In addition to the insects, the phylum Arthropoda contains several other classes, of which the most medically significant is the Arachnida, which includes spiders, scorpions, ticks and mites. All arachnids have a body that is divided into a fore- and hind-part, which may be fused into a sac-like structure. There is no separate head and no antennae. Mouthparts consist of paired chelicerae and pedipalps, and sometimes a central hypostome, all of which may be varied in appearance depending on feeding habits. The adult arachnid has four pairs of legs attached to the front part of the body, but wings are never present. Some arachnids possess venom glands incorporated in the mouthparts (spiders, Fig. 14.1) or at the hind end of the body attached to a needle-like sting (scorpions, Fig. 14.2). Some mites (Fig. 14.3) will cause an allergic skin or respiratory reaction (e.g. house dust mite and storage mite), others will burrow into the tissue (e.g. scabies mite) or may transmit disease (e.g. scrub typhus mite). Ticks (Fig. 14.4) and a few mites will feed exclusively on blood and may act as vectors of a range of diseases.
KeywordsBlood Meal Lyme Disease Scrub Typhus Facial Paralysis Erythema Migrans
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