Last summer I wrote about an intriguing find I discovered on a blade of grass. I wasn’t able to identify the hard, odd-looking, black-as-coal structure but wondered if it might be an egg case of some kind. A short time after that I discovered a second one in the same area — a roadside ditch next to a cow pasture.
Thanks to Alberta entomologist and author John Acorn I now have an answer. It’s probably the egg case of a horsefly or deer fly. (You know, those pesky summer spoilers that can take a chunk of meat out of your arm or leg or wherever they decide to dine.)
A check with Bug Guide confirmed the ID. Apparently these flies deposit their egg masses on vegetation that overhangs water or wet ground (the latter in the case of the ones I found). When the eggs hatch the larvae drop to the ground. (Or into the water, I guess.)
WikiPedia added more to the story …
Both males and females engage in nectar feeding, but in addition to this, females of most species are anautogenous, meaning they require a blood meal before they are able to reproduce effectively. To obtain the blood, the females bite animals, including humans, while the males are harmless. It takes the female about six days to fully digest its blood meal and after that it needs to find another host. It seems that the flies are attracted to a potential victim by its movement, warmth, and surface texture, and by the carbon dioxide it breathes out. The flies mainly choose large mammals such as cattle, horses, camels, and deer, but few are species specific.
And that’s where the story ends for now. I still have questions. For example, how to the eggs get out of that hard case? Does it dissolve after a time? Do they eat their way out? Can they survive in both water and land?
More will be revealed, I’m sure.
The Lens and I 
Those miserable things shouldn’t have any problem biting their way out.
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Thanks for the chuckle, Terry! With 100-800 eggs per case that’s a lot of bite. 🙂
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I read about a creature like this recently — one that puts its egg cases above water, so that the larvae can drop down into it when they hatch. I tried to find the post, but couldn’t. Nevertheless, those clever insects know what they’re doing, and this is another great example.
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We’re surrounded by so many “mysteries” — more questions than answers — it makes getting up each day another adventure!
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I really don’t like horseflies. They pack a mean bite and have a tendency to land on your head. Out comes my hat when I am hiking! 🙂 –Curt
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I’m with you, Curt! It’s been years since one has dined on me — I certainly give them a wide berth. One only needs to see cows or horses running across a pasture to know what’s driving them. Ouch.
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Don’t know if you been to East Africa, Sally, but they are very much like the tsetse fly. –Curt
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Nope. But it sounds like you have and got to meet those little devils up close and personal!
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Definitely worth it, but you are they hard to kill!
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🙂
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