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Terri posting

Strepsyptera are a very strange little order of insects. They're closely related to beetles, forming a sister clade to the order Coleoptera. But while beetles are one of the most diverse and successful groups of animals on the planet, there's only about 600 described species of strepsypterans, and you've probably never heard of them; they have hardly any relationship to humans at all.

Strepsypterans are parasites on other insects, usually bees or wasps. They enter the abdomen of a host as a larva, usually spreading in nests or colonies where their hosts congregate. Once inside the host, they cause the host's body to develop a cyst that the parasite lives in, inside the host's abdomen. There, they start growing; the main genus of strepsypterans, Stylops, can grow big enough to take up most of the space inside the host's abdomen. This typically renders the host sterile, as their reproductive organs are squeezed by the growing parasite inside them.

Male parasites will, upon metamorphizing into their adult forms, leave the host. They leave behind their moult, which continues to take up space inside the host's body. The males are extremely short-lived (a few hours is typical), and like many insect species, they can't feed. Their only goal is to find a female to mate with.

The adult females, by contrast, never fully metamorphize and never develop wings. They retain a grub-like body plan, simply becoming very large (by the standards of something that lives inside a bee's abdomen). To help males find them while they're still inside the host, they stick their head out of the host, in between the plates of their abdominal exoskeleton, and spew pheromones to signal a male to arrive.

Mating happens through hypodermic insemination. Males have a sharp, stiletto-like penis that is simply stabbed into the female, sometimes on her exposed head, sometimes through the abdomen of her host into her body. The sperm, injected directly into the interior body cavity of the female, then simply find their way to her eggs. The brood incubates and hatches still inside the female. As you are probably guessing, the newly-hatched larvae start their lives by eating their way out of their mother, which gives them enough of an energy reserve to crawl out of their mother's host in search of a new host to parasitize.

this kind of metamorphosed-male + maggot-adult-female (paedomorphism is the technical term) situation isn't wholly absurd in the insect-sphere. Not common, but not unique.

ok so groundwork.
Insects (generally) get older in a series of molts, where they shed their hard exoskeleton to reveal a soft one, and then use air to inflate their body size as much as possible before the new exoskeleton hardens. Hard-outside means growing like soft-bodied creatures isn't feasible, they can only get so much bigger before their exoskeleton blocks further growth.
There are hemimetabolan insects, that start as a nymph (basically a small version of the adult body) and moult a few number of times, usually only getting wings on the final, adult, moult.
Then there are holometabolan insects which do the maggot --> pupa --> adult thing, like butterflies and beetles and bees. Being able to change the body-plan to such a massive degree is great because it means that adults & babies can eat different food and live in different places and therefore not get in each others' ways. But it is super tiring and intense to turn yourself into goop and then turn your goop-body into a shiny new adult body. and also when you're goop you can't eat, so if you didn't eat enough beforehand, you're screwed. there are also other problems, like how the gut bacteria gets reset during the process. Pupae can't run from danger. so on.

So when you're a female stylops & live inside the body of a host, metamorphosing into a winged form is kinda useless unless you're going to go live outside the host, but the host provides valuable nutrition and protection that'd be great to have when you've got eggs. So the female stylops may not leave for extra food ("maximal resource extraction") or maybe they're better protected inside the host, or maybe the male's sharp penis is sufficiently dangerous the female uses the host as a barrier against it.
The males metamorphose, though, because they can use the wings to find the females, and it doesn't matter if they live very long since they don't need to care for the eggs after mating.

so mostly you see maggot-females, who stay in the less-mobile but more safe, nutritionally robust grub form, and winged males that have the sole purpose to find the females and mate.
Outside of stylops, you see this arise (seemingly) from nutritionally-poor diets (Platerodrilus), spatially-restricted habitats like under tree bark (Ptinella errabunda) (which, alongside Plastosciara perniciosa, can live as both a true-adult and maggot-adult depending on what's better).

source of much of this post, and worthwhile reading for more:
McMahon, D. P.; Hayward, A. Why grow up? A perspective on insect strategies to avoid metamorphosis. Ecological Entomology 41(5):505-515, 2016.

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