3.3. FTIR test
Fourier transform infrared spectroscopy (FTIR) was used to identify the non-protein inorganic crystals on the cocoons found in SEM. All infrared spectra were recorded on a Nicolet 6700 FTIR spectrometer coupled with a single bounce diamond attenuated total reflectance (ATR) accessory. Data were collected from 500 to 6000 cm− 1 with a resolution of 4 cm− 1. The spectra were recorded using a liquid nitrogen cooled mercury cadmium telluride detector. 64 spectra were co-added for each measurement. The cocoons were punched into round pieces of 3.5 mm diameter and then pressed on the diamond with a high pressure. At least three samples from three different cocoons of each species were used for collecting the spectrum. The spectrum of calcium oxalate hydrate purchased from Sigma-Aldrich was also collected for comparison.