Haemoglobin is usually associated with vertebrates, but chironomid larvae carry haemoglobins free in the haemolymph rather than packed into cells. These molecules bind oxygen at very low partial pressures and release it as the larva works, which is why bloodworms can occupy soft sediment at the bottom of lakes and slow rivers where dissolved oxygen approaches zero for long periods.
The larvae live in tubes of silk and sediment and undulate to draw water past themselves, collecting detritus and algae. Their tolerance makes them useful indicators: an invertebrate community dominated by chironomids and worms is characteristic of organically enriched or oxygen-poor water.
Adults are the source of the enormous lakeside swarms seen at Lake Victoria, the Great Lakes and elsewhere. They have no functional mouthparts and cannot bite; the mating swarm is essentially the whole of adult life, lasting days at most, and the biomass returned to shore feeds birds, spiders and bats.