BAG-WORM or BASKET-WORM, the caterpillar of the moth Thyridopteryx ephemeraeformis, so called from its habit of spinning, as a protection, a bag of silk with which it moves about head downwards, and within which it ultimately pupates. Common in the northern United States, this destructive creature is particularly partial to shade-trees, which may be completely defoliated through its activities. The adult female is wingless and never leaves the puparium, within which she lays her eggs.