T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Fly (Hairy Kelp fly) Chaetocoelopa littoralis

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Section: Schizophora
Subsection: Acalyptratae
Superfamily: Sciomyzoidea
Family: Coelopidae
Genus Chaetocoelopa
Species C. littoralis
Binominal name: Chaetocoelopa littoralis
Synonyms: Coelopa littoralis, Coelopa monstruosa, Chaetocoelopa huttoni
Common names: Hairy Kelp fly, Hairy Seaweed fly,

Chaetocoelopa littoralis is a native fly that is a member of the genus Chaetocoelopa which are commonly called Kelp flies. They are found in the wrack zone of the seashore where kelp is deposited on the sand. 
Chaetocoelopa littoralis larvae are important decomposers in the coastal ecosystem, consuming decaying brown algae, particularly rotting kelp. These flies can be very abundant in this habitat and on landing sometimes they form enormous aggregations on shoreline rocks or nearby in shrubs or tussocks.
They are small flies with a body length of about 7 mm. Their long legs are hairy enabling them to walk on water and to surface after being washed off the deposited seaweed by a wave.
The female lays her eggs in small batches into fresh washed up seaweed. After hatching the larvae feed on the now rotting, bacteria-laden seaweed. The larvae go through three larval instars before pupation. Pupation usually occurs in the sand higher up the shore. There can be several generations a year.

Chaetocoelopa littoralis adults and larva are an important food for some coastal birds like the pipit (Anthus novaeseelandiae) and New Zealand dotterel (Charadrius obscurus) that forage amongst kelp drifts.

A typical aggregation of kelp flies.

Notice the long, hairy legs that enabling it to walk on water

Thanks to Wikipedia for text and information http://creativecommons.org/licenses/by-sa/3.0/