Thaindian discusses new findings that indicate Brazilian (Forelius pusillus) ants may have a form of self-sacrifice to ensure the general welfare of their colony:

The authors said that given the typical nest populations of 100,000, the sacrifice of up to eight workers a night is worth paying for the security of the nest.
Tofilski said that the ants are probably older workers approaching death who are more dispensable than their younger nest mates, and so are programmed to perform riskier tasks as they age.