When the eggs make contact with your skin, they hatch and burrow into your flesh, like blowflies or lice. While you sleep, they’ll crawl out and migrate across your body in the fashion of bedbugs, seeking the best hiding and feeding spots. The ideal for them is accessing your digestive system.
As they grow, they split and multiply asexually, each generation enhancing the odds that a questing worm arrives at its goal, as well as making the infestation that much harder to permanently root out.
Still, this presents the first major fork in their evolution.
What happens if the larva achieve their goal and migrates to the digestive system, or better yet, the eggs were lucky enough to hatch there in the first place, perhaps because they had laced the hosts food?
Then they hatch in the intestines and grow fat there, unconstrained by the need to move or hide. There’s much less work to do in here — it can simply gorge itself on the hosts’ food (a theft which encourges them to eat more and more) while it grows longer and thicker. Eventually, there’s enough larva crowding their guts to bulge the stomach. But that doesn’t mean the growth will stop.
Unchecked, the larva will continue replicating even after the host loses strength or ability or willingness to keep feeding. Even after no more space remains in the guts, they will simply burst outward and infest the rest of the internal organs. As you might imagine, no host will survive long with a chest so full of hungry spawn unleashed; maggotslime drowns their lungs and their ribs burst as the heart is devoured.
This is the the natural, “ideal” process of infestion — the parasitoid lifecycle.
A special note is to be made of alternatives to the gut. It’s the ideal, but it’s not the only place the maggots can take up residence. The reproductive system can provide refuge — mainly in cases where the host has a womb. Advanced manipulation of hormones may even persuade the body to direct nutrients to the passenger of its own accord.
This isn’t the only possible outcome of the maggots arriving in the digestive or reproductive system, but before explaining the alternative, it’s illustrative to examine what happens if they can’t achieve this.
So let’s suppose they never make it to the guts. This means they must survive by subtly draining blood and fat deposits. What happens next depends on the host’s response.
Most obviously, they may notice the bites, the growing weakness and fatigue, the inflammed sickness of an immune system expelling its invaders. If they examine themselves carefully, they will notice the wounds, the blisters — they might even catch a worm in the midst of its wriggle-scurry across an exposed stretch of skin, darting to the cover of a patch of hair.
If the host scratches at the wounds, squishes or tears out the larva, and wipes themselves down, then they will create selection pressure for larva to adapt and hide themselves from notice.
Thus, energy will be diverted from just feeding on the host to camouflage. This happens on a chemical level — sucking up DNA and hormones to imitate the chemical signature of your biology and hide from the immune system’s notice — and eventually it also happens on a physiological level. At an extreme, the worms will eat your muscles, biting into their attachment sites with teeth and tail-spikes, or cutting open your vessels and routing your blood through themselves. Bit by bit, your flesh stops being yours. It still responds to your nerve signals — for now, so long as that aligns with its own agenda.
This is the endosymbiont lifecycle.
But again, this is an adaptation — it cannot happen if there’s no need for it to happen. You may have seen photos of people with necrotic wounds that have decayed to the point of having maggots visible in their cavities. And of course, those awareness campaigns of telling you to check for suspicious lumps wouldn’t be necessary if everyone noticed these things. It is entirely possible, given the right host in the wrong circumstances, for the infestation to develop to an advanced stage essentially untreated.
Thus, the maggots may gather themselves into a tumor-nest and extract nutrients with little if any pretense of mutualism.
Now, it’s easy for this to essentially mirror one of the cases we’ll discuss later on, so what’s essential for a unique outcome here is that eventually, the disease becomes taxing enough that either the host is forced to finally seek some treatment for it, or the host begins to outright fail in gathering sufficient nutrients for the parasite.
Importantly, this all must happen before the larvae have finished gathering the resources needed for their metamorphosis
As a result, the parasite is finally forced to divert resources from extraction and growth toward reinvestment in the survival of its host. This can mean synthesizing hormones like dopamine and adrenaline to motivate behavior, it can mean repairing atrophied muscles and necrotic flesh with the same techniques of replacement-imitation outlined in the endosymbiotic lifecycle.
But this outcome is a bit different. What distinguishes this is that here, the parasite isn’t hiding, and it isn’t integrating into the hosts’ biology. It is colonizing it. The parasite has centralized itself into the tumor nest, and it only extends larva-tendrils into the the limbs or hormone-signals into the bloodstream as something more akin to puppetry.
So that’s a decent enough name for this: the quasicolonial lifecycle.
But remember the first fork in the road? We didn’t finish exploring the rest of it.
So, imagine the larva had arrived in the digestive (or reproductive) systems by migration. This naturally implies that while some can live out the easy life in the guts, their sisters are still scraping by outside. What are they doing? In most cases, the intestinal larva will pump covert hormonal messages into the blood, coordinating the colonies elsewhere in the host.
This means that remaining larva will be directed toward endosymbiotic behavior. This can easily tranistion toward outright endosymbiotic or quasicolonial behavior, especially if the environment demands parasite adaptation to survive, or the host makes an effort to get rid of the infestation. (But I repeat myself.)
Still, unique behavior can emerge in cases where the host is accomodating and the environment fruitful. This is balancing on a knife’s edge. The parasite is integrated enough into the hosts’ biology to discourage outright parasitoid consumption, yet no so intertwined to advance to total assimilation.
This is the oviposition lifecycle.
Now that we’ve enumerated the different modes, allow me to finally illustrate the actual difference between them. Recall how I’ve been repeatedly referring to the parasites as larvae — you’re left to wonder, then, about the chrysalis and emergence. Larva must eventually spin themselves a cocooon.
- In the parasitoid lifecycle, the coccoon engulfs the entire corpse (or agonized corpse-to-be).
- In the endosymbiont lifecycle, the cocoon engulfs the host (or perhaps only one colonized limb at a time).
- In the quasicolonial lifecycle, the cocoon engulfs the tumor-nest (an ideal time to remove it — thus the cocoon often attaches near vital organs).
- In the oviposition lifecycle, the coccoon strains against the guts or womb, but is adapted for the host to lay it.
In short, this parasite species is profoundly adaptive. The host can be its prey to be savored from without, or a mere nest to occupy then abandon, or an assimilated extension of itself, or its fertile breeding stock.