Last week we visited the Mojave Desert to meet the fascinating Blue Death-Feigning beetle. As we continue our escape from the chilly Mid-Atlantic, we travel to the Sonoran Desert in Arizona to meet the largest scorpion in North America, the Giant Hairy Desert Scorpion. I know, I know, scorpions are not insects, they are arachnids, a large and diverse group of arthropods that includes spiders and ticks. But what the heck, these critters with jointed legs and exoskeletons are close relatives of bugs and other insects, so let’s meet one of the most interesting of the clan.
Hairy Desert Scorpions, giants of the scorpion realm, can grow to be more than six inches in length. By day they usually reside in galleries excavated in the surface of the earth or beneath rocky overhangs or pieces of wood. Their subterranean galleries may reach as much as eight feet in length. By night these hunters seek insects and other small animals moving across the desert floor. On the underside of the scorpion, comb-like structures called pectine and sensors on its legs detect vibrations produced by prey moving about the desert floor. Pectine are also loaded with chemoreceptors to capture odors wafting in gentle desert breezes. These sensors enable the scorpion to detect potential victims in the dark desert night. Prey are captured with large claw-like appendages at the front of the scorpion. These appendages, called pedipalps, have movable pinchers technically known as chelae that grasp their prey and guide it into the waiting maw. At the base of the pedipalps, grinding mouthparts called chelicerae pulverize the scorpion’s victim and conduct the macerated tissue into the digestive tract.
What about that impressive stinger at the tip of the scorpion’s tail? The scorpion’s sting serves two purposes, prey capture and defense. A fascinating study of prey capture by the Giant Hairy Desert Scorpion revealed that 100% of prey captures involved use of the sting. After capturing a prey item with pedipalps, the scorpion systematically finds a place to thrust its sting into the body of the victim to administer a dose of venom. The other purpose of the scorpion’s sting is defense, primarily from hungry reptiles and mammals inhabiting the desert. How lethal is the venom of the Giant Hairy Desert Scorpion? One standard measure of toxicity of a compound is called the LD50; that is the lethal dose of a substance to kill 50% of a test population of animals (usually mice). Units are expressed as a ratio of milligrams of compound to kilogram of body weight of the test subject. One published account puts venom of the Giant Hairy Desert Scorpion at an LD50 of 198 mg of venom/kg of mouse body weight. Compare that to our friend the honeybee whose venom is some 30 times more potent at 6mg of venom/kg of mouse body weight. However, if you come across an unknown species of scorpion resist the urge to test the potency of its venom, as some are known to be lethal to humans. The Fattailed Scorpion, Androctonus australis, of the Middle East and North Africa is one of the deadliest scorpions on the planet, accounting for several deaths each year. With an LD50 estimated between 0.32 and 0.75 mg/kg, its venom is more toxic than that of many rattlesnakes. Yikes!
Repeated teasing with a paint brush finally elicited defensive stings by this Hairy Desert Scorpion. The scorpion’s sting penetrated the bristles of the brush for a surprisingly long time.
“Prey Capture by the Scorpion Hadrurus arizonensis Ewing (Scorpiones: Vaejovidae)” by Kathleen Bub and Robert F. Bowermanm, and “Fine Structure of Scorpion Pectines for Odor Capture” by Zhiwu Han, Daobing Chen, Ka Zhang, Honglie Song, Kejun Wang, Shichao Niu, Junqiu Zhang, and Luquan Ren were two fascinating articles used as references for this episode. Thanks to Chris Sargent and Todd Waters for providing the inspiration for this episode and for keeping our arthropods happy and thriving in the University of Maryland’s Insect Zoo. The following excellent web pages were also consulted in preparation of this episode: