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The authors favoured relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata and Arachnida, as well as of some clades within the arachnids. The diagram below summarizes their conclusions, based largely on the 200 most slowly evolving genes; dashed lines represent uncertain placements.
The group's origins lie within an arachnid sub-group defined by the presence of book lungs (the tetrapulmonates); [1] [2] the arachnids as a whole evolved from aquatic chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. [3] There may be more than 120,000 species. [3]
A pedigree chart is a diagram that shows the occurrence of certain traits through different generations of a family, [1] [2] most commonly for humans, show dogs, and race horses. [ citation needed ] Definition
The idea of a tree of life arose from ancient notions of a ladder-like progression from lower into higher forms of life (such as in the Great Chain of Being).Early representations of "branching" phylogenetic trees include a "paleontological chart" showing the geological relationships among plants and animals in the book Elementary Geology, by Edward Hitchcock (first edition: 1840).
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Acarology – study of the taxon of arachnids that contains mites and ticks. Arachnology – scientific study of spiders and related animals such as scorpions, pseudoscorpions, harvestmen, collectively called arachnids. Entomology – study of insects. Coleopterology – study of beetles.
Close-up of the chelicerae of a harvestman. The chelicerae consist of three segments, with the distal two forming a pincer [3] equipped with a row of teeth. Based on developmental gene expression data, the proximal segment is thought to be homologous to the femur and patella of the legs, the middle segment to the tibia through the tarsus of the legs, and the distal segment to the apotele (the ...