Larvae of the midge. They are red because their blood plasma contains haemoglobin like human blood, which increases its ability to take up oxygen. This is of value to the larvae, which commonly burrow in the oxygen-poor mud bottom of pools and rivers. They feed on algae and detritus.

Bloodworms are long, with a distinct head, and segmentation of the abdomen. Prolegs (leglike projections) are found on the first thoracic and last abdominal segments. Gills are present on the last abdominal segment, and often on the segment preceding it. On average they measure 6 mm/0.2 in in length.

Bloodworms frequently build tubes of mud around themselves, which may be attached to stones. They constitute a major part of the diet of fish, hence they are often used as bait by anglers.

Not all midge larvae are red. Those that do not live in mud tubes, but frequent the surface waters, are green, and some species have blue bands.