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Structure and Evolution of Invertebrate Nervous Systems$
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Andreas Schmidt-Rhaesa, Steffen Harzsch, and Günter Purschke

Print publication date: 2015

Print ISBN-13: 9780199682201

Published to Oxford Scholarship Online: March 2016

DOI: 10.1093/acprof:oso/9780199682201.001.0001

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Arachnida (Excluding Scorpiones)

Arachnida (Excluding Scorpiones)

(p.453) 38 Arachnida (Excluding Scorpiones)
Structure and Evolution of Invertebrate Nervous Systems

Tobias Lehmann

Roland R. Melzer

Marie K. Hörnig

Peter Michalik

Andy Sombke

Steffen Harzsch

Oxford University Press

Arachnids are terrestrial predatory chelicerates, including spiders and mites, with more than 110,000 described species within these two groups. The conquest of land led to an enormous adaptive radiation, and in the Carboniferous all extant groups were present. Terrestrialization resulted in several dramatic morphological modifications, including often loss of opisthosomal appendages, transformation of gills into book lungs, and gain of internal fertilization. Extant arachnids are characterized by two tagmata, prosoma, and opisthosoma. Only scorpions retained an appearance similar to the extinct eurypterids, characterized by a division of the opisthosoma into meso- and metasoma. The emergence of the common arachnid ancestor from sea to land also triggered dramatic changes in the organization of the sense organs. For example, cuticular sense organs such as trichobothria and slit-sense organs evolved, which play an important role in orientation and perception of vibrations. Furthermore, a variety of other tactile and chemosensitive hairs are present, mainly on the appendages. Some taxa such as spiders or ricinuleids carry tarsal or pore organs, which are cuticular depressions equipped with thermo-, hygro-, and/or chemoreceptors. For perceiving visual stimuli, arachnids possess a number of median and lateral eyes. The lateral eyes (maximum of five pairs) may be evolutionarily related to compound eyes, which are still present in other chelicerates (Xiphosura). The median eyes can be very prominent as in jumping spiders, or can be lacking as in anactinotrichid mites. In this chapter, the structure of the chelicerate nervous system with a strong bias on their visual system is summarized.

Keywords:   spiders, mites, visual system, internal fertilization, eyes, chemosensitive hairs

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