Springtails have a ventral tube, the collophore, near the bases of the third pair of legs that is thought to be used in maintaining water balance.
misinterpretations allowed: Both opilionids and some mites have an eversible ovipositor behind the legs that could be confused with a collophore. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Some apterygote insects and hexapods (diplurans, proturans) retain paired remnants of abdominal limbs called styli (possibly a coxal branch) that are often associated with eversable sacks—probably osmoregulatory in function.
Tritonymphs and adults of acariform mites primitively have three pairs of segmented appendages in the acetabulum of the genital opening called genital papillae. Larvae lack these structures (and genital acetabulae), protonymphs have one pair, and deutonymphs two pairs. In most mites with genital papillae, their structure is simple, button- or knob-like. The maximum number of genital papillae is reduced to two pairs in many acariform mites and they are lost in most derived Prostigmata.
Several different kinds of arachnids (e.g., pseudoscorpions, whipscorpions) brood their developing eggs in a sack-like structure on their venter. Mites never do this, although some oribatid mites are known to carry eggs on their backs. Spiders typically carry or guard a silken egg case. Some centipedes wrap their bodies around their developing eggs or young hatchlings.
Isopods and amphipods have expanded coxal bases that are used to carry and shelter their eggs. The pouch-like region protected by the coxal plates is called a marsupium.
Scorpions have a pair of ventral, comb-like processes called pectines. These are better developed in males than in females and may function in mate location and other sensory behaviors.
Only acariform mites and opilionids have an elongate ovipositor that can be extruded from the genital region in the middle of the body venter. In mites, the ovipositor ends in 3-fingers and in opilionids in 2 processes. Male opilionids have a penis that is everted from the genital opening.
Misinterpretations allowed: Springtails have a short tube, the collophore, that is produced ventrally from the middle of the body, and could be confused with an ovipositor, but it is cylindrical and is not associated with sclerotised plates that open outward.
Solfugids have 3–5 pairs of small, racket-shaped structures on the basal segments of legs IV.
The heteromorphic deutonymph of the Astigmata (in older literature called hypopus or hypopode) is a specialized dispersal morph with atrophied mouthparts and usually with a well developed sucker plate on the venter. A few Mesostigmata associated with millipedes and centipedes (Heterozerconoidea) have a pair of ventral suckers, as do many male astigmatans.
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
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