Mite

Mites are small arthropods belonging to the class Arachnida and the subclass Acari (also known as Acarina). The term 'mite' refers to the members of several groups in Acari but it is not a clade as it spans two different groups of arachnids; it also excludes the ticks, order Ixodida. Mites and ticks are characterised by the body being divided into two regions, the cephalothorax or prosoma (there is no separate head), and an opisthosoma. The scientific discipline devoted to the study of ticks and mites is called acarology. Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. Their small size makes them easily overlooked; some species live in water, many live in soil as decomposers, others live on plants, sometimes creating galls, while others again are predators or parasites. This last group includes the commercially important Varroa parasite of honey bees, as well as the scabies mite of humans. Most species are harmless to humans but a few are associated with allergies or may transmit diseases. The mites are not an exact taxon, but the name is used for members of several groups in the subclass Acari. The phylogeny of the Acari has been relatively little studied, but molecular information from ribosomal DNA is being extensively used to understand relationships between groups. The 18 S rRNA gene provides information on relationships among phyla and superphyla, while the ITS2, and the 18S ribosomal RNA and 28S ribosomal RNA genes, provide clues at deeper levels. The third edition (2009) of the standard textbook A Manual of Acarology uses a system of six orders, grouped into three superorders: Most fossil acarids are no older than the Tertiary (up to 65 mya). Earlier fossils are too few to enable mite phylogeny to be reconstructed from palaeontological evidence, but in 2002 an oribatid mite (Brachypylina) from the Early Ordovician (c. 480 mya) was found in Oland, Sweden. The first find of Parasitiformes from the Mesozoic was of an argasid tick larva in Cretaceous amber (90–94 mya) from New Jersey. Other fossils including the first opilioacariform mite are preserved in Baltic amber of Eocene age (44 mya). Members of the superorders Opilioacariformes and Acariformes (sometimes known as Actinotrichida) are mites, as well as some of the Parasitiformes (sometimes known as Anactinotrichida). Recent genetic research has caused a change in the naming scheme, however, and recent publications have changed the superorder Parasitiformes to an order. Other recent research has suggested that Acari is polyphyletic (of multiple origins), with ticks and spiders more closely related than ticks and mites. The cladogram is based on Dabert et al 2010, which used molecular data. It shows the Acariformes sister to the Solifugae (camel spiders), while the Parasitiformes are sister to the Pseudoscorpionida. Mites are tiny members of the class Arachnida; most are in the size range 250 to 750 μm (0.01 to 0.03 in) but some are larger and some are no bigger than 100 μm (0.004 in) as adults. The body plan is similar to that of ticks in having two regions, a cephalothorax (with no separate head) or prosoma, and an opisthosoma or abdomen. Segmentation has almost entirely been lost and the prosoma and opisthosoma are fused, only the positioning of the limbs indicating the location of the segments. At the front of the body is the gnathosoma or capitulum. This is not a head and does not contain the eyes or the brain, but is a retractable feeding apparatus consisting of the chelicerae, the pedipalps and the oral cavity. It is covered above by an extension of the body carapace and is connected to the body by a flexible section of cuticle. The mouthparts differ between taxa depending on diet; in some species the appendages resemble legs while in others they are modified into chelicerae-like structures. The oral cavity connects posteriorly to the mouth and pharynx.