Acariformes

Introduction

The Acariformes is one of the three major divisions of the Acari, the other two being the Parasitiformes (including ticks) and the Opilioacariformes. There are more than 30 000 described species of acariform mites, and they inhabit all imaginable biotic and abiotic habitats from hot springs to hydrothermal vents, from leaves to lizards, and from the ears of your pet cat to the eyebrows of your grandfather. They include such familiar scourges as dust mites, spider mites, scabies and chiggers, as well as a huge range of lesser-known parasites, predators, and fungivores. They are an ancient group of arachnids with representatives among the earliest assemblages of terrestrial invertebrates (Rhynie chert, Gilboa formation) (e.g. Kethley et al. 1979).

Characteristics

A number of developmental characteristics unite the Acariformes. Development is anamorphic, meaning that body segments are added between moults. Also, prior to moulting, the legs of the subsequent instar are formed within the body rather than within the hulls of the previous instar's legs. There are also several characteristics of acariform setae that differentiate them from other Acari. Another name for the Acariformes is the 'Actinotrichida', which refers to the fact that their setae have a layer of optically active chitin, 'actinochitin', that is birefringent under polarized light. Other mites - the Parasitiformes and Opilioacariformes - lack actinochitin and are sometimes grouped as the 'Anactinotrichida'. Trichobothria, specialized setae that are sensitive to vibrations and air currents, are found in the Acariformes but not in other mites. Other modified sensory setae unique to the Acariformes include solenidia and eupathidia. Lindquist (1984) provides a list of the apomorphic characters of this group.

Discussion of Phylogenetic Relationships

As can be gathered from the illustrated tree, monophyly is not the rule in the Acariformes. Acarologists have traditionally split this taxon into three major groups, the Endeostigmata, Trombidiformes (also called Prostigmata) and Sarcoptiformes (= Astigmata + Oribatida). However, the Endeostigmata is likely to be an artifical group composed of primitive members of both the trombidiform and sarcoptiform lineages. OConnor (1984) performed a cladistic analysis that showed the Prostigmata to share some derived setal and developmental characters with the endeostigmatan taxa Sphaerolichidae and Lordalychidae. He therefore suggested that the Prostigmata and Sphaerolichida (= Sphaerolichidae + Lordalychidae) make up a monophyletic group, and gave this group the name Trombidiformes. Within the Sarcoptiformes, the Astigmata and Oribatida have traditionally been represented as taxa of equal rank; however, it is quite apparent that the Astigmata are derived from within the Oribatida, making the latter taxon paraphyletic (e.g. OConnor 1984, Norton et al. 1993). Recent chemotaxonomic studies provide further evidence of paraphyly. A novel defensive compound is known only from Astigmata and middle-derivative Oribatida, but is absent in several early derivative oribatid taxa (Parhypochthonius, Gehypochthonius and Nehypochthonius) (Sakata & Norton 2001).

Other Names for Acariformes

• The "mite-like" mites

References

Kethley, J.B., R.A. Norton, P.M. Bonamo and W.A. Shear. 1989. A terrestrial alicorhagiid mite (Acari: Acariformes)from the Devonian of New York. Micropaleontology 35:367-373.

Lindquist, E.E. 1984. Current theories on the evolution of major groups of Acari and on their relationships with other groups of Arachnida, with consequent implications for their classification. pp. 28-62 in D.A. Griffiths and C.E. Bowman (eds.) Acarology VI, Vol. I. Ellis Horwood Ltd., Chichester.

Norton, R.A., J.B. Kethley, D.E. Johnston, and B.M. OConnor.1993. Phylogenetic perspectives on genetic systems and reproductive modes of mites. pp. 8-99 in: D.L. Wrensch and M.A. Ebbert (eds.) Evolution and diversity of sex ratio in insects and mites. Chapman & Hall, New York.

OConnor, B.M. 1984. Phylogenetic relationships among higher taxa in the Acariformes, with particular reference to the Astigmata. pp. 19-27 in D.A. Griffiths and C.E. Bowman. 1984. Acarology VI, Vol. I. Ellis-Horwood Ltd., Chichester.

Sakata, T. and Norton, R. A. 2001. Opisthonotal gland chemistry of early-derivative oribatid mites (Acari) and its relevance to systematic relationships of Astigmata. International Journal of Acarology. in press.