Does a mosquito's hum alternate between D and F?
Dear Straight Dope:
My mother is a music teacher with almost perfect pitch. She can hear and point out the musical notes in car horns, door bells, etc. She swears that mosquitoes alternate between buzzing two notes (I believe she said D and F) as they fly. Has any research been conducted on this matter, or on the tone of any bug's buzzings? Please advise me on this matter.
There's actually a lot of work on wingbeat frequencies in insects, though many mosquitoes are a special case, since males locate females by homing in on the sound. In most insects, the frequency has no particular significance, and is related to all sorts of factors leading to substantial variation between and within species. Mosquitoes are under selective pressure not to vary their wingbeat, but some variation is inevitable--a female that has an empty stomach does not have the same problems generating lift that a fully engorged female has, and may not have the same wingbeat frequency. Bear in mind that if wingbeat frequency is used in mating, then no two species occurring in the same time and place should have the same frequency, or they risk wasting time and energy--or worse, hybridizing.
I would be surprised if the two notes your mother heard were generated by the same mosquitoes. More likely she's hearing individuals of two different species, one that hums in D, and the other in F. It would be odd for an individual mosquito to fluctuate its pitch to that degree, unless it were some sort of peculiar courtship signal.
The pitch may also vary if a mosquito is flying in a special pattern. I recall some insects that will "pulse" as they either fly along slowly or attempt to hover. That is, they'll stay at one frequency while slowly descending, and then boost the frequency and move quickly back up to the starting position, and so on indefinitely.