Plate from 18th century encyclopedia showing illustrations of aquatic worms and flies, and catapillars. Aquatic worms, just like earthworms, are hermaphrodites. This means there's not really a male or female. When aquatic worms mate, each worm is then able to lay eggs. Aquatic worms lay eggs inside small cocoons in the mud, on rocks, or on plants. Eggs are usually laid in late Summer or early Fall. The subphylum Hexapoda (from the Greek for six legs) constitutes the largest (in terms of number of species) grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods.