ABSTRACT Defensive fluids obtained from the Garden Tiger, Scarlet Tiger and Burnet Moths and from the Monarch Butterfly and the Seven-Spot Ladybird Beetle, haemolymphs taken from the wasp, the Burying Beetle and the Rat Flea, and fluids from the eggs of the Garden Tiger, Gypsy Moth and Oak Eggar Moth were examined in the electron microscope after negative staining. Except for the ladybird all the defensive fluids, haemolymphs and egg fluids contained particles 100–150 Å across which frequently had rectangular or square outlines. In some species the particles were occasionally arranged in groups of 2 and 4 and in the egg fluid of the Garden Tiger they were arranged in rows which were aligned side by side to form strands of considerable length. The particles are believed to be protein. They correspond in size to the 16 s component of the haemocyanins of Limulus and Homarus and the protein particles found in Calliphora by other workers.