Fireflies are interesting creatures that glow in dark due to biochemical reactions. They come in special season time and stay for a really short time period (probably while mating). Anyway if you enjoy the show of fireflies and would like to make them stay longer then you should consider making artificial swarm. Karl decided to experiment with simple low power solution.
The circuit is based on Attiny13a microcontroller which has 8 legs. It is powered from CR2032 coin-cell battery. Chip carries single green LED and 1MOhm resistor. This resistor is connected parallel to LED. This resistor is necessary because LED also is used as light sensor. Fireflies most of the times are in sleep mode and occasionally checks ambient light. If it is dark enough, they stat flashing. I would expect some sort of swarm behavior when fireflies detect near by pattern and so adjust flashing that would create multiple flashing patterns. Anyway this is great weekend project to do with kids.