Ichneumon #2

2008 March 22

This is the second Ichneumon wasp (photographed last June), that I mentioned in last week’s posting:


The very long ovipositor (in this case, longer than her body!) is pretty characteristic. While it looks dangerous, it isn’t a stinger and she’s harmless (to us! More on this later . . .) – the ovipositor is used for laying eggs in awkward places.


While it is certainly a member of the Superfamily Ichneumonoidea, Bug Guide isn’t a lot of help beyond that. It could be one of the Family Ichneumonidae, the “true” Ichneumon wasps (about 3100 species in North America), or one of the related Family Braconidae, the Braconid wasps (about 1700 species in North America). The thing is, both the Braconid family and the Ichneumon family are highly variable, and there are a lot of Braconids and Ichneumons that look a lot like each other.


To tell whether it is a Braconid or an Ichneumon, we have to look at the pattern of veins in the wings. It took a little doing to find a good wing picture[1], but this one is not too bad:


And, if we compare it with this drawing[2]


we can see that the vein layout matches the Ichneumon pattern. Specifically, Vein 2m-cu is present, if it was a braconid wasp, this vein would be missing.

Well, enough about trying to identify it. The big question is how do they live, and what is that loooong ovipositor for? This brings us to the singular habits of wasps[3].

Ichneumon wasps are all parasites of other small creatures, usually larvae of one type or another. They lay their eggs on, near, or in their victims, and once the eggs hatch they gradually eat their way around inside, delaying eating any of the really vital organs of their tormented host until they are ready to pupate[6]. At that point, they usually (finally!) kill the host, pupate on or in the corpse, and then emerge as adults.

I gather that their parasitic lifestyle is why there are so many species of Ichneumon and Braconid wasps: some of them are general parasites of many species, while others are highly specific parasites of just one or two host species. Braconids in general are usually smaller than most Ichneumons, with the Braconids tending to parasitize smaller insects, like aphids[4]. So right away, we get different species depending on how big the host is.

Then, there is the question of host lifestyle: the wasps have wild variations in the ovipositor design, depending on how they have to lay their eggs in order to get them into the hosts. Some of them land directly on the host and plant an egg in them, in which case the ovipositor is relatively short and designed for a quick inject-and-go style. The ones with very long, robust ovipositors, like this one, are more likely to use them to penetrate rotting wood, laying their eggs in the grubs tunneling around under the bark. They walk around on the surface of the wood, listening for vibrations of grubs moving around. Then, when they find one, they pinpoint it, and quickly bore down through the wood and get the eggs as close to the (now doomed) target as they can. There’s some question about how they bore through the wood, they evidently don’t have an obvious wood-boring mechanism on the ovipositor tip[5].

Finally, there’s the question of host immune systems. If, say, a caterpillar’s immune system is up to snuff, it can reject the wasp egg, so the wasp has to be able to counter the immune response. This is where it starts getting weird: it seems that the braconids and ichneumons have symbiotic viruses that they inject into the host along with the egg, and the virus suppresses the host immune system so that the egg can grow. The thing is, the virus actually grows in the wasp (it has been inserted into the wasp genes), so the wasp larva that hatches out already has the virus in itself, so when it grows up it will have the virus to inject into the next host. This whole thing really drives the wasps to be different species, because they can only lay eggs in a host that will have its immune system suppressed by their particular strain of virus. So, we pretty much have every host species with its own species of parasitic wasp to infect it, and voila! Thousands of specialized species of wasp!

And, one last, almost unrelated note:
When I searched for “ichneumon”, this bit came up on Wikipedia — “The ichneumon is the enemy of the dragon. When it sees a dragon, the ichneumon covers itself with mud, and closing its nostrils with its tail, attacks and kills the dragon. The ichneumon was also considered by some to be the enemy of the crocodile and the asp, and attack them in the same way.” That’s evidently based on something Pliny the Elder wrote. Of course, it turns out he was talking about the Egyptian Mongoose (which is also called an Ichneumon), and this is a garbled version of how the real animal kills snakes. What the connection is between the Ichneumon mongoose and the Ichneumon wasp, I really can’t say.

[1] While this wasp wasn’t all that big in terms of mass, it was awfully long, so I was photographing it without the macro lens so I could get the whole insect into a single frame. Now, many months later, I think that was a mistake, I really needed the macro in order to see the wing veins properly. These days, I think I’d go ahead and take detail pictures with the macro in addition to a “long shot” to get the whole wasp.

[2] The drawing is from this document (**Warning! Huge PDF file that takes forever to download!**), which is a key for distinguishing between the subfamilies of ichneumons and braconids. Unfortunately, it is for the ones found in England, so it misses a lot of the ones in North America.

[3] “The Singular Habits of Wasps” is the title of a story by Geoffrey A. Landis that involves Sherlock Holmes, Jack the Ripper, aliens, and parasitic wasps (with a brief cameo by H. G. Wells). It is in Landis’ collection Impact Parameter for anyone who wants to read it.

[4] The size difference isn’t a reliable way to tell them apart, though. The smallest Ichneumons are smaller than the largest Braconids. Also, the parasitizing evidently doesn’t stop with the first wasp: if you read to the bottom of this page, you can read about another kind of wasp that lays eggs in the wasps that parasitize the aphids!

[5] If I catch another one this spring, maybe I can mount it up and sign up for some electron microscope time at work, so I can look at the ovipositor tip under very high magnification.

[6] Darwin was evidently particularly repulsed by these wasps. He wrote the following in a letter: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.”

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