ATR is perfectly suited for the detection of calcium oxalate since it probes only the superficial layer where the salt is. As seen in Fig. 9, the two main diagnostic bands identified for calcium oxalate hydrate are the O-CO out of phase bonding at 779 cm− 1 and the asymmetric C=O stretching at 1315 cm− 1.[28] All the crystals observed on the cocoons from SEM pictures are calcium oxalate. The role of bonding between calcium oxalate and silk in the structural properties of cocoons requires further investigation and may open a new field on biomimetic inorganic/organic composites.