Porges, Arthur – The Fly (1952)
An engineer out looking for radioactive ores to mine stumbles across a spider web, which allows him to muse a bit about the intersections of engineering and nature – the web is “the instinctive creation of a master engineer, a nearly perfect logarithmic spiral, stirring gently in a slight updraft” which makes him wonder “how a speck of protoplasm, a mere dot of white nerve-tissue which was a spider’s brain, had antedated the mind of Euclid by countless centuries.”
While he watches, a bluebottle fly gets stuck in the web, waits until the spider approaches, and then “[t]here was a metallic flash as a jointed, shining rod stabbed from the fly’s head like some fantastic rapier” to impale and devour the spider. Beginning to doubt whether this thing was a real fly at all, he rather unwisely catches it in his hand, and it blasts him with radiation and makes its escape (he realizes this after his geiger counter, pointed at his palm, goes crazy).
And that’s it. The protagonist, like the reader, is left entirely in the dark as to whether this was a robot, cyborg, alien, or whatever. This is a nice change of pace.