Us, and stylops do it too. When bees have a female stylop in them and the parasite is ready to give birth, the bee will visit flowers as normal. But instead of collecting pollen from the flower, the bee presses its abdomen against the flower, making it easy for the parasite to give birth in a place where lots of her babies will come into contact with other bees they can then parasitize. Ants infected with virgin female stylops hang out at the tips of blades of grass, where the parasite’s pheromones will travel farther, increasing the likelihood that an adult male stylop will smell her and come mate. Infected leaf hoppers don’t even get mind control; they get body control! Their genitals shrink and wither away to nothing, so they can devote as much nutrition as possible to their parasite instead of to pursuing reproduction.

Adult male stylops have really weird eyes, even by insect standards. They’re sort of like the compound eyes most insects have – but only sort of. Each individual eyelet is some fifteen times larger than on a normal insect, and stylops have commensurately fewer eyelets in each eye. Also, hair grows between the eyelets. The overall effect is that their eyes look like blackberries. The internal anatomy of each eyelet is different from those of normal insects, and scientists suspect that stylops’ eyes may function fundamentally differently. Whether these eyes offer any advantages or disadvantages relative to normal compound eyes is an open question; these guys are really hard to study.

The last bit of weirdness is just a little thing, but it bothers me. Most insects with wings have two pairs of them. Often these pairs serve different purposes. In files, for example, their forewings are used for flight, while their hindwings are modified into little clubs called ‘halteres’. You wouldn’t even know they’re there unless you were looking for them. They help the fly balance as it flies. Stylops aren’t closely related to flies, but they have independently evolved halteres too – except that in stylops, the halteres are the forewings and the hindwings are large and used for flight. There’s nothing wrong with that per se, but it’s backwards from the way I’m used to thinking about insect wings. Like, imagine if you found a new kind of mammal. It’s not closely related to dogs, but the males look a lot like dogs – except its front legs bend like a dog’s back legs do, and vice-versa. There’s nothing improper about that, but it flies in the face of the way you expect the universe to operate. It’s fine, but it’s wrong and it’s gross and I hate it.

An adult male stylop. The red arrows point out the halteres. Note the blackberry-like eyes too.
Image credit: Duane D. McKenna and Brian D. Farrell, released 

At your gaming table, these awful parasites don’t parasitize other insects, obviously. They parasitize people! You get a little worm on you, no bigger than an inchworm or the smallest maggot. You don’t notice as it crawls into your body, either painlessly boring its own hole or entering via an open wound. Over the next few days (parasitism narratives necessarily move faster in fiction than in real life), it crawls through your body, eventually winding up in your abdomen. There it drinks your blood and grows and grows until a heavily-armored head pokes out a hole in the side of your belly. You can’t remove it without a fine surgical suite; its body is bonded to your organs.

If it’s a male, you’re lucky. It soon matures and bursts forth from your abdomen. If someone saves you from bleeding out, you’ll be fine. If it’s a female, it doesn’t leave. It just gets bigger and bigger. It presses on your organs and distends your stomach. You’re nauseous, bloated. You develop stretch marks as your abdomen swells. You’re peeing constantly, since it presses on your bladder. You develop cravings for certain foods that contain nutrients the parasite’s not getting enough of. You guys, I think Alien might have been partly about pregnancy!

If you’re in an area that’s suffering a major outbreak of your fictional stylop-inspired monsters, there will be males flying around looking for females. The males are flying insects the size of cats. They don’t have mouthparts, but they do have sharp penises like old-school can openers that they’ll use to fight anyone who messes with them. But what you’ve really got to watch out for are the hosts infected with females who are about to give birth: swarms of tiny larvae spilling from the armored heads protruding from listless, swollen humans. And they’re only listless until the mind-control switches on. Then they go from zero to trying to hit you over the head so they can dump their larvae all over you and infect you.

The penis of a male stylop. It’s that hooked spiky thing. On the left (A), it’s retracted. On the right (B), it’s ready for use.
Image credit: Peinert et al (2016), released under a CC BY-SA 4.0 license.

The PCs might show up at a village or space colony or whatever that’s fallen out of contact with the outside world. As they approach, they’re passing through territory that has a few tiny larvae crawling through it. Unless the players take enormous pains to keep from getting anything on them (perhaps before they’ve even noticed the larvae are there), one or two of them will be infected before they even arrive at the village. What they find when they get there is terrible: larvae-spewing mind-control zombies guarded by can-opener-penis-wielding cat-sized flies.

After the initial combat is over, what you have next is a problem! The PCs should expect that they’re probably infected. If they have surgery droids or healing magic with them, they can probably cure themselves before the larvae have a chance to bond with their organs. Or maybe the PCs won’t do that, which sets up a need for some high-stakes field-hospital surgery later in the adventure. As always, don’t encourage the players to do what you think they ‘should’. If they’re at a loss, you can lay out options, but otherwise just stick them in a nasty bit of business and react to whatever they do.

Do the PCs try to destroy the village or space colony to contain the threat? How good a job do they do? Do they take any steps to prevent themselves from spreading the parasite to the outside world when they return? In short: it’s the movie Alien!

A stylop eye.
Image credit: Keum et al (2018), released under a CC BY-SA 4.0 license.

Finally, note that this blog post describes a sort of generic stylop. No individual species is quite like this. They all vary from the template one way or another. Note too that I kept the jargon to a minimum, which does introduce some technical inaccuracies. For example, larval and adult female stylops don’t actually have heads, they have ‘cephalothoraxes’, which are different but serve the same function.

Check out Shanty Hunters, my award-winning TTRPG about collecting magical sea shanties in the year 1880, then singing them at the table with your friends. The lyrics of the shanties come to life and cause problems for you and for the crew of the ship you sail aboard. It’s up to you to find clues in the song and put things right!

Encyclopedia of Insects, 2nd Edition by Vincent H. Resh and Ring T. Cardé. 2009
Quick Guide: Strepsiptera by McMahon et al.Current Biology. 2011
Traumatic insemination and female counter-adaptation in Strepsiptera (Insecta) by Peinert et al. Nature: Scientific Reports. 2016
Bee-Parasitic Strepsipterans (Strepsiptera: Stylopidae) Induce Their Hosts’ Flower-Visiting Behavior Change. Yuta Nakase and Makoto Kato, Journal of Insect Science. 2021
The Insects of Australia by E.F. Riek. 1970
The unusual visual system of the Strepsiptera: external eye and neuropils by Buschbeck et al. Journal of Comparative Physiology A. 2003.
Xenos peckii vision inspires an ultrathin digital camera by Keum et al. Light: Science and Applications. 2018.

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