The prelarva is either elattostasic (= elattostatic; noun: an elattostase), i.e., has developed legs and no or reduced mouthparts, or is calyptostasic (= calyptostatic; noun: a calyptostase), i.e., lacks legs and mouthparts. However, the extent of mouthpart and leg reduction varies.
A complete set of immature stages does not always develop.
The tritonymph, for example, is absent in Mesostigmata, there is only one nymphal stage in hard ticks (Ixodida: Ixodidae), while the soft tick genera Argas and Ornithodoros (Argasidae) have up to eight nymphs.
The deutonymph usually does not develop in Astigmata, but in some families is represented by the hypopus (plural: hypopi or hypopodes). Hypopi are produced during unfavourable environmental conditions, particularly when food is scarce. They are non-feeding resistant forms in which the four pairs of legs are either functional (motile hypopus, = mobile hypopus) or non¬functional (inert hypopus). Motile hypopi try to find new food sources and are phoretic, i.e., they use other animals as a means of transport. They either have well¬-developed ventral suckers for attaching themselves to arthropods or clasping organs for clinging to mammal hairs. Inert hypopi may remain inside the protonymphal cuticle until favourable conditions return.
The larva of most members of the Parasitengona (Prostigmata) is parasitic on arthropods or vertebrates. Once fully fed, it drops off its host to find a sheltered place in which to moult. For terrestrial species, this is often in soil or under stones. Watermites choose aquatic plants, freshwater bivalves (Mollusca) or sponges (Porifera).
The calyptostatic protonymph (nymphochrysalis) develops inside the larval exoskeleton, and the deutonymph inside that of the protonymph. A calyptostatic tritonymph (teleiochrysalis) follows the deutonymph and finally the adult emerges. Unlike the larvae, the nymphs and adults live freely in soil habitats where they prey on invertebrates.
Male and female nymphs can sometimes be identified. Examples are the deutonymph of some Parasitidae (Mesostigmata) and the tritonymph or both nymphs of certain members of the Psoroptidae (Astigmata).
On the whole, immatures resemble their respective adults (homostasis), i.e., they are homeomorphic (= homomorphic; noun: a homostase). The most obvious differences are that they are less heavily sclerotized, have a different arrangement of shields and lack certain setae and genitalic structures. When a stage is much different to the others (heterostasis), it is described as heteromorphic (noun: a heterostase). Examples are Parasitengona larvae, Astigmata hypopi and adults of higher Oribatida (the Brachypylina). Some immature Brachypylina (e.g., the Damaeidae) carry masses of soil and leaf litter debris on their body (Figure). Adult and nymphal oribatids (e.g., the Liodidae) can be eupherederm, i.e., they carry part of the cuticle (scalp) of the preceding stage or stages (Figure).