The order Uropygi (Thelyphonida) contains animals that are essentially really buff scorpions with whips on their butts instead of stinger tipped tails. They are often called whip scorpions for this reason. Another popular name for uropygids is vinegaroon because they use this whip to spray a mixture of acetic acid (vinegar) and other compounds when threatened or harmed. Their bodies are bulky and well armored, with powerful crushing structures on their pedipalps. These spine-like protrusions are used to capture and hold prey before tearing it apart. Similar to amblypygids, their elongated first pair of walking legs are held out in front to help with navigation. All of these features can make vinegaroons appear larger and scarier than they actually are, at least to us. The largest species in the world, Mastigoproctus giganteus, only reaches a maximum length of 3.3 inches, which is not huge compared to other arachnids.
Fearsome as they may look, vinegaroons fit with our running theme of tender arachnid sex. When it’s time to make babies, the male gently grasps the female’s antenniform legs and then turns around so that they are both facing the same direction. He deposits a spermatophore on the ground and then grabs it, turns around again, and places it into the female during an abdominal embrace. She can then fertilize her eggs and will lay them within a few months. To do this, she digs a burrow and seals herself inside. Eggs are laid into a sac on the underside of her abdomen, which she holds aloft so as not to drag the eggs on the ground when moving. She does not eat during this time, which is impressive because it can take several months for the eggs to develop and hatch. Newly hatched vinegaroons ride on their mother’s back until their first molt.
Most species from this order inhabit tropical regions, but a few, like M. giganteus, live in arid desert environments like many of their true scorpion cousins. M giganteus can be found in the American Southwest and Mexico where it serves an important role as a predator of pests such as cockroaches and crickets. This vinegaroon was the subject in a study by Schmidt et al. (2000) to better understand the composition of the spray. It was often believed that vinegaroon secretions contain formic acid in addition to acetic acid, but this is false. The study analyzed the fluid within the pygidial glands of vinegaroons of different ages and sexes and found that none contained formic acid. The major components of vinegaroon spray are acetic acid, which gives it the strong vinegar smell, octanoic acid, and water. The purpose of this fluid cocktail was also reevaluated in the study. It was only ever used in self-defense, and even then only if that vinegaroon was directly attacked. It appears as though predators are not deterred by being sprayed inside the mouth. However, any contact with sensory tissue such as the eyes, skin, or an arthropod’s feelers can be irritating enough to put off the attacker. In one particular trial, the researchers placed a sulfugid (camel spider) in with a first instar (between first and second molt) vinegaroon and it got a face full of spray. After that, it ran around the cage frantically trying to clean its face with sand and refused to touch another vinegaroon after that. Adult vinegaroons have little to fear, for they are powerful predators themselves and have tough armor to protect them. In the event that they are attacked, though, the spray is an effective way of letting the predator know to back off.
These animals demonstrate that you don’t have to be big, aggressive, or venomous to make it in this world. When faced with danger, you don’t have to fight. Just be really irritating and maybe you’ll be left alone!
1. Schmidt, Justin O., et al. “Chemistry, ontogeny, and role of pygidial gland secretions of the vinegaroon Mastigoproctus giganteus (Arachnida: Uropygi).” Journal of insect physiology 46.4 (2000): 443-450.
2. Rowland, J. Mark, and John AL Cooke. “Systematics of the arachnid order Uropygida (= Thelyphonida).” Journal of Arachnology (1973): 55-71.
3. Harvey, Mark S. Catalogue of the smaller arachnid orders of the World: Amblypygi, Uropygi, Schizomida, Palpigradi, Ricinulei and Solifugae. CSIRO publishing, 2003.
4. Harvey, Mark S. “The neglected cousins: what do we know about the smaller arachnid orders?.” Journal of Arachnology 30.2 (2002): 357-372.
5. Pocock, R. I. “Arachnida.” The Fauna of British India, including Ceylon and Burma. London: Taylor and Francis, 1900. 100-131.
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