spiders chelicerates , therefore arthropods. arthropods have: segmented bodies jointed limbs, covered in cuticle made of chitin , proteins; heads composed of several segments fuse during development of embryo. being chelicerates, bodies consist of 2 tagmata, sets of segments serve similar functions: foremost one, called cephalothorax or prosoma, complete fusion of segments in insect form 2 separate tagmata, head , thorax; rear tagma called abdomen or opisthosoma. in spiders, cephalothorax , abdomen connected small cylindrical section, pedicel. pattern of segment fusion forms chelicerates heads unique among arthropods, , first head segment disappears @ stage of development, chelicerates lack antennae typical of arthropods. in fact, chelicerates appendages ahead of mouth pair of chelicerae, , lack function directly jaws . first appendages behind mouth called pedipalps, , serve different functions within different groups of chelicerates.
spiders , scorpions members of 1 chelicerate group, arachnids. scorpions chelicerae have 3 sections , used in feeding. spiders chelicerae have 2 sections , terminate in fangs venomous, , fold away behind upper sections while not in use. upper sections have thick beards filter solid lumps out of food, spiders can take liquid food. scorpions pedipalps form large claws capturing prey, while of spiders small appendages bases act extension of mouth; in addition, of male spiders have enlarged last sections used sperm transfer.
in spiders, cephalothorax , abdomen joined small, cylindrical pedicel, enables abdomen move independently when producing silk. upper surface of cephalothorax covered single, convex carapace, while underside covered 2 rather flat plates. abdomen soft , egg-shaped. shows no sign of segmentation, except primitive mesothelae, living members liphistiidae, have segmented plates on upper surface.
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