Spider Venoms
Although the members of the Uloboridae, Scytodidae (spitting spiders) and a couple of very rare families lack venom glands, the great majority of spider species have them and couldn’t survive without them. It was initially thought these glands were modified digestive glands but it is now clear this cannot be correct since a spider’s digestive glands are mesodermal in origin whereas the venom glands are actually invaginations of the outer tissue layer, the ectoderm. Spiders generally use their venom conservatively as a defence against aggressors and to immobilize prey and keep them in good condition to be eaten at a later date. On the other hand, they produce and use much larger quantities of digestive fluids
On this page we will examine the chemical and toxicological nature of the main substances found in spider venoms, their actions in the tissues of victims of spider bites, their potential uses as insecticides, and the medical procedures that are presently in use to help people who are suffering serious envenomation. It has been enstimated that overall there could be as many as ten million individual toxins in the world’s spiders, an ‘average’ spider possessing about 100 of them, mostly working synergistically. Exactly which combination of toxins a particular spider uses varies with its habitat, its usual prey and mode of catching them, and the inevitable changes in venom composition that occur as spiders evolve into new families, genera and species
Spider Venoms
Spider Venoms
The anatomy and physiology of spider venom glands-
In mygalomorph species the venom glands are located within the large chelicerae these spiders possess. There are probably muscle fibres in the walls of these glands and the ejection of venom may also be encouraged by the many muscle fibres that fill the chelicerae, these muscles also facilitating penetration of the fangs as the spider bites. Venom secretion is under the control of the nervous system but can be induced in a spider that has been anaesthetized by a brief exposure to an atmosphere of carbon dioxide gas. All that is needed is to stimulate the venom glands with electrical impulses from a pair of small electrodes placed on each side of the chelicerae. Venom collected in this way often looks turbid because the stimulation has caused regurgitation of gut contents but clear, uncontaminated venom may sometimes be obtained by fitting thin plastic tubes to the fangs, assuming the fangs are large enough for this technique to be feasable
Spider Venoms
Many of the components of spider venoms are highly toxic to arthropods, which is to be expected because the main prey of a typical spider are insects and other spiders. The reader may therefore wonder why a spider doesn’t poison itself before it has even put its venom to use. The answer to this question is not a simple one because of the variety of toxins in a single venom sample and their chemical and functional diversity. However, it is probably correct to say that most of them are stored in the venom glands in inactive precursor form. They presumably only become active as they are being released from the venom glands but there is not much information available as yet as to the actual activation process. Most probably, the toxin is stored as part of a larger molecule and pieces of this precursor molecule are enzymically cleaved as the venom is being injected into the victim.
Spider Venoms
