In arthropods, body regions (tagmata) are functional units composed of several segments variously modified and fused. Myriapods have two regions: head and trunk (abdomen) with numerous pairs of legs. Terrestrial crustaceans have three regions, but the thorax has numerous legs and the abdomen is not strongly set off, so they appear to have a head and trunk.
Velvet worms, tardigrades, and some insect larvae have a more or less well defined head and a trunk with lobopods or prolegs.
Insects, springtails and other hexapods (as well as first instar millipedes) have three body regions: a head with antennae and often eyes, a thorax with three pairs of legs and up to two pairs of wings, and an abdomen that has no true legs (styli or prolegs may be present—see Character 6d) and is usually clearly segmented.
Misinterpretations allowed: Mite larvae are hexapod and could be misinterpreted as having a thorax. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Diplurans, proturans, and thysanurans often retain remnants of the abdominal legs of their crustacean ancestors in the form of 1–2 segmented styli.
Scorpions (Scorpionida) have a unique set of body tagmata: prosoma, mesosoma, and metasoma.
Most arachnids have only two major body regions. The anterior region (prosoma or cephalothorax) has the mouth parts, legs, and eyes and is used for feeding, sensing and locomotion. The posterior region (opisthosoma or abdomen) carries out the rest of the body functions. In spiders and some whipscorpions a narrow waist or pedicel separates the two regions. In pseudoscorpions, opilionids, and other arachnids the juncture is broad, but unlike in mites, opisthosomal segmentation is clearly present.
Misinterpretations allowed: Mites have lost a clear distinction between prosoma and opisthosoma, and have two unique body tagmata: gnathosoma (capitulum) and idiosoma (body). Mites are retained when you use this feature if 'Allow Misinterpretations' is enabled. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Spiders (Araneae) have a 'wasp waist,' the pedicel, joining the prosoma to the opisthosoma.
Misinterpretations allowed: Narrow waists also occur in some wingless insects, but these arthropods have a head with antennae and often with compound eyes. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Some mites have a tripartite appearance, with a well-developed capitulum and 1–2 constrictions behind the legs.
Mites have no head, but some have a small head-like anterior region (the capitulum or gnathosoma) that bears the chelicerae and palps.
Misinterpretations allowed: Although the overall appearance of these mites can be somewhat insect-like (especially if they are larvae with only three pairs of legs), the capitulum does not bear eyes or antennae, and the body is not segmented. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
In many mites only one body region—the idiosoma—is easily distinguished.
Scale insects, the immature stages of white flies, and some parasitic mites are often blob-like and even the remnants of legs can be difficult to distinguish. Tardigrades have a cephalic segment, but no distinct head and lobopods, rather than segmented legs.
Misinterpretations allowed: Pseudoscorpions and opilionids have broad prosomal-opisthosomal junctures and could be confused with mites. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Some early derivative groups of oribatid mites have the idiosoma completely divided between legs II and III, a condition known as dichoidy. This type of body division is unique to the Acariformes.
Tardigrades have a cephalic segment, but no distinct head and lobopods, rather than segmented legs.
Misinterpretations allowed: Pseudoscorpions, opilionids, and mites do not have a head, and often lack clear separation between the body regions that they have and could be misinterpreted as belonging to this couplet. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Seed shrimp (Ostracoda) have clam-like bivalved carapaces. Water fleas are enclosed in a carapace and head shield, although usually transparent and less likely to be confused with box mites.
Misinterpretations allowed: Some oribatid mites or beetles could be confused with ostracods. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
Lucid key player updated Sept. 2025
idtools.org
