Pathophysiology
A primary concern of the bite of a spider is the effect of its venom. A spider envenomation occurs whenever a spider injects venom into the skin. Not all spider bites involve injection of venom into the skin, and the amount of venom injected can vary based on the type of spider and the circumstances of the encounter. With very few exceptions, such as the so-called camel spider (which is not a true spider), the mechanical injury from a spider bite is not a serious concern for humans. Some spider bites do leave a large enough wound that infection may be a concern, and other species are known to consume prey that is already dead, which also may pose a risk for transmission of infectious bacteria from a bite. However, it is generally the toxicity of spider venom that poses the most risk to human beings; several spiders are known to have venom that can be fatal to humans in the amounts that a spider will typically inject when biting.
All spiders are capable of producing venom, with the exception of the hackled orb-weavers, the Holarchaeidae, and the primitive Mesothelae. (Other arachnids often confused with spiders, such as the harvestman and sun spiders, also do not produce venom). Nonetheless, only a small percentage of species have bites that pose a danger to people. Many spiders do not have mouthparts capable of penetrating human skin. While venoms are by definition toxic substances, most spiders do not have venom that is sufficiently toxic (in the quantities delivered) to require medical attention and, of those that do, only a few are known to produce fatalities. (For details that substantiate these claims, see the remainder of this article.)
Spider venoms work on one of two fundamental principles; they are either neurotoxic (attacking the nervous system) or necrotic (attacking tissues surrounding the bite, and, in some cases, attacking vital organs and systems).
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