Order Acari (mites)

Mites. Over 30 000 species have been described so far. These are medically and economically very important animals as they can transmit or cause disease in both plants and animals, e.g., scabies and mange. They tend to be only a few millimetres long. Most have four pairs of legs, but larvae have only three pairs.

The follicle mites are highly specialised and live in the hair follicles and sebaceous glands of mammals including man. The body is soft and long and the legs have become attachment organs. Demodex folliculorum is commonly found in man, and though it is not believed to cause any disease, it is associated with certain skin disorders, e.g. black-heads, acne and impetigo. Demodex canis (left) is found in dogs. The life cycle is usually completed in around two weeks. After mating the female lays eggs in a sebaceous gland. Then she climbs to the top of the follicle and dies. This block the entrance to the hair follicle and so stops another female entering to lay eggs.

Sarcoptes sp., itch mites, infect man, domestic animals and poultry. In humans Sarcoptes scabei (right) causes scabies. The adult female mite is less than 0.5 mm, and the male half that. She burrows into the skin using her mouthparts and cutting surfaces on her front legs. She lays eggs as she burrows (see below right). The larvae crawl out of the burrows to feed around the hair follicles. Then they moult and mate and the life cycle starts again. The whole life cycle takes around two weeks to complete. The female can lay up to 25 eggs every 2 or 3 days.

Secretions from the body of the mite cause itching, and the scratches caused by the itching can become infected. Getting rid of scabies is fairly easy using modern medicines and following the instructions carefully. Diagnosing scabies infections is more difficult, as it requires the inspection of a skin scrape through a microscope. Only the presence of a mite can prove infection. However if one child has scabies and the others are itching, infection can usually be assumed. Physical contact with an infected piece of skin is required to transmit the mite to another individual. An old method of locating the exact infection sites was to put a drop of ink on the skin, then rub it off. The ink will rub fairly cleanly off undamaged skin, but will occupy the burrows of the mite and so show up as dark patches.

On the left is Cheyletus sp. a hunting mite. This mite hunts down and kills pest mites such as dust mites. It is often found on bedroom floors, and even in bird's nests. It can cause allergies in some people.

Below is Acarus siro, the flour mite. It is found in any type of flour or seeds. The female can lay 500 - 800 eggs in her life time, at a rate of 20 - 30 per day. To test if flour is infected just spread some on a flat surface indoors and leave for 20 - 30 minutes. When you return if the flour is infected the surface will have become uneven due to the movement of the mites.

On the left is Humerobates sp., a beetle mite. It is a soil-enriching mite.

Below is Tyrophagus sp. a forage mite. This mite infests and spoils food. It can cause allergies in humans, cats and dogs.