The moment of prey capture by Hadrurus arizonensis, the Arizona hairy scorpion, in captivity at the Harvard Museum of Natural History.
The scorpion's prey is a common house cricket (Acheta domesticus). After first using its chelae (the claws) to snatch and grip the cricket, the scorpion extended its metasoma forward and immediately subdued the struggling insect with a brief injection of venom.
All scorpions have venom, which evolved primarily for prey capture and only secondarily for self-defense, and all species except one have neurotoxic venom, a mixture of peptides, salts, and other complex molecules. The two venom-producing glands are housed in the vesicle, the bulbous part adjacent to the stinger, which is technically termed the aculeus (from Latin acus "needle").
At the moment of envenomation, venom stored in the vesicle is pumped through the aculeus along two ducts that open to the outside in a pair of elongated, slightly subterminal apertures near the tip of the stinger. While the vesicle is covered with long sensory hairs, called trichobothria, the aculeus has its own sensory structures in the form of minuscule pits containing tiny hairs. In the opinion of three researchers who recently investigated a scorpion's aculeus with a scanning electron microscope, these are probably contact chemoreceptors that perceive mechanical or chemical stimuli while the aculeus is being inserted into the body of prey. (See Rainer Foelix, Bruno Erb, and Matt Braunwalder, Fine structure of the stinger (aculeus) in Euscorpius, Journal of Arachnology, 42(1), 2014, pages 119–122. A photograph from their research is available, courtesy of Jan Ove Rein, at The Scorpion Files Newsblog.)
Scorpions sting selectively, a behavioral adaptation that helps them to conserve venom, which is biologically costly to produce. They always seize prey with the chelae first and tend to sting the victim only if its size, strength, and/or struggles make it difficult to subdue with the claws alone. While this pattern of sting use has been anecdotally known for a long time, it was not until the 1990s that the first experimental confirmation of it was made (Jan Ove Rein, Sting use in two species of Parabuthus scorpions (Buthidae), Journal of Arachnology, 21(1), 1993, pages 60–63). Other research has since further confirmed and elaborated on the behavior and biology of sting use in scorpions, including one set of experiments involving a Hadrurus species closely related to the species H. arizonensis pictured above (Martin Edmunds and Richard Sibley, Optimal sting use in the feeding behavior of the scorpion Hadrurus spadix, Journal of Arachnology, 38(1), 2010, pages 123–125).
Recent investigations have shown that some scorpions make two different types of venom and use either according to necessity: a biologically cheap low-grade pre-venom and a biologically expensive venom that is much more potent. Scorpions can also regulate the amount of venom they inject. (See Zia Nisani and William Hayes, Defensive stinging by Parabuthus transvaalicus scorpions: risk assessment and venom metering, Animal Behaviour, 81(3), 2011, pages 627–633.) Combined with selective use of the stinger, these traits let scorpions optimize their venom output in significantly beneficial ways.