Jeffrey Lockwood, in "The Infested Mind," quoted at The Dish.
Too bad that book's not out on Kindle. I ended up buying "Wicked Bugs: The Louse That Conquered Napoleon's Army & Other Diabolical Insects," which is.
ADDED: "Wicked Bugs" also uses the verb "to boil" to describe grasshoppers:
A plague of locusts swept across the American West in the summer of 1875.... When the larvae hatched in the spring, it looked like the ground was boiling with them....Here's a nice illustration, from the Kansas Historical Society website:
A locust... is little more than a grasshopper under pressure.... Grasshoppers usually forage alone, spreading out across large areas when food is plentiful. But during a drought, the creatures might be crowded together, and that proximity brings on chemical changes that cause the females to lay very different eggs. The nymphs that hatch from those eggs grow longer wings, have a propensity to live more closely together and travel in dense packs, and are themselves capable of laying eggs that can survive longer periods of dormancy. They even change color. In essence, a fairly benign, stable grasshopper population transforms itself into something entirely different— a migratory plague of locusts capable of swarming and devouring everything in its path.
This explains why the settlers claimed to have never seen these particular locusts before the ominous swarms arrived, and why plagues of locusts have always been seen as having some divine origin. They are entirely unfamiliar creatures, having transformed themselves from ordinary grasshoppers to larger, darker, never-before-seen invaders.