Regarding the calcium content of butterworms, I would say that it is unproven. The figures widely quoted may simply be misinformation. Every website that gives information about the nutrition of butterworms lists the following:
Moisture 58.54%
Ash 1.04%
Protein 16.20%
Fat 5.21%
Calories/Fat 87.73
Calcium (ml/100 grs) 42.90
At first glance, this looks like an official nutritional analysis, but a lot of this is just rubbish! Here is why it is rubbish, in my opinion:
(1) There is no such thing as "calories/fat". If we assume that they mean "Kcal/g fat", then this calculates out to about 11 Kcal/g of dry weight, which is completely out of line with other insects. So either the units are wrong or the value is wrong.
(2) The unit for the calcium (ml/100g) is also total boloney. This isn't a valid unit for measuring ANYTHING. It's possible that the correct units might be g/100g, making the calcium 42.9%. This just cannot be right.
(3) There is no information about phosphorus content. Without knowing the Ca
ratio, there is no way to say if butterworms are a high-calcium feeder or not.
(4) Regarding fat, if the butterworm is 5.2% fat by total weight, and moisture is 58.5%, then the fat by dry weight would be about 12.5%. This is much lower than waxworms (46.4%) and a bit lower than crickets (13.8%). Frankly, it is so low that I question whether the analysis is really correct. The purpose of a grub is to store energy in preparation for metamorphosis; this energy is normally stored as fat. Can we really believe that the butterworm has a lower % fat than a cricket?
For additional comparisons of fat and calcium values, see:
Caudata Culture Articles - Nutritional Values
I didn't include butterworms in the nutritional analysis on that page because I couldn't find any analysis for them, other than the oft-quoted one, which I think is highly suspect.