There are about 35,300 species of spiders worldwide, but only about 30 species have venom deadly for the people. Spiders are assigned to 116 families.
Spiders represent one of the most adaptable species, resistant to variations of the environmental factors, being amongst the first creatures to inhabit devastated or regenerating areas (like burned forests or volcanic lava). Ecologically, there are three groups of spiders: web spiders, gallery spiders and wandering spiders.
Most spiders have a diameter of 0.5-1 cm (0.2-0.4 in), but the largest spider, Goliath bird-eater spider (Theraphosa blondi) (a tarantula) has a body diameter of 9 cm (3.6 in) and a leg span of 28 cm (0.9 ft). It lives in the rainforests of South America and can weigh up to 120 grams (as much as 2 mice). Most spiders weigh around 2 grams! And most other tarantulas around 7 grams.
Amongst the world's most dangerous spiders (due to the venom) are those belonging to the genus of widow spiders, Latrodectus (like the red back spider (Latrodectus hasselti) of Australia), recluse spiders (Loxosceles) of Americas, and Sydney funnel web spider (Atrax robustus) (believed to have killed 13 people). Some Latrodectus spiders have venom 15 times more powerful than that of the rattlesnake.
The Latrodectus and Atrax spiders produce neurotoxins, thus their venom has the same effect like that of the cobras, for example. Some of their neurotoxins do not act on humans, and scientists look for methods of using these chemicals in insect pest control.
Instead, the venom of the Loxosceles spiders or the six-eyed sand spiders (Sicarius) of meridional Africa has a necrotic (tissue-destroying) effect, resembling the venom of the rattlesnake.
Other spiders may not be deadly but have nasty bites, like the white tailed spider (Lampona cylindrata) of Australia and wolf spiders, whose venom causes swelling, mild or strong pain and itching, from 30 minutes to 4 hours.
The spider's venom causes pain because its molecules target the capsaicin receptor, also named TRPV1, called after capsaicin, the alkaloid from chili peppers responsible for the burning sensation. Three protein subunits (peptides) from spider venom, named vanillotoxins, were identified so far as responsible for activating TRPV1.
The name of tarantula comes from the port of Taranto (southern Italy). The locals correlated the convulsions and uncontrolled jumps of the victim of a spider bite with the local dance of tarantella, and hence the name for the tarantula spiders. Paradoxically, even if they are by far the largest spiders, they do not make the list of the deadly spiders, as their venom is weak. They rather frighten because of their huge fangs, 1-2.5 cm (0.4-1 in) long. The tarantula bite has been compared to that of a wasp.
And the venom of the spiders is not entirely a negative issue. Some traditional medicines use spiders, and ancient Romans appreciated spider venom for its preventive and even curative qualities in the case of some diseases and even bad mood.
The venom of the Sicarius spider is investigated as a treatment for cancer.
The powerful venom of the Brazilian wandering spider (Phoneutria nigriventer) induces hours-long erection. The toxins provoke general pain and higher blood pressure, but also this uncomfortable erection. Their investigation could lead to the development of "spider Viagra". The active compound was a peptide (short protein) named Tx2-6. In humans, it increases the nitric oxide levels within corpora cavernosa, the two main cylinders made of spongy tissues running along the penis. The nitric oxide is implied in the neuronal pathway of an erection.
In the end, the cGMP enzyme affects the smooth muscles of the penis' corpora cavernosa, so that the blood fills up this tissue resembling a honeycomb, expanding it (till about 10 times more in humans).
But the PDE-5 breaks down the cGMP, putting an end to the fiesta: the smooth muscles contract and the blood goes out of the penis, which turns back to its limp state. Viagra, Cialis and Levtra act against PDE-5. But Tx2-6 works differently: it acts in the first steps of the erection initiation, raising the nitric oxide levels. Combining Tx2-6 with Viagra would satisfy the needs of many more patients having erectile dysfunction.