Biggest Spider In The World
Spiders differ from insects, crustaceans and other members of the phylum Arthropoda by having two body parts rather than the three of insects and crustaceans, and the multiple body parts of other arthropods. They also completely lack antennae, the only arthropod group to lack these sensory organs. Spiders use structures called chelicerae in feeding rather than the mandibles found in other arthropod groups.
Spiders, in the order Araneae, share the class Arachnida with other organisms such as scorpions (order Scorpiones), pseudoscorpions (order Pseudoscorpiones), camel spiders (order Solifugae), vinegaroons (order Thelyphonida), tailless whipscorpions (order Amblypygi), daddy long legs (order Opiliones), and mites and ticks (order Acari). All these arachnid groups are thought to have evolved from an ancestor similar to eurypterids (order Eurypterida). These marine chelicerates were common in the seas from 500 to 245 million years ago (Ordovician to the Permian). Some eurypterids grew to two meters long. These giant aquatic arthropods were formidable predators of the ancient seas and had similar morphological characteristics as extant scorpions. In fact, some of the earliestscorpion fossils resemble more recent eurypterid fossils. Scorpion fossils from the Silurian period probably were aquatic because gills are apparent on these fossils. The arachnids that evolved from a eurypterid-like ancestor moved from the marine environment to the terrestrial environment, but largely maintained their predatory nature.
The earliest fossil arachnids include certain scorpions that date back to the Silurian periodbetween 440 and 410 million years ago. The earliest true spiders appear in the fossil record in the Devonian Period about 380 million years ago. Early terrestrial arthropods had certain adaptations that gave them an advantage in the transition from an aquatic to a terrestrial environment including jointed legs to support the body against the pull of gravity and an exoskeleton to protect them from the desiccating effects of the air. The respiratory structures of the early aquatic chelicerates were probably similar to the book gills found on horseshoe crabs. These book gills are, in turn, similar in structure to the internal book lungs of extant terrestrial arachnids. It is tempting to imagine that as the early chelicerates were making the evolutionary transition from an aquatic to a terrestrial existence, the external respiratory structures became internalized as the book lungs.
The order Araneae currently includes over 40,000 species. This number includes only those species known to science. New species are being discovered every year, and the true species diversity is probably closer to 80,000, according to Dr. Norman Platnick of the American Museum of Natural History. Spiders are found in 108 families and over 3,600 genera.
