“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…”
– Charles Darwin in a letter to a friend, 1860 –
In a garden somewhere near you, a caterpillar sits on a leaf, munching away at the edge. Suddenly, a wasp descends and stings the caterpillar. Over the next few weeks, the caterpillar will go through the usual molting processes and construct a chrysalis for its transformation. But this caterpillar will never become a butterfly, for when the wasp stung the caterpillar, she injected an egg inside it. Once the egg hatches, the wasp larva will find itself surrounded by its favorite food: the caterpillar’s organs. After it has eaten its fill, the larva will pupate inside the host’s body — and from the chrysalis will then emerge not a butterfly, but an adult wasp.
Dramas like this play out all around us during the spring and summer months. Although it may seem like a tale straight from a horror film, it’s just business as usual for parasitoid wasps like the Ichneumonidae. This insect family, whose lifestyle so deeply disturbed Darwin, is among the most species-rich branches of the tree of life. And yet, despite the group’s enormous diversity, there is still fairly little known about its ecology, distribution, and evolution. The 25,000 species described today probably only represent a quarter of the total number. In other words, this single family likely contains more species than all vertebrates combined.
If there are so many ichneumonid wasps out there, why aren’t they more familiar to us? Why don’t we talk about them more? One reason for this may be that Ichneumonids aren’t the human-stinging, angry-sounding wasps that disrupt our picnics or build nests on our patios. Another reason might be that, until recently, we could only call these wasps by their Latin name.
A common name
To advance Ichneumonidae research and to popularize the family, a conference was hosted at the Natural History Museum Basel last year. It attracted ichneumonid specialists from five continents who shared their latest research findings. The experts also spent time brainstorming ideas for a vernacular name for the Ichneumonidae.
Some suggestions included “seeker wasp,” since they are often seen searching for their next host, “bastard wasp,” as their hosts might call them, and “wolverine wasp,” as they are referred to in Finland, where wolverines are top predators. But in the end, “Darwin wasps” won the vote. The scientists hoped this name would attract a broader interest for their beloved wasps, and they appreciated the reference to the quote in which the famous evolutionary theorist questions the existence of a benevolent creator.
Ironically, the new name also draws attention to the fact that Darwin himself omitted many relevant traits when he referenced the wasp family. For one, Darwin wasps don’t just parasitize caterpillars. Some species attack moth cocoons, whereas others specialize on various life stages of flies, beetles, spiders, or even other wasps. There are even hyperparasitoids, which have evolved to attack hosts that are already inside another host. The Ichneumonidae are immensely diverse — each species has evolved complex adaptations to specialize on a certain type of host.
For example, the subfamily Rhyssinae prefers wood-dwelling hosts. A mother wasp finds the larva by tapping a tree branch with her antennae and listening for the hollow echo of a feeding tunnel. After locating her prize, she uses a tiny saw at the end of her ovipositor to access the doomed host.
Beyond extraordinary host-finding strategies, Ichneumonidae have evolved a plethora of other bizarre traits. The genus Genaemirum has a face shaped like a shovel, which it uses to scoop sawdust as it bores head-first into trees. Other species have adopted pet viruses that suppress the host’s immune system. Males from the subfamily Diplazontinae wrap their antennae into tight supercoils around the female’s antennae, probably to spread sweet-smelling pheromones directly onto her scent organs.
Broader impacts of Darwin wasps
Darwin wasps represent one of the world’s largest adaptive radiations (a rapid splitting into many species). For most of these species, we are only starting to learn where they live and what they eat, never mind the intricacies of how they interact and coevolve with their hosts. Studying them can help us understand how lineages evolve and diversify.
Moreover, Darwin wasps play a vital role in agriculture. By regulating insect populations, they protect our food crops and reduce the need for harmful pesticides, making them important agents of biological control.
Find Darwin wasps near you!
As a PhD student in Zoology at the University of Basel, I study host–parasite coevolution. I also volunteer at the Natural History Museum, where I am currently describing a new species of Darwin wasp that parasitizes aphid-eating hoverflies. In this project, I work closely with the museum’s curator of insects, Seraina Klopfstein, who organized the conference in Basel.
February 12, 2020 would have been Darwin’s 211th birthday. We want to celebrate by spreading the word about these incredible insects that boggled the mind of an evolutionary genius. Join us! Take a picture of a Darwin wasp near you and post it under the hashtag #DarwinWasps. For help with identification, post it to the Hymenopterists Forum on Facebook, or email me directly: email@example.com. Your finding might even be a valuable contribution to our limited knowledge about the distribution and life history of these wasps.
How to find and recognize Darwin wasps? Here are some tips:
· Worldwide (except Antarctica and open ocean)
· Transition zones, e.g. between an open field and shaded forest
· Sweep along bushes and undergrowth with an insect net
· Size: very variable! From a few millimeters to several centimeters in Switzerland (most about the size of a honey bee)
· Shape: depends on host; from quite robust to very slender
· Antennae: long (>16 segments), flexible. Darwin wasps are one of only 2 wasp families with such long antennae
· Wings: “horse head” cell: If you have a microscope or a magnifying lens, look at the front wings: if the wing veins form a pattern that looks like a horse’s head, you can be quite sure you’ve found a Darwin wasp.
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