The size of the cricket and leaving it there could definitely be the issue. What would you suggest I do in the meantime if he is this malnourished? Have no where to get pin head crickets, but I did place a small cut piece of meal worm in there. Do you think he will eat this? If not, I do not know what else I can do to make him eat and see if this is the issue.I'm not believing that conclusion. Why ever would you get the idea that the ICU is working? Because you put it in an ICU and it seems to you it may be doing better? Ever heard of confirmation bias? Ever considered that your dying sling may just be desperately trying to get out of that thing and therefore be moving more? But that's not even it, that's just a minor point. Main point is, you put a spider in a highly unhealthy environment. What do you think it will get out of it? Moisture? You could have done that better in it's enclosure just by moistening the substrate, but from what you are saying I see no reason at all to conclude that lack of moisture was the problem to begin with.
In another thread you posted that said sling hasn't been eating for 5 months except for 2 crickets (that were actually seriously oversized if your pic is anything to go by) and hasn't molted. Then you come home one day and it looks very thin and the enclosure happens to be dry and you conclude 'dehydration'.
Other, more likely scenarios:
- The sling got attacked by one of the comparably huge crickets you left in there and sustained a microscopic, but nevertheless serious, bite wound.
- The sling is seriously malnourished and weakened because it just couldn't tackle the oversized crickets. That would explain why it couldn't molt either. This seems actually very likely.
An ICU, especially one as ridiculously wet as yours, is generally a good way to kill a spider. If you really wanted to give it water you could always drop something on his fangs. It needs to drink, not soak.