Translucent caterpillar of the grass demon butterfly (Udaspes folus) playing host to a parasitic horsehair worm. (Photo by ygurjar)
Adult horsehair worms are aquatic and free-living (do not require a host), but larvae are parasites of arthropods. The host is usually killed when the worm emerges after maturing.
Uninfected grass demon butterfly caterpillar:
Visible are the tracheae (respiratory tubes) radiating way from the spiracles (breathing holes), but no worms are present. (Photo by aniruddha_singhamahapatra)
