Calcium oxalate crystals of plant leaves pass through chewing invertebrate larvae virtually unchanged

Abstract The function of biomineralization in plants has been widely discussed. Herbivore defense is generally considered an one important function. This has been demonstrated for, e.g., needle‐shaped raphides in combination with toxic contents, widely found in families such as Araceae. For calcium oxalate (CaOx) crystals and druses (crystal aggregates), which are abundant in leaves of many eudicotyledons, herbivore deterrence has been suggested in the literature, but there is little direct evidence. We used light microscopy and micro‐computer‐tomography to illustrate the biominerals in several temperate shrubs and trees and followed their path upon consumption by caterpillars. We show that a range of different caterpillars consume leaves full of crystals and druses without any apparent harm and that the CaOx druses pass through the animals virtually unchanged. Our observations document the ineffectiveness of CaOx for herbivore deterrence. At least specialized insects are capable of overcoming these obstacles.