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You may think that beaches provide an inhospitable environment for insects with the turbulence of the waves and the alternate flooding and drying caused by the tide. However, a few species can thrive, living on rotting seaweed or sheltering in rocky crevices.

Some species of insects thrive on the inhospitable shoreline and beach. The seaweed fly lays its eggs on seaweed, causing a mass of maggots to thrive. Beetles will make burrows and lay their eggs deep inside while swarms of midges can be seen flying overhead.

Most aquatic insects live in comparatively still waters or near freshwater ponds, lakes, and rivers. Not many insects are successful at establishing themselves in fast streams, and there are even fewer in the intertidal zones of the shore.

While the salinity of the shore accounts for a small number of successful insect colonizers, the turbulent action caused by the waves is the main reason, as insect life is usually abundant in saltmarshes and brackish water.

Photo of seaweed fly

Seaweed Fly

One insect which has successfully established itself on the seashore is the seaweed fly. Found easily throughout the year, it lives and breeds in the heaps of brown seaweed thrown up along the high tide mark.

This rather flattened, blackish, hairy species, which looks like a housefly at first glance, is attracted to the seaweed within a day or so of it being beached. When the seaweed decomposes, it becomes slightly warmer, and the female flies will lay their eggs in it.

The eggs hatch into legless maggots about one day later. Their feeding spreads the decay and heats up the seaweed again so that more flies are attracted.

The entire heap becomes a mass of maggots in warm seaweed, able to continue their growth even throughout the coldest winter.

Adult flies also spend much of their life in the seaweed. Between laying the eggs and the emergence of the adults is about a fortnight, giving ample time for the next generation to be produced before another high spring tide removes the seaweed.

The larvae are able to withstand immersion in salt water for as long as six days and can be washed out to sea and then returned on fresh weed.

Adult seaweed flies can sometimes be seen carrying out mass migrations along the shore, and in years when their numbers are unusually large, they have been found some miles inland. Seaweed flies appear to be attracted to trichloroethylene and carbon tetrachloride.

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A number of beetles belonging to the genus Bledius live in the intertidal zone near the high tide mark, feeding on various algae.

The females, which dig in the sand where there is a coating of mud, have to face the saltiness of the water but also a lack of oxygen. To counteract this, they constantly dig tunnels, which, with the tidal movements, keeps the tunnels ventilated.

The females lay their eggs in batches in multiple chambers and protect the eggs and larvae from intruders until they are large enough to build their own tunnels. If the mother is taken away, the eggs and larvae perish.

Bledius species live in colonies and throw up small mounds of sand about 12mm (1/2in) high, and these can be seen in a continuous line for miles along the shore.

The beetles stay in their burrows when the tide covers them, but at other times can be seen flying low in large numbers along the beach. These species are preyed on by an active ground beetle Dyschirius, which constantly runs in and out of their burrows.

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Photo of rocks, seaweed, and beach


Several midges regularly breed in the intertidal zone, and you can often see them in swarms over seaweed. Species of Clunio have a fascinating mating sequence.

The male emerges as an adult from tubes built on seaweed in rock pools. The male emerges before the female and, after finding a suitable partner, must help her out of her pupal skin before mating can take place.

The males have to remove her as the female is incapable of freeing herself from the pupal skin without the help of the male.

The male helps the female out by grasping the female and tearing off the pupal skin with their genital claspers.

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Insects and Rock Crevices

The ground beetle, Aepus marinus, can sometimes be found running about on stones and seaweed at low tide. When the tide comes in, it hides under stones or in rock crevices. This is where you are most likely to come across both the larvae and the adults where they live in large colonies.

The larvae are small, pale, and, like the adults, carnivorous, feeding on the crevice-inhabiting springtail Anurida maritima.

The larvae of Aepus marinus pupate in late summer and spend a fortnight in this stage, deep in the rock crevices. They appear to be able to withstand immersion in salt water, although fissures in the rock may have air pockets that help them to survive.

Another crevice-inhabiting insect is the marine bug Aepophilus bonnairei, which belongs to the family Saldidae, which are regularly found at the margins of fresh and salt water.

A. bonnairei has no hindwings, and only short forewings, so cannot fly. It lives in family groups, typically in slate and shale crevices in the lowest Fucus zone.

Aepophilus bonnairei is predatory, feeding mainly on marine worms. They are fairly sedentary because their crevices are so low down the shore that they have little opportunity for hunting outside.

They have areas of minute water-repellent hairs around their breathing holes (spiracles) which trap air so they can breathe under water.

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During mating, the male flies in a series of gliding flights carrying the female with him for five to fifteen minutes at a time, as she cannot fly alone.