6.3 Chelizerata

Arachnids are also derived from annelids; however, a changed number of body segments are united to tagmata in the different orders. In scorpions and spiders, the prosoma carries the feeding organs and the legs, whereas the opisthosoma remains without organs for locomotion. The male and female gonoporus is located on the 8. postoral body segment in all orders. The tritocerebrum with the first postoral commissure innervates the chelicerae corresponding to the first pair of extremities. The second pair, the pedipalps, are segmented and carry sense organs (fig. 6.2, page 244).

Morphology of hard and soft ticks

The acari, the ticks and mites, feed on the blood of vertebrates. The body is non-segmented and the reduced head, the capitulum or gnathosoma, carries the chelicerae and pedipalps (fig. 6.16 and 6.18, pages 260 and 262). The cerebrum and the ventral nervous system are united, as are the segments with and without legs, designed as the idiosoma. The capitulum of hard ticks or ixodidae is located anterior to the idiosoma, whereas that of soft ticks or argasidae is ventral to the idiosoma (fig. 6.20, page 264).

The morphology of both is derived from spiders (fig. 6.15, page 259). The chelicerae move like telescopes and its claws, which are located laterally on the tip, grasp backward by means of muscles arising on the inner apodemes. The ventral hypostoma with numerous hooks directed backward is sometimes divided lengthwise but without any muscles and represents an anterior ventral process of the capitulum. The salivary glands correspond to mandibular glands. The extended sucking pump is usually designed as a "pharynx" but corresponds to the cibarium of insects. The hard ticks or ixodidae carry a dorsal tectum, which is small in females but covers the idiosoma completely in males. A pair of glands, Gene’s organ, is located in the furrow between the capitulum and tectum in the female. Secretions from this organ cover the eggs singly with a waxy layer and protect them against desiccation.

For blood-feeding, the ixodidae place themselves on the host’s skin and distend the idiosoma backwards. The argasidae place themselves on their ventral side extending the idiosoma dorsally. For the oviposition, both types borrow into the ground. However, individuals of the ixodidae oviposit only once and close the hole with its empty body, whereas the argasidae oviposit several times and return to the surface for further blood-feeding (fig. 6.20, page 264).

Ixodes ricinus is a three-host tick, with the larvae, nymphs and imagoes feeding on three different kinds of hosts. The general biology of hard ticks is given in the context of piroplasms (see chapter 2.7.3, page 113). The anatomy of the capitulum (chelicerae, rostrum of male and female), a pair in copulation, the Haller’s organ on the first walking legs and the construction of the spiracle are characteristic for hard ticks (fig. 6.16, page 260). The internal anatomy of the intestine, including the resorption of food by phagocytosis, and the highly differentiated salivary glands with three types of acini are particularly important for the development of Theileria spec. (fig. 6.17, page 261).
Ornithodorus moubata, a tropical soft tick, lives in huts with loamy soil or the holes of warthog, aardvark (Orycteropus afer) and wild muridae. It can be easily reared in the laboratory and can feed on heparinized blood through artificial membranes. Its biology is given in the context of relapsing fever (chapter 4.3.2, page 218). The anatomy of the capitulum, the internal anatomy of the intestine, the excretion organs and the construction of the spiracle are presented in fig. 6.18, page 262. The male internal genital organs are peculiar with respect to the development of the spermatophore. For its transport at copulation, the rostrum is used (fig. 6.19, page 263).
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