ONLY a few species of spiders and insects are known regularly to inhabit spider webs. Known associations involve predation or commensalism. Spiders of the family Mimetidae invade the webs of other spiders to devour the owner of the web. Small spiders of the genus Argyrodes (Theridiidae) are often seen in the webs of larger spiders, where they seem to consume only small insects ignored by the spiders that constructed the web; this relationship is thus commensalism and not parasitism2. The insect Ranzovius (Hemiptera: Miridae) seems to be a commensal in the webs of certain spiders3; however, R. fennahi may be predaceous on spider eggs4. The tropical hummingbird, Phaethornis superciliosus, has been observed feeding on insects trapped in webs of Nephila clavipes (Araneidae). The hummingbird–Nephila association may be parasitism because although only very small insects are taken by hummingbirds, they may remove a portion of the host's web for nest material5. I report here the kleptoparasitism of the prey of web-building spiders by scorpionflies of the genus Panorpa (Panorpidae), and I consider a likely selective context for the evolution of this unusual feeding strategy in scorpionflies. Field observations were conducted in south-eastern Michigan near Ann Arbor during 1971–74 (ref. 6).