HMC Bee Lab: A fly in bee’s clothing

Here’s a challenge: can you spot which one of the insects below is a bee, and which are the imposters?

Volucella bombylans, Eristalis arbustorum, and Syrphus ribesii [1]

Trick question… none of them are bees! There are a few details that can clue you in- these flies have far larger eyes than the bees they’re mimicking, and have two wings instead of a bee’s four.

If you know what to look for, it seems like these clues might make it pretty simple to identify these flies-masquerading-as-bees… right? As I’ve learned this summer, things can be a bit more difficult in the field.

Part of our bee field experiments involved counting how many bees arrived at certain flower patches. Knowing what these not-bees look like was important for making sure I wasn’t accidentally counting flies as well. We had to sit still, and at least a couple feet away from the patch we were observing to avoid disturbing the bees. So, the “bees” were far away, partially hidden by flowers, and moving! Even when I moved in to take photos after I had finished counting, most of my pictures ended up looking like this:

Some very blurry photos of buckwheat flowers. There’s a fly somewhere in the second one! [2]

Thankfully, there’s an easier way to tell who’s a bee. Morgan told me to look at the way the insect was moving. It turns out, the honey bees we were trying to count tend to hover around a flower before landing- they often seem to change their minds last-minute and go somewhere else. In contrast, the not-bees I observed tend to be very decisive when flying, and (ironically) make a bee-line for their destination.

With this new identification trick (and Morgan’s expertise) I was able to build a small collection of not-bee photos:

The first two “bees” are from different Bombyliidae fly species.

The last one might look the most fly-like of the three, but

it actually looks very similar to a kind of carpenter bee (Xylocopa

californica) that I’ve seen around the field station! [2]

So, why would flies disguise themselves as bees? (Other than to thwart the efforts of field researchers, that is.)

One reason might be what’s known as Batesian mimicry- a harmless species mimics a dangerous one in order to ward off predators. For example, many types of hoverflies mimic stinging bees and wasps. Although the flies themselves can’t sting, the insects they’re mimicking can. Even if the fly’s disguise isn’t perfect, the predator might not want to risk a sting and decide to leave it alone. Similarly, if a predator bothers a social bee like a honeybee, it could potentially face the wrath of the colony. Honey Bee-mimicking flies could be using this to their advantage.

However, there are some fly species that use a bee disguise for more nefarious purposes. Some types of robber flies mimic the appearance of bumblebees to fool both their predators and their prey. Bumble bees don’t hunt, so small insects generally don’t see them as a threat. However, robber flies eat other insects- including bees! Their bee-like appearance catches their prey off-guard. So, despite looking pretty similar to a large bee, robber flies don’t exactly replicate their behavior!

A robber fly and a rather unfortunate bee. [3]

Something I find fascinating about mimicry is that these flies aren’t just putting on a bee costume– as a species, they’ve evolved to resemble bees! I think the difference between bee-mimicking robber flies and other types of robber flies shows how incredible this adaptation can be.

Left: A robber fly in Claremont.

Right: A bee-mimicking robber fly. [4]

Whereas the fly on the left isn’t fooling anyone, the fly on the right is able to mislead predators, prey, and probably also some humans! Even if the disguise is imperfect, sometimes looking “enough” like a bee is all these mimicking flies need. Potential predators and prey might not have the chance to count its wings!

Unfortunately, not the way insect mimicry works. [5]

HMC Bee Lab

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Tuesday, July 20, 2021

A fly in bee’s clothing

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