Structures can vary both in melanosome type and melanosome organization. There are five main types of melanosomes (shown as schematics in bottom row, each viewed from the side and in cross-section (A–E)) and two main types of structural organization (shown by microscope images of barbule cross-sections, middle row: single-layered (A) and photonic crystal (B–E)). A single-layered structure with thick solid rods (A) gives rise to the dark, black-blue iridescence of a brown-headed cowbird (Molothrus ater). This type of structure generally gives rise to ‘weak’ iridescent colors, with low color saturation and brightness. Photonic crystals (B–E) with multiple layers of melanosomes generally give rise to ‘brilliant’ iridescent colors, with high saturation and brightness. Thin solid rods (B) in a multilayer configuration (also called a one-dimensional photonic crystal) produce the iridescent colors of the Nicobar pigeon (Caloenas nicobarica). In the elegant trogon (Trogon elegans), the iridescent green color is produced by hexagonally packed hollow rods (C). Sunbird (here the variable sunbird, Cinnyris venustus) barbules contain melanosomes stacked in multilayers, with solid platelet-shaped melanosomes serving as the building blocks (D). The fifth melanosome type is a hollow platelet (E), which forms multilayer configurations in many hummingbird species (here a ruby-throated hummingbird, Archilochus colubris). The five types of melanosomes are characterized by different combinations of three key modifications: thin melanin layers, hollowness, and platelet shape, which are indicated as symbols under each melanosome type. Thin melanin layers are present in four melanosome types, but they are achieved in different ways, as is shown by the schematic in (F). A simplified diagram of each melanosome type (F, right) shows how solid forms translate to a single melanin layer, while hollow forms create two thinner melanin layers intersected by an air layer. All photographs (top row) are under a Public Domain License (details in Supplementary file 1n).
© 1977, Durrer et al. Transmission Electron Microscope images from Durrer, 1977, reproduced with permission. It is not covered by the CC-BY 4.0 licence and further reproduction of this panel would need permission from the copyright holder.