The bioinorganic fraction of the dermal granules of Molpadia intermedia from a wide variety of habitats has been studied by chemical analysis, electron microscopy, X-ray diffraction, thermal analysis, visible and infrared spectroscopy and magnetic susceptibility. It was found to have many similarities to polynuclear iron(III) proteins. Silica was found to be an integral component of the granules. The proportions of the major elements, P and Si, were found to vary with habitat as well as the minor elements, Ca and Mg, whereas the Fe content remained relatively constant. Models are proposed for the microstructure of the granules and the processes of their growth.