Some of the tiny wasps parasitise aphids, laying a single egg into their living hosts. The young wasps develop inside the aphid, where they pupate, then eventually emerge. This is a bit of mousear, with a live aphid (the little, green one on the stem) and an aphid mummy (the bloated brown one, on the leaf). You can see the 'trapdoor' at the back of the aphid mummy, where the young wasp has cut its way out.
The wasps responsible really are tiny, so we're unlikely to spot them, unless we're taking five minutes out to peruse the goings-on in some shrubbery or similar (it does happen!).