Most springtails have a bifurcate process near the end of their abdomens, the furcula, that gives them their spring. The furcula can be very long or short, and rarely is absent.
Japygids (Diplura) and earwigs (Insecta) have pincers at the rear of their abdomens. Japygids use these pincers to capture and hold prey, and then curl the abdomen to bring the prey in range of the mouthparts. Some mites have distal opisthosomal lobes and/or hypertrophied posterior setae, but not pincers.
Cerci (singilar, cercus) are sensory appendages at the rear of many Insecta, Diplura, and Symphyla. Cerci are usually composed of several to many segments, but may be pincer-like or setiform. The uropods and caudal rami of Isopoda and Copepoda look rather like cerci. Aphids have subterminal tubes that could be confused with cerci.
Misinterpretations allowed: Mites never have rami, uropods, or caudal filaments, but sometimes have whip-like posterior setae or setae born on short horn-like projections and are allowed here as misinterpretations. Similarly, the posteriormost legs of some Chilopoda and the furcula of some springtails are somewhat cercus-like. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Polyxenida millipedes have a pair of distinctive columns of tightly bunched hairs (pencil tufts) at the ends of their bodies. Similar structures are found in a few cyphothalmid opilionids.
Most spiders (Araneae) produce silk, even those that don't spin webs, and the silk is produced from palp-like (primitive spiders) to peg-like abdominal structures, usually at the distal end of the abdomen.
Misinterpreations allowed: Microwhipscorpions (Palpigradi) have a distinctive, multisegmented, whip-like flagellum at the rear of the body. Unfortunately, the flagellum often snaps off in preserved specimens. The long terminal filaments of some harpactacoid copepods could be confused with a flagellum and is allowed here. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
The flagellum of schizomids (Schizomida) is short and finger-like (nymphs, females) or knobbed (males).
The abdomen of some insects (e.g., thrips) terminates in a tube-like narrowing of the body, or sometimes a tube-like ovipositor. Ricinuleids and amblypygids have a more or less well defined ring-like postabdomen. A few mites have an elongate, tube-like posterior or appendage.
The genitalia of insects are distal on the abdomen (modified limbs of abdominal segments 8–9) and often modified into an ovipositor in adult females, sometimes used defensively as a sting.
The genitalia of all arachnids, including mites, are produced basally on the opisthosoma (body segment VIII). When mites have an ovipositor, it occurs in mid-body. However, some mites (e.g., spider mites and their relatives) have a posterior aedeagus, typically a minute sclerotized internal structure.
Misinterpretations allowed: A few mites have tail-like processes that could be confused with an ovipositor, e.g., the strange Australian Xanthodasythyreus and some feather mites. (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
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