This is a post by our Research Associate Elsa Youngsteadt about some rare parasites she found.
In a lab full of people quietly staring through microscopes, a startled yelp occasionally breaks the silence. And I admit, it sometimes comes from me. Diving into the magnified world can be a lot like watching a scary movie on a big screen.
Usually the freak-out happens when I’m doing something soothingly monotonous, like counting tiny scale insects on leaves. I get in a groove and forget that I’m seeing everything 30 times bigger than it really is. Then a spider dashes crazily across the field of view: It appears enormous, threatening, bristly, unpredictable, and very, very close. This is when the yelping happens. But of course the beast is actually smaller than a pencil eraser and nowhere near my face, so I feel silly, compose myself, and get back to the scale insects.
But sometimes, the startling thing is something I’ve never seen before, something wild and creepy and totally distracting. This is what happened earlier this year when I settled in to identify some newly collected bees from my yard. These were dead bees, mind you, fresh out of a cyanide jar and expected to sit still and not be startling.
The first few toed the line. There was a shiny, bulbous, blue-green mason bee, and a plain little matte-black sweat bee. A ground-nesting Andrena still had flakes of clay on her jaws. All normal and non-threatening.
Andrena bee with strepsipterans peeking out of its abdomen. Photo: Matt Bertone
Then, the yelp. Spreading across the rear end of the next bee was a mass of very alive, squirmy, maggoty little larvae, looking not-so-very little. I knew in split second what they were, but I admit to quickly popping the bee back in the cyanide jar to finish things off before looking again.
I was witnessing the birth of a brood of strepsipterans—a group of parasites whose wacky life cycle involves a weird, blobby insect eating a live bee from the inside out, while that insect’s own babies eat her from the inside. At the time I caught my bee, the strepsipterans had just finished feeding on their mother and emerged into the world, via a hole in their mama’s head (technically, in her cephalothorax). How else?
Had the bee avoided my net and continued to fly around visiting plants, the larvae would have hopped off on the next flower. (Really, hopped: Despite the initial maggoty impression, these are leggy, nimble little insect tykes.) Each one would have waited at the flower-airport to board a fresh bee, which would carry it back to an underground nest tunnel. There, the strepsipteran would find the bee’s egg or larva and burrow in—disappearing inside within a few hours.
The strepsipteran then loses its legs and eyes and grows along with its young bee host. Eventually it fills the bee’s abdomen, stunting its organs and altering its development. Female Andrena bees, for example, develop the angular cheeks and hairy abdomens typical of males. (They also become very hard to identify.) One researcher even reported finding a bee with a single, giant eye arching over its head like a headband–perhaps a strepsipteran-induced developmental glitch.
Strepsipterans emerging from Andrena bee abdomen. Photo: Matt Bertone
As adults, male strepsipterans have wings and return the outside world to fly around for a few hours before they die. Females, however, remain ensconced in their bees, with only their heads visible from the outside. (That’s the smooth, yellowish tissue you can see sticking out of the bee’s abdomen in the photos.)
The rest of her body is “a great sack full of eggs,” wrote entomologist W. Dwight Pierce in 1909. The thousands of eggs simply float around inside the mother’s body, absorbing nutrients from her blood. Eventually, they hatch and trek to the outside world via the “brood canal” that exits their mother’s head. Which is where they were when they made me shriek.
This whole stranger-than-fiction setup is more than just a titillating insect freak-show. As parasites, strepsipterans may have an important role in maintaining insect diversity. Parasites tend to regulate their hosts’ populations in ways that prevent any one species from becoming outrageously common—leaving room for more species overall. That process keeps ecosystems diverse and food webs stable.
Stepsipteran larvae emerging. Photo: Andrew Ernst
I would bet that strepsipterans are among the parasites that help maintain diversity, although their role has never been measured. There are about 600 species of strepsipterans in the world, and they can also be found in planthoppers, cockroaches, grasshoppers and wasps. In the bee and wasp species that have been checked, 10 to 30% of individuals carry strepsipterans and are unlikely to reproduce.
That means strepsipterans are common enough to regulate the abundance of their hosts. In a tantalizing example, the European paper wasp (Polistes dominula) left its strepsipterans behind when it moved across the Atlantic to North America—and none of the North American species will parasitize it. So researchers have suggested this wasp’s fast reproduction and dominance over native North American wasps may be due, in part, to the loss of its parasites.
Of course I have no way of knowing where my one bee with her alarming brood of squirmy strepsipterans fits into the big picture. But I’d like to think she’s a good sign. I’d like to think that if I kept looking I would find that my messy yard is teeming with a diversity of slightly creepy parasites, keeping their hosts in line and yielding an endless supply of microscopic scary movies.