Spring Spider ( Marpissa muscosa )
|Heymons , 1901|
A group of arthropods (Arthropoda), which is characterized by the possession of a specifically transformed extremity of the first head segment, is referred to as jaw-claw carriers , antennae or chelicerates (Chelicerata) . These extremities, known as chelicerae , also represent the first extremities of the body. Antennas, such as those found in crustaceans and tracheal animals and homologous to the chelicerae and pedipalps , are missing.
Among the chelicerates include world well-known 100,000 species, including the horseshoe crabs and the arachnids belonging scorpions , spiders , harvestmen and mites . The extinct sea scorpions , which were the largest arthropods ever to live, also belong to this group. With the exception of the horseshoe crabs and woodlice spiders , all recent species of this group live on land or have subsequently returned to the water (such as the water mites or the water spider ).
The jaw-claw carriers are a very diverse group within the arthropods, which is why it is difficult to name common characteristics of the species. The most important common feature they have in common are the chelicerae mentioned above, which arise on the second head segment (not on the third, as had been assumed for decades). The next pair of limbs is at the horseshoe crabs are already running the first pair of legs, with all other groups, it takes as pedipalps various tasks. The following four pairs of extremities are primarily designed as walking legs in all groups. In the flagellated scorpions and flagellated spiders , the first pair of legs was transformed into tactile organs, the so-called antennae legs.
The body of the jaw bearer is usually divided into two sections ( tagmata ), a front body ( prosoma ) and an abdomen (opisthosoma). All of the extremities listed above as well as the most important sensory organs are located in the animals' front body, the extremities of the abdomen are usually completely transformed and have completely different functions (sexual organs, spinneret glands , fan lungs ). The digestive organs, the internal genital organs and the tubular heart are housed in the opisthosoma.
Originally the antennae had complex eyes, but these are only present in the horseshoe crabs. The other groups have a maximum of five pairs of individual eyes.
Reproduction and development
There are also various variations in reproduction. Since most species live on land, there is very often internal fertilization by penis-like structures (for example in spiders, in which the male and female sexual organs fit into one another as in the lock-and-key principle). The males of other groups such as the scorpions and most of the mites deposit sperm packets ( spermatophores ) which are ingested by the females.
The kinship relationships within the Cheliceraten are still largely unclear and the subject of controversial discussions. In particular , there is no agreement on the classification of the morphologically strongly modified woodlice spiders ; there are several alternative views on the classification within arachnids . The systematics of the Chelicerata in the classical phylogenetic system according to Weygoldt and Paulus (1979) is shown below:
Relationships within the Chelicerata according to Ballesteros and Sharma (2019).
- Jawbearers (Chelicerata)
- Woodlouse spiders (Pycnogonida, also Pantopoda)
- Scorpions (Scorpiones)
- DT Anderson: Invertebrate Zoology. 2nd Edition. Oxford Univ. Press, 2001, ISBN 0-19-551368-1 , chap. 14, p. 325.
- RSK Barnes, P. Calow, PJW Olive, DW Golding, JI Spicer: The invertebrates - a synthesis. 3. Edition. Blackwell, 2001, ISBN 0-632-04761-5 , chap. 8.4, p. 174.
- RC Brusca, GJ Brusca: Invertebrates. 2nd Edition. Sinauer Associates, 2003, ISBN 0-87893-097-3 , chap. 19, p. 653.
- J. Moore: An Introduction to the Invertebrates. Cambridge Univ. Press, 2001, ISBN 0-521-77914-6 , chap. 14, p. 207.
- EE Ruppert, RS Fox, RP Barnes: Invertebrate Zoology - A functional evolutionary approach. Brooks / Cole, 2004, ISBN 0-03-025982-7 , chap. 18, p. 554.
- UW Hwang, M. Friedrich, D Tautz , CJ Park, W. Kim: Mitochondrial protein phylogeny joins myriapods with chelicerates. In: Nature. 413, 2001, p. 154.
- P. Weygoldt: Evolution and systematics of the Chelicerata. In: Experimental and Applied Acarology. 22, 1998, p. 63.
- WC Wheeler, CY Hayashi: The phylogeny of the extant chelicerate orders. In: Cladistics. 14, 1998, p. 173.
- Maximilian J. Telford & Richard H. Thomas: Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment. In: Proceedings of the National Academy of Science USA. Vol. 95, 1998, pp. 10671-10675.
- Arthur D. Chapman: Numbers of Living Species in Australia and the World. 2nd Edition. Report for the Australian Biological Resources Study. Canberra 2009, ISBN 978-0-642-56861-8 . (on-line)
- Jesús A. Ballesteros & Prashant P. Sharma: A Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error. Systematic Biology, syz011, February 2019. doi: 10.1093 / sysbio / syz011 ( PDF )