The stiltleg sunworm came from the extremely rare mating of a lobed solarworm and a sunworm. The resulting creature then moved towards the Flisch Marsh in large swarms, and there it stayed... and changed.
The stiltleg sunworm has gained many new changes. For one, three pairs of its nine legs have grown larger, while the others are now so small they are no longer used. (small legs are behind large ones in picture, as such they are not shown).
Another new change is the wings. Despite the odd look of them, their bizarre pattern has a point to it- The darker parts are the parts where photosynthesis occur, while the lighter parts are translucent, so as to let the younger or smaller stiltleg sunworms survive better.
That brings up yet another adaption. When the original hybrids moved to the Marsh, they came in large numbers, and stayed in large numbers. Because the marsh is uninhabited, it means that their only source of food is the poop of the stiltleg sunworms closest to them. (hence the long "mouth") The translucent parts of the wings allow the young or small members of their enormous group to get sunlight, thus energy, thus waste, which the other stiltleg sunworms will eat!
The mating process is no longer possible, seeing as how the worms will barely move their entire lives aside from moving their wing up or down (depending on position of the sun) and moving their mouth. As such, the mating actually occurs in a larval stage! When the worm is born, its legs are not yet grown, so it has quite a bit more agility than its parent. It will immediately scuttle off to find a mate. After the mating is finished, they will go eat their first meal, dig their legs in the ground, and start growing. After they are fully grown, they will lay their eggs in the mud, and the process will start all over again!