The House Dust Mite (sometimes abbreviated as HDM) is an extremely small ‘critter’ with eight legs (mites are therefore not insects; they are more closely related to ticks and spiders and belong to a class called acari, which also includes ticks). They measure, on average, just 16 hundredths of an inch in length!! They are responsible for a lot of allergies (and asthma attacks) for people the world over. They feed on organic debris, including dust and shed human skin cells, etc. and so survive very well in homes, and especially in mattresses and pillows and other similar bedding items.
Additional Information and Photo: House Dust Mites (University of Florida)