Order Araneae - Araneida: The Spiders

Order Araneae/ Araneida: The Spiders

The different kinds and types of spiders belong to the Order Araneae, the largest entirely carnivorous group of animals in the Arthropoda, consisting about 40,000 species and rank seventh in total species diversity among all the other groups of organisms (Sebastian and Peter 2). These organisms differ from other animals of the Class Arachnida in possessing a sac-like abdomen joined to the cephalothorax by a slender pedicel. (Comstock 218). According to Comstock, spiders have been derived from an ancestral stock in which the abdomen was distinctly segmented. (218). Only a few spiders nowadays have a segmented abdomen, most of them have no definite indications of segmentation.

Although the phylogeny of spiders is still controversial, specialists have classified and distinguished the spiders from one another by their several unique morphological traits (including spinnerets, pedipalps and genitalia) and as well as behavioral, and physiological characteristics in relation with their ancestral stock which separate them to different groups or clades consisting of a particular common ancestor and its descendants in a monophyletic arrangement. These characters are often studied and derived from the examination of chromosomal, protein and genomic (DNA or RNA sequence) data that traces these characteristics way back through its ancestors. Relationships between the groups of species, genus, families, etc. can be studied by the distribution of derived characters and molecular similarities within the group through cladistic techniques. ("Classifying Spiders").

According to Platnick, spiders currently comprise 110 families, 3600 genera and nearly 39,000 species. (qtd in Coddington 21). Strong evidence supports monophyletic relationship between the organisms of the Order Araneae: cheliceral venom glands, male pedipalpi modified through sperm transfer, abdominal spinnerets and silk glands, and lack of the trochanter-femur depressor muscle. (Coddington 21). Order Araneae are divided into two sub-orders: Mesothelae and Opisthothelae, the latter being further divided into two infra-orders: the tarantulas and their allies (Mygalomorphae) and the so-called "true spiders" (Araneomorphae). (Coddington 21).

Suborder Mesothelae

Mesothelae is the suborder of spiders that include the extinct families Arthrolycosidae, Arthromygalidae and the only living primitive family of Liphistiidae. These spiders are characterized to have segmented plates on top of the abdomen, narrow sternum on the ventral side of the prosoma, the presence of ganglia in abdomen, anterior median pair of distinct spinnerets (four pairs), downward and forward striking direction and parallel alignment of fangs, presence of two pairs of book lungs and absence of venom glands (Leroy J. and Leroy A. 15-21). One example of a species of Family Liphistiidae

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