Of all insect larvae I have come across, these have been the trickiest to spot. Red Lily Beetle adults were mating and laying eggs on my Asiatic lilies this afternoon. I was sitting next to the lilies and I was intent on spotting the larvae. It took a while. They look exactly like wet caterpillar poo.
The bright orange eggs are laid in rows under the leaf, and when the larvae hatch they stay together and start eating the leaf where they have been born, their slimy droppings are deposited on their back, forming a faecal shield which grows in size with the larvae. The faecal shield disguises the larva, presumably helping to protect them from predators. When small, larvae fed on the underside of the leaf, leaving the upper leaf surface withered. As they grow they disperse from their siblings and they may move to the upper side of the leaf, eating the whole leaf or flowers as they grow. If they feel disturbed, they completely retreat inside their fecal shield, when they relax, their shiny black head and short legs are visible (above). The fully developed larvae drop to the soil where they make a white cocoon. There is a single generation a year, the new generation of beetles emerges and feeds before overwintering in the soil.
A string of lily beetle eggs
A cluster of young larvae - the leaf has been turned upside down to take the photo. The withered end of the leaf can be seen in the background