In almost all metazoa, the pro-germline cells get segregated from other stem cells at an early stage of development and thrive and differentiate in their neighborhood. This is important in order to preserve the germline. This [post](http://biology.stackexchange.com/questions/8016/how-are-new-people-created-from-the-dna-of-an-aged-person-i-e-why-are-we-young/8017#8017) provides some basic explanation.

However, even drosophila have adult multipotent stem cells and help in the formation of midgut as reported by [this](http://www.nature.com/nature/journal/v439/n7075/abs/nature04333.html) study.