The Schmidt sting pain index is a pain scale rating the relative pain caused by different hymenopteran stings. It is mainly the work of Justin O. Schmidt, who was an entomologist at the Carl Hayden Bee Research Center in Arizona.
Schmidt's original 1983 paper was a way to systematize and compare the hemolytic properties of insect venoms. A table in the paper included a column that rated sting pain, starting from 0 for stings that are completely ineffective against humans, progressing through 2 for familiar pains such as those caused by common bee or wasp stings, and finishing at 4 for the most painful stings. Only the bullet ant, Paraponera clavata, was given a rating of 4, although later versions of the index added two more species.
Schmidt repeatedly refined his scale, including a paper published in 1990, which classifies the stings of 78 species and 41 genera of Hymenoptera, and culminating in a book published in 2016.
The Schmidt sting pain index arose from the pursuit of a larger hypothesis: that the evolution of sociality in Hymenoptera was dependent on the evolution of venom that was both painful and toxic. Pain is a signal of damage in the body, but molecules that produce pain are not the same as toxic molecules, which actually cause damage. Although the painful signal acts as a deterrent, intelligent predators learn the dishonesty of this signal with repeated exposure. For the early Hymenoptera that were primarily solitary, the pain alone would allow them the chance to escape. Moreover, solitary insects do not provide a high energy reward for predators, and therefore predators do not expend significant effort to hunt them.
However, colonies of Hymenoptera are a nutritionally rich target. If there were no defenses, predators would devour the defenseless society, leaving few surviving individuals. Therefore it was theorized that the development of toxicity in Hymenoptera facilitated the development of sociality, which brings various benefits, including the shared raising of youth, individual task specialization, inter-colony communication, and food storage. Assays for toxicity were already well characterized, so the evolutionary connection between toxicity and sociality could be studied. Schmidt developed his pain index to relate the amount of sociality to the level of pain.
Schmidt's pain scale of Hymenopteran stings is organized into levels, ranging between 1 and 4, with 4 being the most painful. However, insect stings that feel very different can be put into the same level. Thus, later versions of the scale always include a brief description of his experience being stung by each type of insect.
While Schmidt's published scientific papers use a 1 through 4 number scale, an entry in The Straight Dope reported that "implausibly exact numbers" such as "bullhorn acacia ant at 1.8" were "wheedled out of him" by Outside magazine for an article it published in 1996.
In September 2015, Schmidt and Michael L. Smith were co-awarded the Ig Nobel Physiology and Entomology prize for their Hymenoptera research.
The Starr sting pain scale is based on the Schmidt index.