As humans we cannot vomit on attackers as a form of defense. Well, most people surely cannot anyway. However, vomiting as a form of defense during enemy attacks is common in the insect world. For example, caterpillars of the small mottled willow moth will not hesitate to projectile vomit on their hostile enemies. These caterpillars are also known as beet armyworms. Many caterpillars possess defensive hairs that are usually quite effective at fending off enemies. However, the beet armyworm covers enemies in a special vomit that deters enemy attackers. A variety of different ant species are the most common predators to beet armyworms. The beet armyworm vomit is particularly bothersome to ants, but it only deters them from attacking, it does not kill them. This vomit could be thought of as a sort of pepper spray used by beet armyworms.
When a small number of ants approach one or more beet armyworms, the armyworms will promptly vomit their digestive contents onto the nearby ants. The compounds in the armyworm vomit cause ants to frantically clean their heads, at which point their plan of attack fails. The compounds that are contained within beet armyworm vomit act as surfactants. For example, the vomit envelops over ant bodies, including their heads. In order to prevent death by drowning, the ants must frantically clean themselves of the digestive fluids. When the vomit covered ants become preoccupied with cleaning themselves of vomit, beet armyworms make their getaway.
Although this method has proven to be quite effective against ant attacks in laboratory settings, the vomit will obviously not suffice to protect the armyworms from large numbers of invading ants. However, the vomit does protect beet armyworms from a number of different solitary insects. Upon analyzing the caterpillar vomit, scientists found that it acts as a surfactant, which is a fluid that covers an entire object on contact, as opposed to simply dripping off like water. Insects make frantic efforts to remove this surfactant substance from their bodies after being attacked by beet armyworms. Surprisingly, researchers found that beet armyworm vomit acts as a surfactant no matter what the caterpillars eat. This is the first time an insect’s bodily fluid was found to act as a surfactant. But researchers believe that the method of vomiting surfactant substances as a from of defense may be widespread among insects.
Do you believe that surfactants would deter spiders from attacking caterpillars?