Wasps: The Only Placental Invertebrate

At one point during my (extended) university years, I attended a campus screening of an insect documentary. Maybe it was Life in the Undergrowth? Probably, because, well, David Attenborough. Anyway, although the show delivered intriguing vignettes about ants, beetles, flies, and many other six-legged beasties, the audience received the overwhelming impression of a single take-home message. In the silence after the credits rolled, one commenter expressed it for all of us:

"F*ck wasps."

Now I'm not usually given to flights of colorful metaphor, but ever since that moment, every time I have learned something new about wasps, that viewer's succinct expression pops into my head.

Wasps are despicable.

Sure, some wasps sting humans, and that doesn't feel good. I know, because in first grade I got a wasp stuck behind one knee in the skirt of my uniform*, and thereby learned that these creatures sting repeatedly.

But stings, however painful, are a mere evolutionary afterthought to the real nightmare of wasps: the ovipositor. A female wasp uses her ovipositor to inject eggs into the living bodies of other animals, where her offspring incubate, hatch, and then assiduously consume their (still-living!) host from the inside out.

Say it with me: EWWWW.

Not all wasp species engage in this "parasitoid" lifestyle. In fact, any wasp that stings almost certainly does not also do the freaky egg-injection thing, because evolution has turned their ovipositors into stingers. In the grand scheme of wasp evolution, oviposition came first; stinging is a recent innovation.

Evolution came up with the sting because of the eggs, though. If Wasp Mom just dropped off her kids at the local caterpillar and flew away, the caterpillar's immune system would go ballistic on the baby wasp intruders. So evolution mixed up a cocktail of chemicals for Mom to inject along with her eggs, a sort of "baby care package" if you will.

This package can include a paralytic venom, which weakens the host so it's easier for the babies to eat. Over time, some of these venomous wasps evolved from parasitoids into simple predators, who paralyze food and carry it back to their babies instead of burying their babies directly in the food. These are the nest-building wasps, and they include the yellowjacket that employed its arsenal on my tender five-year-old popliteal fossa.

I encourage you to delve into the nearly inexhaustible and totally incredible literature about parasitoid wasps (including the "Russian doll warfare" so delightfully named by Carl Zimmer, and the news that parasitoid wasps may be the most speciose group on the planet). No matter how disturbing it gets, you've got to give them this: they know how to take care of their kids.

Despite the tender maternal care, however, these children are not helpless--ahem--parasites. Evolution has gifted Baby Wasp with some fantastically freaky survival techniques as well.

Rather like the embryos of animals that bear live young, wasp embryos develop entirely inside someone else's body. So they don't need a hard shell to protect them from the environment. Heck, they don't even need a soft shell. Nestled in the hemocoel (essentially the bloodstream) of a fellow insect, they're safe from wind and rain and surrounded by nutrition.

All they need is a placenta to absorb it.

Yup, wasps are the only non-mammals known to possess a placenta.** Of course, this doesn't indicate kinship with mammals! Evolution simply solved a similar problem in a similar way. Shortly after Mom Wasp does her business with the ovipositor, the embryo grows a membrane around itself. As Ahmed Sabri and colleagues explain in a 2011 PLoS paper, this membrane then "invades the host tissues . . . and form[s] a placenta like structure able to divert host resources and allowing nutrition and respiration of embryo."

For readers unfamiliar with the workings of the mammalian placenta, the authors draw an explicit parallel: "Such interspecific invasion, at the cellular level, recalls mammal's trophoblasts that anchors maternal uterine wall." I'm sure this could be construed as a contradiction to my "babies are not parasites" thesis (which I did not expect to be nearly as controversial as it apparently is).

As I noted four years ago, I love babies and I love parasites, so what could be more pleasing than these placenta-bearing parasitoid wasps? I'm sorry, Hymenoptera; I take it back. Wasps aren't despicable at all. They're delightful.



* I switched to public school for second grade, so this was my only year of mandatory school uniforms. The wasp incident didn't endear them to me.

** I first learned about these wasps from the unashamedly pro-insect book Planet of the Bugs by Scott Richard Shaw. I hope that I can do cephalopods half the justice that he did insects.