(by Bruce Halliday)
(This contribution was initially published as feature article in Australasian Arachnology 77 (April 2007)).
Almost all arachnids are predators. The only group of arachnids that has managed to break out of the predatory habit on a large scale is the mites, which have diversified into an extraordinary range of niches. Many are still predatory, but there are also thousands of species of plant feeders, fungivores, saprophytes, pollen and nectar feeders, microbial filter feeders, and internal and external parasites on a wide range of vertebrates and invertebrates. Others have a complex life cycle, in which parasitism and predation occur at different life stages within a species, and others are omnivorous. Mites occur in soil and decomposing organic matter, in fresh water and sea water, high in the air, deep in the oceans, on and in the bodies of other animals, and on plants of all kinds. This ecological diversity has been accompanied by a bewildering degree of morphological diversity. Their diversification has in turn been reinforced by their small body size, which allows them to occupy minute spaces, and makes it necessary to use specialised techniques for their observation and taxonomic study. All of these factors conspire to create an artificial division between acarologists and other arachnologists, to the extent that mites have their own literature, their own anatomical terminology, and their own national and international conferences. One of the outcomes of this division is that research papers on mites are more likely to be found in entomology journals than those devoted to arachnology. This is a reflection of the fact that mites have much in common with insects, especially as pests of cultivated plants and crops.
Classification of the Acari
Mites are the smallest arachnids in terms of body size (although the biggest mites are bigger than the smallest spiders). Adults of most species of mites are in the range 300-800 micrometres in body length. Mites are also distinguished from other arachnids by the complete absence of body segmentation, and by the fundamental organisation of the body. There is no 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 the animal's anatomical structures, including leg bases, central nervous system, ocelli when present, and reproductive and digestive systems, are all fused into a single unsegmented body called the opisthosoma.
Many systems of higher classification have been used for the mites, and many different names have been used for the higher taxa of mites at all levels. In the most modern classification, the mites are placed in the class Acari, within which there are two superorders. The superorder Parasitiformes includes order Opilioacarida, order Holothyrida, order Ixodida, and order Mesostigmata. The superorder Acariformes includes order Prostigmata (= Trombidiformes) and order Sarcoptiformes (Walter 2006; Krantz and Walter 2007).
Orders of Acari (with special reference to the Australian fauna):
Acarology in Australia
The history of the science of acarology in Australia has been reviewed in some depth (Southcott 1982; Halliday 2001b). The main trends have been research on agricultural pest mites and their natural enemies, on mite parasites of vertebrates, including humans and domesticated animals, on mites that have various types of association with Australian insects, and on the native mite fauna in agricultural and forest soils. There is no single comprehensive mite collection in Australia. Instead, there are collections in a number of institutions where the research effort has historically been concentrated.
The South Australian Museum in Adelaide has had the longest history of research in truly Australian mite taxonomy. Research there began with the work of Stanley Hirst, who was publishing from about 1912 to 1930, on a very wide range of mite groups. Herbert Womersley worked in the South Australian Museum from 1933 to 1962 (Southcott 1964). His work includes important publications on chiggers (Trombiculidae), including the vector of scrub typhus; on other parasites of vertebrates; on beneficial predatory groups; on pasture pest mites and their natural enemies; and on the families of Mesostigmata that dominate the fauna of soil and forest litter in this country. No modern study of Australian taxonomic acarology is complete without constant reference to Womersley's publications and collections. Acarology at the South Australian Museum continued with Ron Southcott, who published hundreds of papers from about 1945 to 1999 on a wide range of subjects. He is best known for his major works on trombidiform mites, especially the Erythraeidae (Southcott 1961), Trombidiidae (Southcott 1986) and Microtrombidiidae (Southcott 1994). The Adelaide tradition was continued by David Lee, who published important revisionary work on Mesostigmata (e.g. Lee 1970) and Oribatida (e.g. Lee 1992, 1993).
