As you will have gathered by now, there isn’t much about insects that doesn’t interest me. These animals, primitive by some measures but highly evolved by others, are endlessly fascinating in the variation of their form and the diversity of their life histories.
A particular obsession right now is nest parasites, or kleptoparasitism, among bees and wasps. These insects are not internal parasites, like tapeworms, nor external ones, like ticks. Rather, they usurp the nests and resources of their host species, letting their offspring grow by eating the food the host species has stored for its own young.
Offhand you might consider this an obvious innovation: Why not let another species do all the work? And presumably that’s the principle behind the evolution of nest parasitism. Somewhere in the distant past, some female bees or wasps laid their eggs in the nest of other bees or wasps. Some of the young survived, and over time, natural selection turned a random act of thievery into a habit.
But while it saves the work of building a nest and collecting food for your young, nest parasitism raises a host of new challenges that parasitic species have had to evolve solutions to. How do you find somebody else’s nest in the first place? How do you place your eggs inside it once you’ve found it, given that the owner has every reason to try to stop you? How do your offspring manage to overcome the host larvae, and how do you preprogram all these complex behaviors, given bees and wasps have no opportunity to teach skills to their offspring?
The short answer to these challenges, of course, boils down to “genetic control developed across many thousands of generations of random variation.” But the results often look like sheer genius.
Many nest parasites are highly host-specific — that is, they only parasitize one genus or even just one species. In many cases, the host species are themselves highly specialized, relying on just a few closely related flower species for pollen. Such specialization may seem like a huge risk, since it limits your options so dramatically. But the fact that it occurs so often argues that in fact it’s an advantage: Presumably, focusing on very specific hosts makes it easier to evolve to complex behavior needed for nest parasitism.
Nest parasitism seems to be somewhat less common among the wasps, though nearly all of the roughly 230 North American members of the family Chrysididae parasitize other wasps. (Beautiful insects, often iridescent green, these so-called “cuckoo wasps” have evolved a thick, coarsely sculpted exoskeleton and the ability, unusual among wasps, to curl into a ball when threatened. Both qualities protect against the stings of irritated host wasps.) And I recently ran into a parasitic spider wasp, which, although it is a generalist in terms of the species it parasitizes, has evolved a bizarre specificity about how it does the job.
Ceropales maculata, the spotted cuckoo spider wasp, is a member in good standing of the spider wasp family, Pompilidae. Most members of this family provision their nests with spiders, which the female wasp stings and paralyzes before hauling underground for her young to feed on as they grow. But C. maculata and the dozen or so other members of its genus have given up on catching their own spiders.
Boldly marked with yellow and about a centimeter long, a female Ceropales spends a good portion of her time eating pollen from flowers, which is where I’ve found this species. But when it’s time to lay an egg, she scouts around for a female of a different Pompilid species that is dragging a spider toward her nest. The unfortunate host female will, at some point, probably need to leave her paralyzed prey unattended while she does some excavation on her nest burrow. And when she does, the female Ceropales drops down onto the spider, quickly lays an egg on it, and disappears. Accounts are quite specific that she lays the egg, not just anywhere, but specifically on the book lungs of the spider — breathing organs, somewhat like fish gills, consisting of sheets of tissue densely filled with blood vessels.
Why on the book lungs? Nobody seems to know. Perhaps the blood-rich, oxygen-rich tissue of these organs helps the Ceropales egg stay viable until it hatches. Perhaps the complex structure of the book lungs makes it easier to conceal an egg from the host female, who might otherwise remove it. But in any case, soon after the host female has dragged the inert spider into her nest, the Ceropales egg hatches and the larva consumes, first, the host spider’s egg and then the spider she laboriously captured.
The precision of Ceropales egg placement and the amazing instincts behind this wasp’s ability to recognize potential victims would astonish me if I weren’t getting used to this kind of thing. The diversity of insects and their behavior is so extreme that not much surprises me anymore.