I thought this model could possibly work as a possible way to get a functional system for giant arthropods, but I realized with problems when I was writing an answer to this question Are 'ambiskeletons' viable?.
The placoderms, a clade of extinct fishes in which almost the half of their body was covered by bone plates and for what I know, the soft tissue over the plates was minimum, this would allow to keep the exoskeleton-like appearence.
The placoderms (underwater) could reach bigger sizes than the biggest arhtropods like euripteryds of 2.5 meters long, with one of the biggest species having moderated estimations till to 6-7 meter long and the upper estimations with even to 10-11 meters.
Also the placoderms solved a very mentioned problem for giant arthropods of having a body with organs contained within a hard armor.
So the problems with which I realized are principally biomechanics when extrapolating to terrestrial giant arthropods:
- These were acuatic creatures, so is probable that all the extra weight provided by the armor were sustented by water, which might cause problems on land for be so heavy. So would be necesary to determine how thick it could be before reaching a limit which it falls under its own weight and also just the half or their body covered by the armor.
- The the biological mechanism by which these animals could grow within their armor without shedding as arthropods would, I have assumed that by a stacking of layers of bone dissolving into the lower ones as they grew. But I´m not how that worked and if tis possible for arthropods to use a similar mechanism.
- The "semi" part, for much than placoderms could have an exoskeleton-like look, still having a layer of girded soft tissue over the bones plates in specific plates, so to use this on arthropods, appears the doubt about if arthropods could develop some "skin zones".
- And the mechanic part, I don't know if the surface provided together by the "minimum" internal skeleton and exoskeleton would be enough to anchor a sufficient amount of muscle to be able to move on land. So, is that, to find how much surface muscles require.
Next a crude scheme with something like a giant generic insect, about how this could look, practically an internal rigid support shown in black which could be considered a spine or column as internal muscle anchor point, the column would be directly conected with the "braincase" and specific parts of the body in which some muscle and skin is over the exoekeleton together with the internal muscles to improve the strenght.
So the question is if this system is really viable and a solution for giant arthropods, considering the previous points with a biomechanical focus about the working?