What is the insect that "hums" in hot weather?

Do you mean a cicada? The adults live in trees. And they can hum really loudly:

"Question: How do they make that noise?

Answer: Only the males make that noise (although some females are capable of making sounds). They have drum like membranes on their abdomen that vibrate very quickly, creating the creepy tones. Every species of cicada has a unique call. The frequency and tone of their calls is related to the temperature, and the time of day."

Chirping Insect

Cicadas are one insect that often call out on hot, humid summer nights. Some of them are extremely loud (sometimes over 100 dB), with a high pitched creeky 'veep' sound. They make this sound by vibrating a drum-like membrane on the side of their abdomen.

Crickets are another noisy insect. Their chirps are created by rubbing the edge of their leg against their wing, making the wing vibrate almost like a violin string. The frequency of their chirps is often dependent on the ambient air temperature, and with some species you can determine the exact temperature by counting the chirps in fifteen seconds, then add 40 and you get the temperature in Fahrenheit.

I suspect that your harpsichordy bug is a cicada. Here is a site by Joanna Philips on cicadas http://www.bookmice.net/fleur/cicadas.html and a page that includes her recordings of cicadas she has heard http://www.bookmice.net/fleur/cicadasound.html

Here's another site with the sound of a single cicada http://www.chat11.com/Cicada_Sound

Those are Cicadas (sic ka duh)

The sound you hear comes from the males. They rub their abdomens and produce that sound. The frequency of the sound varies by species, time of day and temperature.

Mosquitoes

They are cicadas, and I have noticed that they have unique sounds in different parts of the country. Here are some cicada recipes:

http://biology.clc.uc.edu/steincarter/recipes.htm

Can you believe it??!!

It is the cicadas, which are mistakenly call locust.....the locust is more like a grasshopper....the noise is produced by the male.

Locusts.

I think the dragonfly does this..

Peace