Key to Australian Freshwater and Terrestrial Invertebrates
Common names: opilioacarans, primitive mites
Opilioacariformes are a small order of weakly sclerotised, soft-bodied mites less than 3 mm in length that are superficially similar to Opiliones (harvestmen, daddy longlegs), hence their name. Mites are distinguished from other arachnids by the complete absence of body segmentation. There is no obvious division of the body into cephalothorax and abdomen. Instead, the mouthparts and associated sensory structures form a discrete anterior structure known as the gnathosoma. All the rest of a mite�s anatomical structures, including legs, central nervous system, ocelli (when present), and reproductive and digestive systems, are all fused into a single unsegmented body called the opisthosoma. Opilioacariformes have many character states that are considered to be ancestral and are closely related to the Parasitiformes. These mites have a hairy anterior portion of the body bearing two or three pairs of simple eyes and bare posterior portion and the possession of a pretarsus, or apotele (scalloped ridge), on the pedipalp, with prominent claws that are usually relatively unmodified and paired like those on the legs.
Distribution and diversity
Opilioacarans have been found in several widely separated locations across northern tropical Australia. They are the smallest arachnid order; worldwide there are around 20 species in nine genera from a single family. The Australian fauna comprises several undescribed species.
Males transfer sperm indirectly by producing a spermatophore (sperm packet) that is placed in the female�s genital opening using his legs or chelicerae. Eggs are laid into the substrate such as soil or leaf litter. The first free-living stage that hatches from the egg is the larva which has only three pairs of legs. Subsequently, the larva moults again becoming a first instar with four pairs of legs. Typically, several moults occur before it becomes an adult.
Opilioacariformes are free-living scavengers and opportunistic predators that ingest solid food, feeding on insects and other mites, as well as fungal hyphae and pollen.
Opilioacariformes are terrestrial mites that occur in moist habitats under stones and in leaf litter in semi-arid climates where they are widespread but relatively rare. They are not known to be economically important.