Evolutionary zinger: Bats, moths and mites
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 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 head 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.
"Newly arrived in the ear of her choice, the female D. phalaenodectes does not immediately set up housekeeping. During the first hour or so she returns from time to time to the midpoint or 'crossroad' between the two ears, pausing there on each visit to probe again in various directions and then returning to the ear first entered. The significance of these back trips can perhaps be surmised , but it has not been fully clarified. Is she laying a trail or posting a sign of some kind to guide the next mite to come? I think that she is, although I cannot read the trail or find the sign myself. In any event, the next traveler and all that follow will normally take the same route as the first mite and with little hesitation at the midpoint will lodge with her in the same inn, never visiting the empty tavern across the street. Yet if mites are persistently placed and replaced in that opposite ear, they will sometimes remain there and establish a second normal colony. Very rarely a bilateral infestation is found in nature."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.-Treat, Asher, An earful of mites, Insect Lives, p. 273
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.