We were in Ferry Beach State Park in Old Orchard Beach, Maine, on 11 August when I spied this tiny drama taking place. One of the kids present was fascinated and another couldn't care less. A harvestman (Order Opiliones) was sucking the juices out of a large fly.
Eventually it got nervous and dragged the fly to a lower leaf, where it continued feeding.
I returned an hour or so later, and the harvestman was finished its meal and was resting on a nearby leaf. (The conical structure beside its body may be a gall produced by a gall wasp - see comment for correct id.)