Introduction
People rarely notice tiny mites; but they are everywhere. With the exception of a very few species, the vast majority of these tiny arthropods go about their lives completely beneath our notice.
Mites belong to Arachnida, the group that also contains the spiders, scorpions, daddy long legs, and a few other interesting types of arthropods. Because all arachnids, including mites, are types of arthropods, they share many traits with insects: they have exoskeletons which they molt as they grow and they have segmented limbs. Unlike insects, no arachnids have antennae (like insects and crustaceans), nor do any arachnids have wings.
Mites typically have 8 legs (four sets of two) and for this reason resemble tiny spiders. Most mites are less than 1 mm in body size - although there are a few exceptions to this, most notably: members of the Ixodida, also known as ticks in English.
For a number of reasons, mites are incredbly successful arthropods that inhabit nearly every ecosystem you can conceive of, and mites can be found in deep ocean vents to snow covered mountain peaks. They live deep in the soil and can be found floating high above earth in air currents. Many species have close and complex biological associations with plants, fungi and other animals that has likely further added to their diversification.
Making sense of such as diverse and tiny group of organisms is difficult. For one, a large number of the species (especially those that are not major pests to agriculture or of medical/veterinary concern) remain undescribed by scientists. Secondly, identification requires examination under microscopes, often with specialized lenses/optics known as phase-contrast that help make visible the tiny transparent anatomical parts that can be difficult or impossible to see clearly even at high magnification. Making this step even more challenging, the mite itself should generally be properly mounted on a microscope slide - a skill that also takes practice to do well. Lastly, the anatomical characters and character systems used in the keys (tools used to identify biological organisms) are complex and have a specialized set of terminology.
Mite Taxonomy
Based on modern phylogenetic studies that use both anatomy and DNA sequenc data, the group known as mites (Acari) appears to contain two large groups, which are generally treated as separate Orders. Some members of these orders resemble each other, but likely represent two independent lineages of Arachnida, and each may be more closely allied to another arachnid group than to each other. One is called the Parasitiformes, and includes the ticks (Ixodida) as well as a diverse group collectively known as the Mesostigmata. The other Order is called the Acariformes, and it is also quite diverse and includes a number of biologically important mites associated with plants (such as spider mites: Trombiculidae) and soil mites (such as beetle mites: Oribatidae).