Venom Metering for defense

In my first blog I touched a little about how metabolic cost can be a restraint for the use of venom. In this blog I am going to go into further detail about how certain species use venom metering to not exceed their metabolic limits for defense. Venom metering is the use of venom in the most economically way possible for both defensive and predatory actions (Hayes et al. 2014).  In many venomous species it is believed that due to the high metabolic rate many species will deliver a 'dry' bite, which is a bite that contains no venom, to defend itself (Hayes et al. 2014). This is a behavioural adaptation to conserve venom for prey and high risk situations. In my next article I will talk about venom metering for offense.
It was shown in the Western Black Widow (Latrodectus hesperus) that it uses venom metering in defense. Hayes, Kelln, and Nelson (2014) found that spiders in low threat levels many spiders either tried to play dead or escape and in high threat levels would bite 60% of the time. It was also found that a majority of single bites when provoked were 'dry' bites. When pinched on the body the Black Widow would deliver 1.8-fold more venom in a bite then if pinched on the legs (Hayes et al. 2014). This shows that the Black Widow is metering its amount of venom due to the higher risk of danger to the body then legs.
Another study on the Dark scorpion (Parabuthus transvaalicus) found that in low threat situations the scorpion would deliver 'dry' stings, but also found that if the threat persisted it was more likely to envenomate (Hayes and Nisani 2011). When in a high threat situation the scorpion would produce 2-fold more venom per sting then during a low threat (Hayes and Nisani 2011). Some scorpion species also has an extra ability in being able to produce a pre-venom. The study found that scorpions with low threat situations delivered the pre-venom first and if the threat persisted it would switch to its other venom, but during a high threat situation the scorpion would skip the pre-venom and use its main venom (Hayes and Nisani 2011). This is due to the pre-venom being less energetic costly then its main protein-rich venom.

Hayes, W. Kelln, W. Nelsen, D. (2014) 'Poke but don't pinch: risk assessment and venom metering in the Western Black Widow spider, Latrodectus hesperus'. Animal Behaviour, Vol. 89, Pgs. 107-114. 

Hayes, W. Nisani, Z. (2011) 'Defensive stinging by Parabathus transvaalicus scorpions: risk assessment and venom metering'. Animal Behaviour, Vol. 81, Iss. 3, Pgs. 622-633.