Head or head-like region

a) head with antennae or eyes

In arthropods, a head is the anterior body region specialized for sensing and feeding. Insects, myriapods, and crustaceans have heads, but arachnids (mites, whipscorpions, spiders, and their relatives) do not. Except for proturans and some insect larvae, all microarthropods with heads will also have antennae (although they may be small) and they will usually have eyes or ocelli near the bases of the antennae.

In some mites, the two anteriormost segments are head-like (the capitulum or gnathosoma), but antennae are never present. When mites have eyes, they are 1–2 (rarely 3) simple ocelli located on the 'shoulders' of the body above the coxae of the anterior legs.

Misinterpretations allowed: The pseudoculi of proturans could be confused with eyes. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)

b) head without antennae

Protura are the only microarthropods with heads that lack antennae as adults. However, the antennae of some insect larvae are minute or seemingly absent.

Misinterpretation possible: Ostracods have well developed antennae, but these may be confused with legs. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)

c) cephalothorax (prosoma)

Most arachnids have an anterior body region that combines the functions of sensing, feeding, and walking and is often covered with a carapace—the prosoma.

Misinterpretations allowed: The exceptions are the mites and ticks (Acari
Factsheet for:Acari
Glossary:Opilioacariformes:
an obsolescent supraordinal term used for the Opilioacarida.
), which have a gnathosoma (capitulum) and have lost a clear distinction between prosoma and opisthosoma, but those with a dorso-sejugal suture may be confusing. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)

d) head-like capitulum present

Only mites and ticks (a kind of mite) have a head-like capitulum (=gnathosoma). Ricinuleids also have a gnathosoma protected by a cuculus dorsally.

e) head apparently absent

The presence of a head with antennae is a simple and effective way to distinguish small insects, hexapods and myriapods from mites. However, in some larval insects, highly modified adults (e.g., scale insects), and some parasitic mites, the head is absent or difficult to distinguish.

Misinterpretations allowed: The head of ostracods and many copepods are obscure, less so for cladocerans. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)