Seeders are fairly large, six-legged beasts that fire Nemacyst spores out of a second mouth on their rear end. They have also been known to jam radio transmissions with their very presence. You can see a Seeder during Act 1, but you have to look carefully.
Seeders are perhaps the most bizarre creatures of the Locust Horde . Each of these living cannons is literally fed its own ammunition. The mouth on its front is the creature's backside. From concept arts, a Drone is in the same hole with the Seeder and has several Nemacyst with him. He shoves them in the Seeder's mouth where it goes through the body immediately out of its other mouth.
The Locusts use Seeders as a sort of living artillery cannon. They can launch Nemacysts as mortars at long range and also launch Nemacysts at close range to jam radio transmissions. When in battle, Seeders remain stationary with the vulnerable front end of their bodies under the ground and their armored rears exposed. Aiming is unnecessary as Nemacysts can seek out their targets once in the air. In some instances, a Seeder will pull its entire body out of the ground when injured or panicked.
The link between Nemacysts and Seeders is confusing, neither is seen without the other, meaning Seeders are needed to launch or produce them. The link between the radio jamming abilities is even more confusing, though it could be the Seeders or the Nemacysts using their ink to jam the transmissions. Nemacysts are used as anti-air, and air-to-ground attack units. The are also apparently used as Locust artillery.