Thanks to Morgan Jackson, who has come through with an ID – this is a spider fly (Acroceridae), also known as hunchback-flies or small-headed flies.
As far as is known all Acroceridae are parasitoids of spiders. They are most commonly collected when a spider from the field is brought into captivity. As in the related families, Bombyliidae and Nestrinidae, members of the family undergo hypermetamorphosis; the adults do not seek out their hosts; instead the first instar larvae is a planidium. In the Acroceridae the planidia seek out spiders. They do not resemble the triungulin of most beetles with a hypermetamorphosis, but do resemble the triungulin of Stylops. The larva can move with a looping movement like a leech or inchworm, and can leap several millimetres into the air. When a spider contacts an acrocerid planidium, the planidium grabs hold, crawls up the spider’s legs to its body, and forces its way through the body wall, usually at an articulation membrane. Often it lodges near a book lung, where it may remain for years before completing its development.
The adults of most species, like various members of the Tabanidae, Nemestrinidae and Bombyliidae, are nectar feeders with exceptionally long proboscises, sometimes longer than the entire bodily length of the insect. Unlike the other families however, when not deploying the proboscis for feeding, the Acroceridae carry it lengthwise medially beneath the body, instead of projecting forward. As a result the proboscis might escape casual notice, though careful inspection may reveal it projecting slightly behind the abdomen.
Another cool parasite!
(Found in the Saskatchewan River Valley, Edmonton.17 August, 2011)