I started work on a grub form the Sunday after I returned from my break, today I made considerable progress on it, crafting the rest of the body, adding definition and then rigging and skinning. As it is quite a simple shape compared to the other forms, the skinning process did not take long.
Aside from the legs, already the grub is about ready for controls. Like with the juvenile, it will require a spline IK in the spine, as it is quite a flexible creature and attempts at bones serving two segments did not work out particularly well.
It might need pose space corrections to give solidity to the segments. So when the segments compress they push upwards for that fatty "squidgyness" the grub will have.
I have considered that it might be best if there would only be one grub due to time constraints, There's no drastic change for the two considered forms apart from maybe the grub getting fatter, which could be achieved either with a blendshape or controlling the scale of certain bones. Or the talk of scale mentioned in one scene could simply be represented by the camera looking at what is simply a bigger grub in the tank. David's shock could easily come from simply how gigantic a little bug like this gets - growing from 5-10cm to almost a metre across the months it is studied.