Litter Critters: Intro to the Mites and the ancestral group, the Opilioacarida

The greatest diversity of soil invertebrates is going to be found in the Acari, or, the mites.  At the mention of the word ‘mite’, plenty of folks will instantly think of ticks – those specialized, blood-thirsty mites in the suborder Ixodida.  Or, perhaps they may think of chiggers, also mites, which are the larvae of some tiny predators in the suborder Prostigmata.  Maybe, just maybe, they may think of clover mites.  Clover mites are also in the suborder Prostigmata, but are plant-sucking pests of people-loved plants.  Few will think on the beneficial mites, such as the helpful little oribatid mites that help recycle nutrients back into the soil, or the fearsome mesostigmatids that prey on plenty of plant-damaging pests. These predators are so helpful for this, in fact, that biocontrol companies will breed and sell them to gardeners as pest control.  

But first things first. What, technically, is a mite?

Mites are a subclass within the Arachnida.  It’s easy enough to count eight legs to put them in with this bunch, but what really makes them their own thing?  

It is supposed that the most ancient of mites had many abdominal segments, and over time these segments have fused.  As a result of this segmental amalgamation (aka tagmosis) the Acari now have two main body parts – the gnathosoma (the ‘mouth’), which is derived from the first two primitive segments and comprises mostly of the mouth parts, and the idiosoma, which is the ‘body’ essentially:  guts, legs, the works.  Mites have eight legs, two pedipalps and two chelicerae, with an incredible range of cheliceral morphs across the different acarine orders adapted to an equally diverse range of lifestyles.

Let’s start with the Opilioacarida, the most primitive group of mites. 

Opilioacarids are considered to be primitive because they have retained many ancestral characteristics that have long since been lost in the other mites.  They have an opisthosoma (abdomen) with 13 segments with remnants of primary segmentation marked on their backs, some have three pairs of ocelli (at most, other mites might have two pairs), all tarsi are divided (that’s the last tip of the legs) – a characteristic shared with other, non-mite arachnids, but not other mites.  The super fancy palpal apotele (a modified structure found at the tips of the palps in other parisitiform mites that looks like a little comb) is simply a pair of claws in the opilioacarids.  The opilioacarids are large (1.5-2.3mm long), can autotomize  (that is, cast off)legs when threatened, and regenerate them on the next molt. 

Given their diversity, any generalization about the diets of mites immediately suggests an exception. Plus the natural history of this group is ripe for exploration. That said, opilioacarids have been shown to feed on pollen, fungus, and arthropod remains, which they eat by taking in particulates.  They can look quite similar to the mite-like opilionids (Cyphophthalmi), which isn’t surprising, given their name.  Though they are found in many habitats such as caves, litter, under rocks in semiarid environments and forest litter in tropical and warm temperate regions of the world, there is still only a single family (Opilioacaridae) with eleven genera and thirty-eight species, over half of which have been described from the New World, mostly from North and Central America.  However, if you do come across one, they are spectacular!  Who could resist a creature with brilliant bands of blue or purple on it’s legs and back?

Illustration and Text by Brittany Benson

© 2015

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