I work on the 23rd floor of a building, and occasionally we see grasshoppers outside our windows. Considerably larger than a few millimeters (3-5 inches in length, I’d guess) and not capable of balloon construction, as far as I know. Can these critters get that high just from rising air currents? Can they fly that high. If so, why would they want to?
Grasshoppers have wings and can fly; they don’t need balloons. They fly in order to find new territory to lay eggs, and I’d guess that 250 feet or so of elevation wouldn’t be that big a challenge to them.