Noun
Any hairy caterpillar which appears in great numbers, devouring herbage, and wandering about like a palmer. The name is applied also to other voracious insects.
In America, the larva of any one of several moths, which destroys the foliage of fruit and forest trees, esp. the larva of Ypsolophus pometellus, which sometimes appears in vast numbers.
Source: Webster's dictionary