Black Fly Larvae and Water Striders

2015 August 29

We were all up at a small stream near Eagle Harbor on July 19, 2015, where Sam and Rosie were busily rummaging around to see what they could catch in their nets. And we noticed a particular spot in the fairly rapidly-flowing stream where objects on the bottom were coated with these little black insect larvae. They were each only about 3 to 4 mm long.

Here are some that were on a leaf that we fished out for a closer look.

The bigger, black ones with fat heads are most likely black fly larvae in the family Simuliidae (the thin one in the very middle of the picture is something else, most likely some kind of midge). Anyway, once we spotted those we were glad not to be there when they hatched out as adults, because these are some of the most annoying of the biting flies that we have in the area, which I am willing to go to some length to get rid of. What these larvae do is hang on to a surface in a stream with moderately brisk flow, and then filter out any debris that washes by. Peridoically they bring up their tail filters and lick off whatevery they’ve caught. On the one hand, this is not a very energetic lifestyle, so they don’t need all that much food. On the other hand, they don’t catch all that much, so they end up short of protein and salts when they mature. Which is why the adult females come after land animals to get their blood, which provides the things that they lacked and make it easier for them to lay eggs.

This is why black flies around here are generally a fairly localized problem. They only seem to fly maybe a few hundred yards from the flowing stream that they grew up in, and so you only hit them close to moderately fast-flowing rivers and streams (unlike mosquitos, which will breed in pretty much any stagnant water they can find). The big emergence of adults is generally in late May through most of June, so that’s a good time to only approach rivers if you are wearing a head net or some other protective gear.

And, at the same time, Sam caught some water striders (along with some tiny fish like the one blurrily visible on the right side of this next picture)

This is a smaller water strider species than the ones I posted back in 2008. I think that these are in the genus Metrobates, a group of smaller water striders with short abdomens (and, apparently, no wings).

Water striders (also known as “Jesus Bugs”, because they walk on water), are carnivorous bugs that skate around on the surface looking for even smaller creatures to eat. Which includes mosquito larvae. Since mosquito larvae breathe through a snorkel that they poke through the surface of the water, the water striders can grab them by the snorkel and eat them. I approve of this. Unfortunately, they can’t get at the black fly larvae because they aren’t on the surface, so we need some other predator for them.

Comments are closed.