There is no oxygen inside the Welif. Do you really think you'll survive long enough to get to anything major? You can't evolve either, you'll suffocate during the process. If the centipede can't swim, the adventure is already over. You may as well try to get to the shore as quickly as possible.
Because there's been some confusion on this point, let me explain for those who are confused: Insects breathe out of spiracles, small openings in their skin. These use the same oxygen combustion process as your lungs. If you're submerged, or your skin is blocked (by heavy rain, being surrounded by guts, etc) you will suffocate. You've already put yourselves in an extremely dangerous situation by attacking the Welif and not simply letting it carry you to new sources of food. With luck, you haven't killed yourselves already - get out of there.
Also, your number one evolutionary priority should be to get a dedicated stomach, mouth and digestive tract. That's why your "metabolic efficiency" is still low - you're digesting inefficiently through the skin on your underside. After that, you need tougher skin. A single scratch to your vulnerable underside will kill you - you won't survive contact with a rat that knows you're there. You cannot hunt while in that vulnerable condition.
Once you have a mouth, you can make it an appendage that can cause damage. Then we can actually fight.
In terms of planning for the future, remember that we do not have specialized organs. We have, at best, clusters of specialized nerves and tissues - digestive tissues, protective tissues, metabolic tissues. If we want to get increased capabilities, we need subsystems that specialize in a certain function. A stomach. Lungs. A mouth. Tough skin. A liver (for dealing with disease). A circulatory system that isn't based on primitive methods insects use (haemolymph). A reproductive system, preferably based off of replication, as we don't know if there are any others of our kind. We have eyes, which are the best sensors we could have, but we could use other specialized senses as well. Take the rat's sense of smell when you have the chance.