First, the compositions of the the food and of the hemolymph are remarkably different in many cases. Second, and more important, the differences are not uniform across the range of individual ions. Here are examples. In this insect sodium is much higher in the hemolymph than in the food, while potassium is higher in the food than in the hemolymph. A different situation obtains in the stick insect Dixippus , which often eats Privet leaves.
Privet comes from shrubs in the olive family.
Here, sodium, potassium and calcium are much higher in the food while magnesium is much lower. Finally, in the Colorado potato beetle Leptinotarsa , there is very little sodium in the common food while potassium and calcium are much higher in the food than in the hemolymph. A clear example of this would be the large change in hemolymph calcium ions following a typical meal in the stick insect. One mechanism to regulate ion concentrations occurs at the level of the alimentary canal. The insect would simply limit the absorption of unnecessary salts.