Arachnida

The Arachnida are a class of Arthropode S Chélicérate S terrestrial, often insectivorous. It is the group which understands, inter alia, the Araignée S, the scorpions and the Acarien S. They are distinguished within their junction by the fact that they have 4 pairs of legs, that they have neither wings nor antennas, and that their eyes are simple (ocelles) and not made up. The majority of Arachnida are oviparous animals and the sexes are generally separate.

The name of the class draws its origin from the arachné Greek word meaning spider.

History

To date, oldest known fossil Arachnida belong to the order of the scorpions. This fossil has approximately 500 million years.

The first Arthropode S with grips of the sub-order of the Chélicérates appeared approximately 540 million years ago, during the Cambrien, with the whole beginning of the era Paléozoïque. The majority of them adapted to the terrestrial life. Then, during the higher Silurien (440 million years ago), appear the first known specimens of scorpion and Araignée S extremely similar to those which we can find nowadays.

One to date listed approximately 80.000 species of Arachnida, including more than 1500 species of scorpions and 50.000 species of spiders living in all the existing biotopes, of the tropical areas to the polar regions. The majority of Arachnida are however terrestrial.

Anatomy and morphology

The internal organization especially interesting by the lungs, or is phyllotrachées , kinds of pockets opening outside by the marks, and containing a series of layers parallel, thin-walled, through which are made the gaseous exchange.

  • 4 pairs of locomotor legs in the adult (1).
  • body made up of two parts: prosoma or céphalothorax (combined head and thoraxes (2)), epistosoma or abdomen (3). In the acarina and oppilated them - also called “faucheux” -, these two parts are amalgamated. In the scorpions and pseudoscorpions on the contrary, the abdomen is prolonged by a postabdomen or metasoma , commonly called “tail”.
  • not of antennas
  • not of wings
  • simple eyes
  • reproductive bodies located on the former ventral face of the abdomen.

The céphalothorax carries the 4 pairs of locomotor legs, as well as the Pédipalpes or maxillipedes, located in front of the prosoma. These pédipalpes has a primarily tactile role. The prosoma also carries a pair of chélicères, prolonged by a hook with venom. The céphalothorax is strongly chitinized in Arachnida, getting for the animal an appreciable protection. However, the rigidity of this carapace constrained members of this class to periodic moults of growth.

The abdomen is segmented in the scorpions, not segmented in the spiders and the acarina. It is not it that slightly at oppilate. It is estimated that the importance of the abdominal segmentation is an evolutionary index: the scorpions, with the very segmented abdomen, are thus regarded as being the most primitive order of this class; the acarina, with the abdomen little or apparently not segmented, are regarded as being the most advanced order of the class. This estimate appuye on these morphogenetic considerations, but also on the more or less great anteriority of the fossils discovered for various Arachnida.

At certain orders like the solifuges, the pédipalpes are very developed and can be confused with legs. They are well however different bodies: the legs include/understand 7 articles when the pédipalpes include/understand only 6 of them. Moreover, the base of the pédipalpes is often equipped with silks in the shape of comb on the interior face, so as to be used as filter in front of the opening bucal. Lastly, the pédipalpes do not play any part in the locomotion.

Taxonomy

Place des Arachnida in the animal kingdom

Classification

The class of Arachnida includes/understands the following orders:

  • Acari - the Acarina S and the Tick S

  • Amblypigi - the Amblypige S
  • Araneae - the Spider S
  • Opiliones - the Opilion S
  • Palpigradi - the Microscorpion S
  • Pseudoscorpiones - the Pseudoscorpion S
  • Ricinulei - the Ricinulides
  • Schizomida - the Schizomide S
  • Scorpiones - the scorpion S
  • Solifugae or Solpugida - the Solifuge S
  • Uropygi - the Uropyge S

Phylogenetic classification

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