Acarology is the study of mites and ticks, animals which are classified in the Order Acari of the Class Arachnida, which is part of the Phylum Arthropoda. As such they have typical arthropod characteristics, including an exoskeleton which they have to shed to grow, and jointed legs. Because of the limitations of their exoskeletons, they are limited in size and are usually quite small. As Arachnids, they have two body parts (cephalothorax and abdomen) and eight legs, along with the other orders in that class: spiders, whip scorpions, scorpions, pseudoscorpions and harvestmen. Acari can be differentiated from the rest of the arachnids by having an abdomen that is broadly joined to the cephalothorax so it often looks as if they only have one body part. Larval mites have three sets of legs but grow a fourth set before they mature.
Mites are found free-living in most terrestrial and aquatic habitats. As well, there are many species of ectoparasitic mites, living on birds, mammals and other large animals. Many mites belong to the suborder Mesostigmata which is placed in the group Parasitiformes along with the ticks (suborder Ixodides). There is another group of mites, the Acariformes, which includes the ectoparasitic mange, itch and feather mites, gall mites, oribatid mites and spider mites. Altogether there are over 50,000 species worldwide of mites and ticks.
Members of the Mesostigmata include predators, scavengers and parasitic forms. Predaceous mites live in leaf litter and soil. Parasitic forms attack birds, mammals snakes and even insects, living on their skin and sucking their blood or body fluids. Most of the parasitic forms that attack humans and their pets, causing dermititis and mange, belong to the Acariformes. They can also infest all sorts of organic products such as cheeses, dried meats, seeds and flour. As such they are important economic pests. Another particularly nasty group of mites are the sarcoptidae, the itch and scab mites which burrow into the skin and cause irritation. Mites have also been implicated in house dust allergies.
Chiggers are another unpleasant form of mite. These animals are parasitic as larvae, burrowing into the skin of humans and other animals, using their saliva to dissolve the host’s tissues, which the chiggers then digest and this causes itchiness for the host. The adults are small and free-living.
A lot of mites attack plants instead of animals and these too are important economic pests. Red spider mites do serious damage to orchards, garden and greenhouse plants.
Water mites live in ponds. The larvae live on aquatic insect hosts such as dragonfly nymphs and the brightly coloured red or green adults live freely in the water. They can often be seen under the microscope, hunting other small pond creatures.
Ticks are a specialised and successful group of mites that specialise in sucking the blood of higher animals such as birds, mammals and reptiles, at various stages of the tick’s life cycle. In addition to the annoying itchiness caused by a tick, there is the danger of disease, for ticks can be vectors of diseases such as Lyme’s disease and Rocky mountain spotted fever. The saliva of paralysis ticks can cause paralysis and death in the host if not removed or if there are too many. Ticks lay their eggs on vegetation and the hatchlings then grab on to a passing host. After a feed, the young ticks drop off and they live on vegetation until ready for the next moult and the next feed. They will feed several times before they mature and lay their own eggs.
Ticks are common in Australia and cause the deaths of many pet dogs and cats. Australian paralysis ticks are deadly to introduced species but native mammals are relatively immune to these parasites. Bandicoots are a natural reservoir of ticks which then move onto cattle and other domesticated animals. The bandicoots can carry quite a heavy load of ticks without harm.
My most memorable encounter with ticks involved a young platypus who was infested with over 700 of the nasty little critters. Over seventy of those ticks were paralysis ticks and would have killed the platypus if they had not been removed. The fact that the platypus lived is a testiment to its natural immunity to these ticks since a single paralysis tick can kill a dog weighing ten times as much as the platypus. Kangaroos, possums and echidnas also have a natural immunity to ticks.
Although parasitic mites and ticks are unpleasant creatures, the vast majority of mites are free-living and a natural part of their ecosystems. They are an important part of many soil and aquatic exosystems and are well adapted to their life styles. They are the sorts of creatures that will probably survive long after we are gone.
Source: Borror and DeLong 1971 An Introduction to the Study of Insects
