Caterpillar Parasites: How Do They Breathe?
You might already be familiar with the idea of caterpillar parasites, or you might not. Either way, there are things you need to know.
The basic parasitic story is this: the mama wasp or fly lays one or more tiny eggs on the back of a random caterpillar. The eggs hatch into little grub-maggots that burrow through the skin and into the body of the host. Once there, they start eating the caterpillar’s fat and non-essential organs (which can’t feel good) until they’re full-grown, when they chew their way out, spread their wings, and fly away to start a life of their own. The caterpillar is of course happy to see them go.
It’s a great day for the caterpillar when the hungry maggots in its guts finally leave. On the downside, all those little grub-maggots exiting its body at the same time has an unfortunate side-effect: instant death. All of those “exit wounds” are inevitably fatal for the caterpillar, and if you’re thinking that this will be the only reference to a Steven Seagal movie in this story, you are correct.
After they eat their way out of the poor caterpillar, the wasp grub-maggots hang around long enough to spin cocoons on the caterpillar’s skin. The cocoons hatch, and the adult wasps fly off to mate and lay more eggs on more caterpillars. All that’s left of the host caterpillar is a loose, blackened sack of digested goo hanging from a stick.
Caterpillar parasites — actually parasitoids, since they kill the host, which proper parasites do not — are one visible tip of a vast, complex, and incredibly violent web of arthropod parasitism that is boiling all around you every single day. There is basically no insect that’s immune to being parasitized, including the parasites themselves, a fully metal arrangement called “hyperparasitism” that I hope we can discuss in the Bughouse at a later date.
But as familiar as I am with the many ways in which caterpillars are attacked by parasites, one question never occurred to me until about a week ago: How do those little grub-maggots breathe when they’re immersed in the mostly-liquid insides of the caterpillar host?
So How Do Caterpillar Parasitoids Breathe?
Think about it — the parasitoid larvae are basically underwater, with no access to external air. How do they pull that off? These are the things that keep me up at night, but there is an answer. Well, answers. My source for much of the following is this truly excellent post from Ask Entomologists.
So how do caterpillar parasitoids breathe when they’re immersed in a sea of caterpillar juice? Here are your answers:
Answer 1: They are able to access oxygen drawn from the host insect’s haemolymph — basically bug blood — that the parasitoid obtains by being immersed in the stuff. Kind of like a fish, although also nothing like a fish.
This works because insects don’t have blood that circulates oxygen in veins the same way you and I do. Instead they have haemolymph, a fluid that just kind of fills up the open spaces inside the bug. Since insects have an exoskeleton instead of internal bones, there’s a lot of space to fill in. The haemolymph — which is green, because it’s copper-based, unlike your blood, which is red because it’s iron-based — circulates around the insides and bathes the organs in dissolved nutrients. It also contains dissolved oxygen.
If you’re an internal parasitoid grub-maggot, you spend your life in a bath of haemolymph. Basically, you’re a sea creature. You need to get at least a little oxygen from somewhere, and it turns out there’s just enough dissolved in the haemolymph to keep you alive long enough to devour your host’s innards. You just soak it up in ways that I am not ashamed to say I don’t understand.
Answer 2. Parasitoids obtain oxygen by hacking into the tracheal system that the caterpillar uses. This is the method favored by the most death-dealing parasitoid out there, the big bristly fly known as Compsilura concinnata. C. concinnata attacks hundreds of different caterpillars, which is unusual in the parasitoiding business. Most stick to a few different species, but this fly goes after almost everyone. Unlike most wasps, it doesn’t lay a bunch of eggs — it only lays one (this to me is actually more disturbing than a swarm of tiny wasp maggot. I have raised caterpillars that were parasitized by both wasps and flies, and the appearance of a multitude of little wasp cocoons is far less disturbing than than the appearance of one big, slimy maggot emerging from the dying corpse of the caterpillar you’ve been raising).
Perhaps because the fly larva is so large, it doesn’t even try to live on the dissolved oxygen present in the caterpillar’s haemolymph. Instead, like a berserk insecticidal HVAC repairman, it hacks directly into the caterpillar’s ventilation system.
Like all insects, caterpillars breathe through intake valves called “spiracles.” A caterpillar’s body has 13 segments, and 9 of those segments sport a spiracle on each side. This means there’s a row of air intake valves along each side of the caterpillar’s body. These little air-holes are part of system of tubing that brings in oxygen for the insect to use. As in you and me, the air-tubes are called trachea.
When the larva of C. concinnata settles into the innards of the caterpillar, the first thing it does is start pulling all of the host’s trachea it can reach into its vicinity. In this way it maintains a supply of outside oxygenated air while it munches on the stomach contents of the caterpillar. When it’s time for the maggot to exit and turn into a pupa (and then an adult fly), it simply detaches from its hijacked air tubes and eats its way out.
Answer 3. Okay, this is where things go, in the words of Lou Reed, from bad to weird. It turns out that caterpillars are not totally helpless when a parasitoid is munching away on its innards. They have evolved a response that attempts to seal off the invader by surrounding it with a kind of wall or barrier. Some parasites have counter-evolved an answer to this by using the sealing material the caterpillar produces to actually build tubes that connect to fresh air. It’s basically a homemade snorkel that keeps the invader supplied with oxygen. I know we’re getting into the weeds here, so let’s just leave it at that. You can read more on the fine blog post mentioned above.
Thanks for reading! There’s much more to be said about parasites and their victims, all of it bad. I look forward to sharing it.
Sources:
https://askentomologists.com/2015/01/26/how-do-insects-breathe-part-2-parasites/
