Hygraula is the only aquatic caterpillar (a moth larva) in New Zealand. The pale, yellowish or greenish caterpillars are easily distinguished from terrestrial caterpillars by the clusters of tentacle-like gills along the body. The caterpillar is sometimes found in a portable case made from fragments of aquatic plants or algae.
Hygraula is most commonly found amongst aquatic plants, including the introduced oxygen weeds. These plants tend to grow in soft-bottom, slow-flowing and unshaded streams.
They are among the few New Zealand freshwater invertebrates that feed on aquatic plants.
Hygraula larvae are usually associated with weedy streams and these commonly have poor water quality with low night time dissolved oxygen levels. Hygraula has relatively low tolerance values of 4 (hard bottom sites) and 1.3 (soft bottom sites).