M is for Midge

Dr Henry Disney (Department of Zoology) has been fascinated by insects since he was four years old. His career has taken him all over the world. Despite losing 75% of his sight in 2012, Disney walks every day to his lab, where use of the latest imaging and magnifying technology enables him to continue his research. Below, Disney answers questions about the tiny insects that can, during summer months, turn a camping holiday on the beautiful west coast of Scotland into a nightmare.

What are midges?

Midges are classified as Diptera – which comes from the Greek for two wings. Diptera fall into three main groups: higher flies, middle flies and lower flies. Midges, like mosquitoes, fall into the lower group, which are the most ancient. They are typified by long antennae which have many segments. Some Diptera are enormously important as a threat to human health: they include many species in which the females suck blood and, in many parts of the world, transmit diseases such as yellow fever and malaria.

What is the life cycle of a midge?

The speed at which midges reproduce is temperature dependent. In the UK, you might get two or three generations a year. In the hot and steamy environment of Cameroon, where I’ve worked as a medical entomologist, you might see a new generation emerging every three weeks. Adult females lay their eggs in the water or on the margins of water. The eggs hatch into free-living larvae which go through several moults before they pupate. The adult emerges and sits on its empty case for a moment to open its wings before buzzing off.

When are midges most visible?

Midges are easiest to spot when groups of them dance in mid-air.  What you’re seeing are the males saying to the females: here we are, where are you? They give off a signal that's partly smell and partly sound. If you watch really carefully, you might see a pair of midges dropping out of the group to mate. Midges swarm near an object such as a branch which gives them a point of reference. Sometimes they gather in such numbers that they make huge towers. So many midges once swarmed on Salisbury Cathedral that the fire brigade was called; it looked as if the spire was swathed in smoke.

How many species of midge are there?

In the UK, alone there are more than 500 species of non-biting midges and more than 150 species of biting midges. Identification of the species is primarily based on details of the male genitalia examined under a microscope. Increasingly this is supplemented by the use of DNA ‘barcodes’.

Why do midges bite?

Only the females bite. They need a protein-rich meal of fresh blood in order to mature their eggs. Both the males and the females rely on sugar meals for energy for flight but the females need more than this to ensure the next generation. Female midges feed on the blood of birds as well as mammals. Each species has its own preferred choice of host.

What is the midge's place in the ecosystem?

Meniscus midges live at the point where air and water meet – a zone known as the surface film. It’s a habitat that supports a whole community of plants and animals, many of them still unexplored. Some minute organisms spend their lives within the surface film; others, like meniscus midges, spend their larval lives feeding on it.

The boundary where air and water connect is rich in resources. The larvae of non-biting midges feed on algae and bacteria, filtering micro-organisms out of the water, but some are predators. The larvae of phantom midges live in the open water and prey on water fleas and small larvae. Adult midges are eaten by all kinds of things - from spiders to swallows. The larvae are eaten by fish, dragonfly larvae, water beetles and other predators.

What can midges tell us about the environment?

The apparent boundary between air and water of ponds and other bodies of water is masked by a layer of lipoprotein leached from organic materials. Within this ‘membrane’ live all kinds of microorganisms – bacteria and so on. Some of it drops in from above and some of it rises up from below. Hundreds of species depend on the ‘membrane’ for food as well as on the prey that inhabits it. Changes to the structure and content of this membrane will affect all these species.

Research has shown that midges are some of the most sensitive indicators of pollution in water. The presence of some species is a sign of a healthy water course with normal oxygen levels; their absence is a sign of lower oxygen levels and can point to pollution. Water authorities sample the numbers and species of midges present in a water course above and below a discharge – for example from a sewage treatment plant – to monitor contamination of the water by organic matter.

Oil, and detergents used to disperse oil, also alter the character of the surface layer – and will have a negative effect on species such as meniscus midge larvae that depend on this delicately balanced habitat.

What more is there to learn about midges?

Some insects have economic and medical importance. For example, there's a huge body of literature devoted to mosquitoes. Anything that bites and transmits disease is likely to attract research funding. A Scandinavian team showed that midge bites could lead to a mild fever but its effects were short-lived and quickly alleviated. Although midges are known as ‘Scotland's secret weapon’, there is no need to worry about being bitten leading to serious problems. However, biting midges have been implicated in transmitting a disease of livestock. In hot climates, midges are known to spread both African Horse Sickness and Blue Tongue virus.

There is still much to learn about midges and novel biological methods of control, that avoid the use of pesticides, for those species posing problems.

How did you get interested in insects?

I was always fascinated by natural history. When I was around four, I disappeared and everyone was out looking for me. I was found sitting among some cabbages watching a caterpillar. An aunt hugely encouraged me and left me a small legacy with which I bought my first microscope. I'm still using it more some 50 years later. My career has been immensely varied - I've worked in medical entomology in Belize and Cameroon. Since my move to Cambridge, I’m occasionally asked to report on specimens from forensic cases - including some involving infamous crimes – as well as pest problems and medical cases. I’ve authored, and contributed to, several books and written hundreds of papers. I’m never bored.

The most important question of all: how do you keep midges at bay if you have to work in areas where they are rife?

The most effective solution for people working outdoors is to wear a loose net over-garment with a hood, impregnated with DEET, over one's normal clothing.  This lasts longer than applying DEET to one's skin or normal clothing.  We used to test these against alternatives when running the annual field course at my field centre in Yorkshire for the London School of Hygiene and Tropical Medicine.

Next in the Cambridge Animal Alphabet: N is for an animal that won't win any beauty contests, but can live for 30 years and may be able to help in the development of new therapies for chronic pain.

Have you missed the series so far? Catch up on Medium here.

Inset images: Adult Dixella in side view (from British Dixidae (Meniscus Midges) and Thaumaleidae (Trickle Midges) by Henry Disney, published by the Freshwater Biological Association); Dorsal view of adult Dixa BM, BL, median and lateral bands on the scutum (from British Dixidae (Meniscus Midges) and Thaumaleidae (Trickle Midges) by Henry Disney, published by the Freshwater Biological Association).

Home page banner image: A chironomid midge. Credit: S Rae