Spiders are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except Antarctica, and have become established in nearly every land habitat. As of November 2023, 51,673 spider species in 136 families have been recorded by taxonomists. However, there has been debate among scientists about how families should be classified, with over 20 different classifications proposed since 1900.
Spider
Palystes castaneus female, dorsal aspect pedipalp trichobothria carapace of prosoma (cephalothorax) opisthosoma (abdomen) eyes AL (anterior lateral) AM (anterior median) PL (posterior lateral) PM (posterior median) Leg segments: coxa trochanter femur patella tibia metatarsus tarsus claw chelicera
Palystes castaneus female, ventral aspect. Nos 1 to 14 as for dorsal aspect. sternum of prosoma pedicel (also called pedicle) book lung sac book lung stigma epigastric fold epigyne anterior spinneret posterior spinneret Legs are labelled I, II, III, IV from anterior to posterior.
A syrphid fly captured in the web of a spider
Arthropods are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated (metameric) segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They are an extremely diverse group, with up to 10 million species.
Protaetia cuprea (copper chafer). Beetles are the most diverse order of arthropods.
Cicada climbing out of its exuviae while attached to tree
Long bristles (setae) of a Tliltocatl albopilosus tarantula
Head of a wasp with three ocelli (center), and compound eyes at the left and right