The blood-sucking tsetse fly, which spreads sleeping sickness between humans, also provisions its young with microbes, but does so inside its own body. It’s an insect that’s trying very hard to be a mammal. Rather than laying eggs, it gives birth to live young. And rather than hedging its bets with a horde of offspring, it devotes its energies to a single grub, which it raises inside a uterus and feeds with a milk-like fluid.
