Encounter with a strange stream insect: yellow-legged water-snipefly


Sculpin Enterprises
WFF Supporter
I was fishing an Idaho river two weeks ago when I ran into a breeding aggregation of the yellow-legged water-snipefly, Atheris ibis. Let’s call it “the Agate Sand River”. I have fished this river for 20 years and never seen these breeding aggregations. A good friend and his son were fishing a nice run in the evening and encountered something that neither they nor I have ever seen. I fished with them the next night and had a chance to see and photograph this phenomenon for myself. We never did see a live water-snipefly but they may appear to be active during the day and we fished this spot in the evening.

On the vertical sides of a midstream boulder, they observed two ovalish aggregations of insect exoskeletons. The largest aggregation was about 18” wide and 12” tall; there was a smaller aggregation on the other side of the boulder. Here is the larger aggregation.

YellowLeggedWaterSnipeFlyAggregation30166 copy.jpg

The insects were tightly packed in a single layer and were attached to each other.

YellowLeggedWaterSnipeFlyAggregationCloseup30169 copy.jpg

A closer view of individual exoskeletons had the gestalt of a fly. When I was back home, I did some research and feel that these are yellow-legged water-snipeflies.

YellowWaterSnipeFlyHusk30100 copy.jpg

According to Wikipedia [https://en.wikipedia.org/wiki/Atherix_ibis] and this scientific paper [https://www.researchgate.net/public...8_Diptera_Athericidae_Old_myths_and_new_facts], egg-laying females of this species form tightly-packed clusters on bridges or vegetation or on the vertical face of boulders in this case. Presumably pheromones bring them together. Velcro-like structures on their feet bind the females to each other and to the surface. The larvae hatch after a 9-12 days and fall into the water. The scientific paper disputes the Wikipedia statement that the larvae feed on their deceased parents. Once in the water, the larvae are detritivores and predators on stream insects. There’s always something….

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Active Member
What a cool find!

Those look like the large yellow-butted biting bastards that are on the Quinault River. Fuckers take 2-3 aerial swats to kill and they’ve got one hell of a chomp.

Glad to have a name to put to them.


Active Member
Great shots of those Steve! And that is a rare find. I've read authors like Dave Hughes or Rick Hafele discuss the importance of their larvae in some trout streams, and I found good numbers of those myself in the Gallatin this year. But I've never even seen one of the adults as far as I know.

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