Millions waiting for a meal

by John Walsby

There is one hazard along New Zealand's coastline that even the Coast Guard is unable to save or protect us from: small black biting flies. In some countries they are called black flies but both here and in Australia we know them as sand flies because they are so common on some of our sandy beaches.

There are about a dozen different species in New Zealand, not all of which bite. Those that do can sometimes be so numerous that they can make a day at the beach or beside lakes and rivers quite miserable, for they often land and bite almost as fast as they can be brushed off.

At other times they are much less of a problem. The weather makes a difference. The little flies are much more active on warm days when humidity is high and some people claim that a sudden increase in sandflies is a sign that it is about to rain.

Sandflies far more numerous along beaches that are close to running freshwater for it is in streams and rivers that most breed. The females lay their eggs on stones or water plants just at or below the waterline.

The legless larvae that hatch after a few days need well oxygenated water and prefer flowing streams although some are found in still ponds where there are good growths of aquatic plants to oxygenate the water. The larvae feed on minute organisms in the water which they strain from the passing current with a pair of bristle toothed combs beside the mouth. To feed most effectively they hang out into the current to sweep their feeding combs through the water.

To avoid being swept away the small larvae anchor themselves securely. From the salivary glands each larva secrete a pad of gelatinous glue to its home base and clings to this with a circlet of hooks at the rear of the body. Fully grown larvae pupate underwater in open pouches woven from silk on to the stone or plant that each is living on.

Most mature from eggs to adults in about six weeks but water temperature and food availability can delay development and those that hatch in the late autumn do not mature until next spring. When pupae are ready to metamorphose into adult flies they leave their silken holsters and float to the water surface.

Like mosquitoes it is only female sandflies that bite us and drink our blood to obtain the protein-rich food they need to make their eggs. The sandfly's bite is always more noticeable than that of a mosquito because a mosquito has mouthparts like a fine hypodermic needle and can often penetrate our skin without us noticing. Only later when our body reacts to the anticoagulant injected into the wound to stop the blood clotting do we notice the irritable soreness.

When female sandflies feed they must bite into our skin to reach a blood capillary and this is usually noticed immediately. Unlike many blood sucking tropical insects sandflies do not carry any serious diseases, so they are not a major concern to our health in New Zealand though they can certainly be an irritating and painful nuisance at the beach.

Back to Case History Menu

Back to Ecological Factors

Rivers Wetlands River Surveys

River Detectives Beaches Home