The Agricultural Scientific Collections Unit at the New South Wales Department of Primary Industries, Orange Agricultural Institute, Orange NSW, has an important collection of predatory Phytoseiidae created by Eberhard Schicha (Schicha 1987; Schicha and Corpuz-Raros 1992), which includes many type specimens. There are also collections of other groups of plant mites including Tenuipalpidae, Tetranychidae and Pygmephoridae. These are now being supplemented by a growing collection of plant parasites of the family Eriophyidae (Knihinicki and Boczek 2002, 2003).
The mite collection in the Australian National Insect Collection in Canberra, has grown as a result of a series of research projects on mite and insect pests and their natural enemies. A long term program of research on red-legged earth mites has included taxonomic studies of the pest and its relatives (Qin and Halliday 1996a, 1996b, 1997), and families of predatory mites that occur in the same habitats, especially Anystidae and Bdellidae (Wallace and Mahon 1973, 1976; Otto 1999a, 1999b, 1999c; Halliday 2005). The result was the formation of large collections of plant-feeding and predatory mites from pastures, both from Australia and overseas. The ANIC also includes large collections of mites associated with dung and dung beetles (e.g. Wallace 1986; Halliday 2000) and with many other groups of insects, but only a minority of these have been documented. There is a representative collection of marine mites of the family Halacaridae (e.g. Otto 1994, 1999d), and some families of oribatids are represented (Niedbala 1987; Niedbala and Colloff 1997). There is a large collection of ticks, most importantly those documented by Roberts (1970). The mite fauna of forest litter is huge and diverse, and a few preliminary studies of ANIC collections of these groups have appeared (Halliday 1997; Halliday et al. 1998). The collection also houses hundreds of unsorted samples of fauna from forest litter and other habitats, and these contain very rich collections of mites that await study.
The Queensland Museum in Brisbane houses a collection of approximately 16,000 slides and 2,500 vials of mites. By far the most significant portion of this collection is Bob Domrow's collection of about 13,000 slides and 1,000 tubes of mites from mammals, birds and reptiles, which was originally housed at the Queensland Institute of Medical Research. Only a small portion of these slides have been registered, and even fewer databased. The remainder of the collection comprises about 1,000 vials of mixed mites (mostly from pitfall trap samples), 500 vials of ticks, and another 3,000 slides that have a strong focus on the Trigynaspida, Podapolipidae and Tetranychidae (e.g. Seeman and Nahrung 2005). The Entomology section also holds a collection of 1,270 Berleseates, mostly from the wet forests of coastal Queensland, which are a rich source of unstudied mites.
The University of Queensland Insect Collection houses a very substantial collection of mites, which was largely built up through the efforts of Dave Walter. Most of the major families are represented, with strength in soil and litter Mesostigmata, predatory Phytoseiidae (Beard 2001), plant-feeding spider mites (Tetranychidae) and false spider mites (Tenuipalpidae), and some of the lesser known families of Prostigmata, including aquatic families. There is also a good range of Oribatid families, and both free-living and parasitic Astigmata. About 25% of the collection has been catalogued in the UQIC database. An outline of this catalogue is available at http://www.sib.uq.edu.au/acarina-catalogue, and contains links to a number of spectacular mite images.
The Museum of Victoria mite collection in Melbourne includes a substantial collection of water mites from Victoria. It includes types and identified material (Harvey 1990a; Harvey and Cook 1988), as well as unidentified water mites from multidisciplinary surveys of river faunas. There are also significant holdings of unidentified and sorted and unsorted mites from soils in a range of Victorian forests and woodlands, collected in pitfall traps during biodiversity and environmental management surveys.
