In The Netherlands both species have isolated populations. I have been beating caterpillars of both species last weekend (over 100 of each) and to my surprise there is absolutely no difference in the appearance of the caterpillars. The differences described in BW are nonsense both species are extremely variable in coloration (blueish, green, bright green, brown and more or less well marked).
Remarkable is the fact that only 4 brown variations of rupicapraria were beaten and 2 of primaria, all others were green. Also interesting is the fact that rupicapraria seems to prefer Crataegus as foodplant and primaria Prunus. Well on both location both foodplants are present in almost equal numbers. Also remarkable is the fact that rupicapraria seems to prefer older more mature foodplants whereas primaria prefers the younger plants.
I wonder if anyone has the same or other experiences with these two species or knows characteristics to separate the caterpillars by appearance.
All the best,