Complex sentence: The larvae construct silken webbing inside which they live in tubes, chewing off needles and pulling them inside the tubes for consumption; the tubes soon fill up with bits of needles, cast skins and frass.
Split into simpler sentences: The larvae construct silken webbing inside which they live in tubes, cutting off needles and pulling them inside the tubes for consumption. The tubes soon fill up with bits of needles, cast skins and frass.