Awesome, thanks everyone for the advice!
I do not want to introduce any light other than the green LEDs. I could cover the light bulb up with black paint perhaps. Great idea about the temp sensor - this could be used to introduce some variability, I will experiment! I will be using a black enclosure and ambient temps here stay well below 38C (100F) A small flash light bulb's heat would certainly be visible enough through IR. SO, heated area isn't too important, I just need visibility (imagine if you were looking at a human body through FLIR but scale this down to insect size - if that makes sense).
As this is the first iteration of this project, I went ahead and bought this.
The Pro Micro - 5V/16MHz: https://www.sparkfun.com/products/12640
The input voltage is 5 - 12V (as opposed to 7 - 12V for the UNO), so I should be fine with my 6V battery, I think, I hope. The number of inputs and outputs will suffice for now (gonna hold off on those fans). Thanks Paul, for the advice on voltage. I noticed if I didn't put resistors in my 555 timer circuit (when I swapped the speaker out for an LED), the LED glowed yellow when it wasn't green and got quite hot (hmm).
The main thing I want is variation of all of these outputs. Maybe one speaker will be making a simple heart beat (square wave), while the others are doing something else (varied beats, no music necessary, although good idea - perhaps I should look into a recording to fit my needs). In the mean time, the LEDs will be alternating slowly (every 5 minutes). Occasionally, all of the LEDs will turn on (need to generate varied shadows).
My first go at this was a simple 555 timer circuit and a tiny speaker from radio shack. Here is where I got the idea:
It turned out perfect! A nice "thump, thump". But, I need some variation (like I said). A vibration here, vibration there. Change in beat, etc.
Shouldn't be a problem to reproduce this with an arduino I figured.
So I received all of my hardware: a ton of green LEDs (bright and diffused), a photocell, a tiny transducer (good for making vibrations), a couple of tiny speakers (8 Ohm, .1W), my arduino compatible Pro Micro (SparkFun) and a few jumpers and such
If you can guess what the purpose of this project is, you're pretty cool! Again, thanks everyone - you're giving me some great ideas. I'm quite excited to tinker with this all night.