Spider
Taxonomy and Evolution
Spiders (order Araneae) are air-breathing arthropods that have eight legs, 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 for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization.
The evolutionary lineage of spiders dates back to almost 400 million years ago. The first true spiders, the Mesothelae, appeared in the Carboniferous period. The more modern suborder, the Opisthothelae, evolved later in the same period and includes the majority of extant spiders.
Anatomy and Physiology
Spiders have a two-part body, the cephalothorax (also called prosoma) and the abdomen (or opisthosoma). The cephalothorax is derived from the fusion of the cephalon (head) and the thorax, and is covered by a single, convex carapace. The abdomen is soft and egg-shaped. It shows no sign of segmentation, except that the primitive Mesothelae, whose abdomen shows traces of the segments that in other spiders are entirely hidden by a hardened exoskeleton.
Spiders, unlike insects, retain their segmentation through a series of somites, or repeated segments. The abdomen is divided into a preabdomen and postabdomen, which can be separated by a constriction, as in the Mesothelae, or without such a clear separation, as in the Opisthothelae.
Silk Production
Spiders are known for the impressive silk they produce, a material with a tensile strength comparable to that of high-grade steel. The silk is produced in specialized glands found in the abdomen. Despite the common belief, not all spiders spin webs to catch prey. Some hunt, some burrow, and others live a nomadic lifestyle.
The silk is composed primarily of a protein very similar to that used in insect silk. It is initially a liquid, and hardens not by exposure to air but as a result of being drawn out, which changes the internal structure of the protein.
Venom
Most spiders possess venom, which they use primarily to kill their prey. The venom of a few species, especially the widow and recluse spiders, may be dangerous to humans. The venom is a complex mixture of biologically active molecules, proteins and peptides.
Reproduction and Life Cycle
Spider reproduction involves complex mating rituals. Males must avoid being eaten before fertilization. Most spiders lay eggs in silk bundles called egg sacs. Spiderlings resemble miniature adults. Spiders may live for several years (up to 20 years in some tarantula species), and many species can survive for long periods without food.
Ecological Role and Human Interaction
Spiders perform ecological roles that are beneficial to humans, such as consuming pests. However, they are often maligned due to their appearance and the behavior of a tiny minority of species that are dangerous to humans.