My submition for :user_worldcraft:

The nestfly spends most of its life under water in its larval form. This form lives in very shallow waters near the waters edge, and moves along the sand using its two large front claws. It has four other legs, which are unable to move, and are not part of the adult. Their function is currently unknown, but believed to have some part in the creatures ability to detect prey. They eat tiny creatures that live in sand.

The adult looks much different from the larva. It has no legs, large eyes, a long proboscis, four wings and a tail that splits into two. On its back is a large hollow, which contains the flys eggs. These nests can contain up to 100 eggs. The adult does not eat as it has no internal organs suited for digestion, but does attack other animals, using its proboscis to suck up their fluids and pump them into its nest, giving the eggs nutrition. Once it has starved to death, it locks itself to a small ridge or rock in the sun. Once the eggs are ready to hatch, they all expand at once, ripping open the parents nest with explosive force, propelling the young into the waters.