Grass-carrier wasp larva
For the past few years when I have washed my windows, I've noticed some dry grass and what appeared to be cocoons and empty pupa skins stuffed into the bottom of our windowsills where the screen slides into the framing. My suspicion was that they were solitary wasp nests. On June 1, 2007 I noticed this yellow larva and 5 small green tree crickets in the framing (note the green leg on its head). Just when I think I've seen all the weird things the insect world has to offer, I find something like this!!! I had a very hard time taking this photo due to the strange angle I had to put my head and the camera, so I was unable to show the entire larva.
***Update*** Thanks to Eric for identifying this critter as a Grass-carrier wasp larva on Bugguide.net. According to him, the building of nests in window tracks is a recent phenomenon, they normally nest in natural cavities.