Whoa, I didn't foresee that happening! Nice to see you are enjoying my little game :)
Yeah, I really didn't communicate finer details of the simulation anywhere, so here's a small rundown of these mechanics:
A bug dies if it's health reaches 0. Health can decrease from lack of oxygen or food, or when attacked. If the bug has enough food, it will recover. At old age bugs can no longer regain health so having so old bug is amazing!
A bug will reproduce if it wants to and has enough food and have reached at least half of their final mass (maturity). They reproduce by splitting, no partner needed. Notice that some bugs might simply not want to make offspring, while others will do at any chance.
FINGER state means that the bug is attracted to your mouse cursor. In FLEE state they also might run away from cursor. I added this as a small way to play with them and also you might be able to guide them towards food or prey. I imagine it being like your finger touching aquarium glass, the fish sometimes come look at it (or get scared away).
In the end, with multiple mechanics it's no wonder that the bugs may fail, since their desires are randomized. If a bug doesn't want to eat when hungry or picks fights when at low health or just priorizes to play with cursor it gets extinct pretty soon. Or, as you had it, you might have the best bug in the world, but it doesn't want to reproduce so no continuation in that line also.
There might be some hidden condition which triggers some behaviour, for example low/high food, health or oxygen, or amount of other bugs in vicinity. That might explain why your bug acted violently when there was multitude of other bugs around, but let a single bug live. Or it might connect to lower oxygen levels caused by overpopulation instead, who knows :)
Wow, this is actually very well thought out. Thanks for the game!
An update for my best bug ever: I recreate the terrarium with his gene, his offsprings survive this time. Because of his high mutation (76?) his offspring are very different. They all have very long life just like him, and rarely reproduce.