Arachnida ( / əˈræknɪdə /) is a class of joint-legged invertebrate animals ( arthropods ), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, harvestmen, and solifuges. In 2019, a molecular phylogenetic study also placed horseshoe crabs in Arachnida.
- Diet and Digestive System
Arachnids are defined as coming from the class of Arachnida. The requirements for this class is to have two body regions; a cephalothorax and an abdomen; 4 pairs of legs; and 2 pairs of mouthpart appendages, the chelicerae and the pedipalps.
All adult arachnids have four pairs of legs, and arachnids may be easily distinguished from insects by this fact, since insects have three pairs of legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps have been adapted for feeding, locomotion, and/or reproductivefunctions. Like all arthropods, arachnids have an exoskeleton. They also have an internal structure of cartilage-like tissue, to which certain muscle groups are attached. Arachnids have no antennae or wings. Their body is organized into two parts: the cephalothorax, and the abdomen.
There are some adaptations for life on land. They have internal respiratory surfaces. These may be trachea (tubes), or a modification of gills into a 'book lung'. This is an internal series of lamellaeused for gas exchange with the air.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only the harvestmen and some mites eat solid food particles. Predigestion avoids exposure to internal parasites. Several groups secrete venom from specialized glands to kill prey or enemies. Several mites are external parasites, and some of them are carriers of disease (vectors). Arachnids pour digestive juices produced in their stomachs over their prey after killing it with their pedipalps and chelicerae. The digestive juices rapidly turn the prey into a broth of nutrients which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus also acts as an additional pump.
The word Arachnida comes from the Greek for 'spider'. In legend, a girl called Arachne was turned into a spider by the goddess Athena. Arachne said she'd win a weaving contestagainst the goddess. Athena won, but Arachne became angry, and started to weave an insult to the gods. Then Athena turned her into a spider for her disrespect.
The subdivisions of the arachnids are usually treated as orders. Historically, mites and tickswere treated as a single order, Acari. However, molecular phylogenetic studies suggest that the two groups do not form a single clade; morphological similarities are probably due to convergence. They are now usually treated as two separate taxa – Acariformes, mites, and Parasitiformes, ticks – which may be ranked as orders or superorders. The arachnid subdivisions are listed below alphabetically; numbers of species are approximate. 1. Acariformes– mites (32,000 species) 2. Amblypygi – "blunt rump" tail-less whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more (153 species) 3. Araneae– spiders (40,000 species) 4. †Haptopoda – extinct arachnids apparently part of the Tetrapulmonata, the group including spiders and whip scorpions (1 species) 5. Opilioacariformes– harvestman-like mites (10 genera) 6. Opiliones– phalangids, harvestmen or daddy-long-l...A scorpion (Sc. Maurus Palmatus)Galeodes, a camel spiderA whip scorpionA Pseudoscorpion, on a printed page
Spiders Temporal range: Pennsylvanian – Holocene, 319–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N An assortment of different spiders Scientific classification Kingdom: Animalia Phylum: Arthropoda Subphylum: Chelicerata Class: Arachnida Order: Araneae Clerck, 1757 Suborders Mesothelae Opisthothelae Mygalomorphae Araneomorphae See Spider taxonomy. Diversity 120 families, c. 48,000 species Spiders ...
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The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about in the Silurian period. Note 2] Attercopus fimbriunguis , from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders ,  which first appear in the Late ...
Arachnids as food. For the cultural and biological process of eating arachnids, see Arachnophagy. Some arachnids may be used for human consumption ( edible arachnids ), either whole or as an ingredient in processed food products such as cheese ( Milbenkäse ). Arachnids include spiders, scorpions, and mites (including ticks ).
Arachnid is a 2001 American horror film directed by Jack Sholder.The film centers on a group of plane crash survivors who are stalked and killed by giant alien spiders. The film stars Alex Reid, Chris Potter, Rocqueford Allen, Robert Vicencio and José Sancho