Spider and Scorpion Venom

Spider fangs have evolved to puncture and cut through insect exoskeletons in order to inject the venom and paralyse their prey. Therefore, the fangs must be sufficiently tough to withstand the impact of the bite, and stiff to break the prey's exoskeleton. The fang's strongest point is at the tip, and the softest point is at the base. Both insect exoskeletons and spider fangs are made of chitlin; the fiber orientation of the fang determines its physiological properties - such as its durability. An opening to the venom canal is on the dorsal side of the fang. The venom moves from the venom gland to this opening, where it is injected into the prey or predator. Scorpions have a similar structure at the end of their tail (Politi, et al., 2012).

A diagram of a spider's fang (Image 1).

A diagram of scorpion anatomy, including the stinger and venom gland (Image 2).

Spiders and scorpions use their venom to catch prey and to protect themselves from predators, like other venomous animals. However, spiders and scorpions do something no other animal can do - inject a pre-venom. Venom synthesis is a metabolically expensive procedure, so the pre-venom is a metabollicaly 'cheap' alternative. Pre-venom is used to deter low-threat predators by inducing pain. The scorpion or spider will not take this risk with a high-risk predator, so they inject the main venom. This shows that venom expendure is regulated (Casewell, et al., 2013).

Funnelweb spider fangs (Image 3).

A scorpion in defense position (Image 4). 

References

Casewell, N. R., Wüster, W., Vonk, F. J., Harrison, R. A. & Fry, B. G. 2013. Complex cocktails: the evolutionary novelty of venoms. Trends in Ecology and Evolution 28: 219-229.

Politi, Y., Priewasser, M., Pippel, E., Zaslansky, P., Hartmann, J., Siegal, S., et al. (2012). A Spider´s Fang: How to Design an Injection Needle Using Chitin-Based Composite Material. Advanced Functional Materials , 22, 2519-2528.

Images

Image 1 - http://static.ddmcdn.com/gif/spider-fang.gif. Accessed on 21/3/14.
Image 2 - http://img198.imageshack.us/img198/6237/scorpionanatomy.gif. Accessed on 21/3/14.
Image 3 - http://www.spiderzrule.com/spiders063/Picture%20029.jpg. Accessed on 21/3/14.
Image 4 - http://img198.imageshack.us/img198/6237/scorpionanatomy.gif. Accessed on 21/3/14.