ARACHNIDS

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ARACHNIDS

 

The ticks and mites compose a group of organisms that are part of a larger group of arthropods known as the Arachnida. Included in this group are the scorpions (Scorpiones) and the spiders (Araneae). The ticks and mites are actually composed of four groups of organisms, most of which are free-living. The Metastigmata is the group representing the ticks (composed of the "Hard Ticks” of the Ixodidae and the "Soft Ticks” of the Argasidae), and all the representatives in this group are parasitic in some part of the life cycle. The Mesostigmata contains the chigger mites (Trombiculids) and the two fowl mites (Dermanyssus and Ornithonyssus); this group also contains many free-living species. The Prostigmata contains the follicular mites (Demodex) and several hair-clasping mites (Cheyletiella). The Astigmata contains many parasitic forms, including the burrowing mites (Notoedres and Sarcoptes), the ear mite (Otodectes), and another hair-clasping mite (Lynxacarusradovskyi).

Ticks and mites are characterized as adults by having four pairs of legs and a fused anterior and posterior body (unlike spiders and scorpions) that appears to lack segmentation. The mouth parts are found on an anteriorly projected structure called the gnathosoma. The ticks and mites breathe either through tracheae or directly through the cuticle. The position of the opening of the major lateral truncks of the tracheal system is used to distinguish the different groups. In the Metastigmata (Ticks) the opening of the tracheal system is through a stigmata that is found behind the fourth pair of legs. In the case of the Mesostigmata, the stigmatal opening is between the third and fourth pair of legs. In the Prostigmata, the stigmatal openings (although difficult to visualize or absent) are located at the base of the mouthparts. The Astigmata have no tracheae ,and hence, no stigmatal opening. Krantz (1978) presented a classification of ticks and mites that differs from that used here which is based on that of Baker and Wharton (1952) and Nutting (1984). In the classification of Krantz, the Metastigmata is equivalent to the suborder Ixodida, the Mesostigmata are placed in the suborder Gamasida, the Prostigmata are placed in the suborder Actinedida, and the Astigmata are placed in the suborder Acaridida.

The basic life cycle of a tick or mite includes an egg, a larval stage (with three pairs of legs), a nymphal stage, and the adult. In some cases, there may be more than one nymphal stage, and often these have discrete names. For example, in the case of Otodectescynotis, the stage that comes out of the egg is called a larva, the first nymphal stage is called a protonymph, the second nymphal stage is called a deutonymph, and this is followed by the adult stage. Some mites have a third nymphal stage that is called a tritonymph. Some of the soft ticks will go through as many as five nymphal stages. The hard ticks tend to have the fewest stages, a larva, a nymph, and the adult.

REFERENCES:

Baker EW, Wharton GW. 1952. An Introduction to Acarology. Macmillan Co., New York.

Krantz GW. 1978. A Manual of Acarology, second edition. Oregon State University Book Stores, Inc., Corvallis, OR, 509 pp.

Nutting WB. 1984. Mammalian Diseases and Arachnids. Volumes I and II. CRC Press, Boca Raton, FL, USA.

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