This is a female scorpionfly (Panorpidae). she has a range of feeding strategies, and one of them is somewhat risky. Once a spider has successfully ensnared a flying insect in her web and parcelled it up neatly for dinner, a sneaky scorpionfly can take advantage of the nicely packaged ready-meal. Stealing food is known as kleptoparasitism and is a widespread foraging strategy in the animal kingdom.
How the scorpionfly manages to avoid becoming dinner herself is a bit of a mystery!