The most basic, original form of something — especially when you're talking about insects — can be described as larval. A caterpillar is the larval form of a butterfly.
In animals that take one form before metamorphosing into their adult shape, that first, immature stage is larval. The larval phase of a frog is a tadpole, and the larval form of a fly is a tiny, white, squirming maggot. You can also use this adjective in a figurative way, for the early stage of non-insects: "My novel is really still in its larval form. It's actually just a paragraph."