Using the tree to understand history

Predicting ancestral characters
Knowing the characters of the organisms at the tips of a phylogeny lets you figure out what the ancestral characters were likely to have been. For example, modern flies and moths have silk glands but beetles and wasps don't.

The simplest explanation is that silk glands evolved only once in the ancestor of flies and moths.

That means that when we imagine the most recent common ancestor of flies and moths (the ancestor at the node between them), we should imagine an insect with silk glands.