Acarology at the Australian Museum (Sydney) began with Rainbow (1906), the first attempt to produce an overview of the Australian mite fauna, and some of Rainbow's types are still in the collection. The tick collection there includes types of some species described by Roberts, among others (e.g. Roberts 1960). There is also a collection of water mites, including some described by Harvey (1987, 1990a, b), but the collection is dominated by oribatids described by Glenn Hunt (Hunt 1996a, b; Hunt and Lee 1995). The slide collection is supported by substantial holdings of non-type and unidentified mite accessional material. including bulk samples from pitfall and litter collecting.
The Western Australian Museum in Perth has strong collections of mites in a range of families. There is an important collection of types of mites parasitic on vertebrates, mostly described in a series of papers that appeared in Records of the Western Australian Museum in 1978-1981, under the general title Parasites of Western Australia (e.g. Fain and Lukoschus 1979, 1981). There is also a large collection of marine mites of the family Halacaridae, especially the species from Rottnest Island (e.g. Bartsch 1993, 1994), a collection of water mites from Western Australia (Harvey 1988, 1996), a good collection of ticks, and a large quantity of unsorted mite material.
There are several excellent modern textbooks of acarology. Evans (1992) is a comprehensive review of mite anatomy, morphology, and behaviour, with detailed coverage of the structure and function of the integument, musculature, circulatory, respiratory and sensory systems, feeding, digestion, physiology, reproduction and mating, and development and dispersal. There is also a summary of classification, with keys to higher taxa down to the superfamily level. Walter and Proctor (1999) thoroughly explores the evolutionary origin of mites, the morphology and systematics of the major mite groups, and the life cycles, development, behaviour, reproduction, habitat and ecological relationships of mites, and uses mites as models to demonstrate a wide range of phenomena in evolutionary biology. The most detailed textbook of mite taxonomy is Krantz (1978), which provides keys down to the family level for all groups. Each family study includes information on biology and behaviour, evolutionary relationships and economic importance, and a taxonomic overview of the family. The current 1978 edition will soon be replaced by Krantz and Walter (2007), which presents the latest concepts in mite classification.
Many research publications in acarology appear in Acarologia, International Journal of Acarology, Systematic and Applied Acarology, Experimental and Applied Acarology, and a range of other acarology and entomology journals. Much important information also appears in the Proceedings of the International Congress of Acarology. The latest of these to be published was from the 1998 Canberra Congress (Halliday et al. 2001). The Proceedings of the 2002 Mexico Congress is in press, and that of the 2006 Amsterdam Congress is in preparation. Other important recent compilations of papers presented at acarological conferences include Schuster and Murphy (1991), Wrensch and Ebbert (1993), Kropczynska et al. (1995), and Bernini et al. (2002).
A variety of valuable information about acarology is available on the internet through the Acarology Home Page. This site provides contact details for acarologists and their institutions, details of acarological societies and publications, and an e-mail discussion list.
One of the subjects that is discussed among acarologists on the e-mail discussion list and elsewhere is the steady decline in the level of basic skills in our science. In 1998 the acarologists of Australia felt confident enough in the strength of their science to host the 10th International Congress of Acarology. In 2007, despite the enormous economic importance of mites, the demonstrated presence of many very distinctive components of our fauna, and the existence of thousands of undescribed species and higher taxa, there is not one full-time professional mite taxonomist in Australia. The research effort is being sustained by people working opportunistically part-time, or voluntarily in retirement, or by overseas acarologists who are working on the Australian fauna. There is no regular training in acarology at undergraduate or postgraduate level in Australian universities. Many challenges lie ahead in Australian acarology, and many important questions in mite systematics remain unanswered. An increase in the level of basic skills in mite taxonomy will be needed if Australia is to avoid mistakes in biosecurity and pest management, to realise the potential of mites as a source of information in biodiversity conservation, and to make an adequate contribution to international efforts to document this fascinating group of animals.
I am very grateful to Dave Walter for permission to reproduce the illustrations, and to Owen Seeman, Greg Daniels, Graham Milledge, and Peter Lillywhite, who provided valuable information about the mite collections in their care.
Anonymous 2003. Ticks.
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