Education, Science.

Evolutionary zinger: The bat, the moth, and the mite

When Fred Rickson taught his section of General Biology at Oregon State, I made sure to attend all of his lectures, as he opened them with his evolutionary zingers, hoping that his students would would be enticed to be more prompt than usual.  My favorite was his zinger about the three-way symbiotic relationship between a bat, a moth, and a mite.

The bat, the moth, and the mite

Some species of bats feed on, among other things, certain noctuid, or night-flying owlet moths. The bat uses its sonar echolocation to detect the moth flying in its vicinity, and homes in and snatches it out of the air into its mouth. Every stable predator-prey population requires that the prey have some means of escaping the predator, just not all of the time. In this instance, the moth can hear the bat’s sonar signal, and when it hears the signal, immediately folds its wings and drops out of the air, or begins flying erratically. Most of the time, the moth evades the bat; sometimes, it is captured and consumed.

Some of these noctuid moths have a symbiotic relationship with a mite of the Dicrocheles phalaenodectes species, which colonizes in the moth’s ears. But when the mite sets up house in the moth’s ear, when depositing its larvae, it destroys the tympanic membrane which is part of the moth’s mechanism to detect a bat’s echolocation signal, rendering the host moth unable to detect a predator bat. This is obviously a deadly outcome for both the host moth and the mite colony. But fortunately for them both, the mite colonizes only one of the moth’s ear, leaving the other free to listen for the bat, which is sufficient for the moth, and its mite colony, to evade the bat as before.

How does the mite make sure only one ear is colonized? It has been surmised, from good circumstantial observational evidence, that pheromone trails are laid around the uncolonized ear, which lead to the colonized ear. Any mite landing on the lead of a colonized moth will follow the pheromone trail to the colonized ear. Also, once the mites establish a colony in the moth’s ear, scouts are sent out to check the other ear, and ensure that no other mites are there. If they find mites, they lead them to the other ear, evacuating the “good” ear. These mite scouts also regularly refresh the pheromone trail.

After first hearing this story, my immediate question was: How the heck did someone figure all of this out? Particularly the bit about the mites sending scouts and leaving chemical breadcrumb trails? More of the story can be elucidated by following the links in the Related Articles section. 

Many other ear mites colonize moths, and are found in both ears, but do not destroy the tympanic membrane, leaving the moth’s echolocation detection intact.  There is one population of Dicrocheles ear mites that colonize both ears, destroying both tympanic membranes: the moths and mites survive because the local bat population has been lost.

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