The Order Harpellales is a group of fungal-like protists within the class Harpellomycetes, primarily known as gut symbionts of arthropods, especially insects like mosquitoes, midges, and black flies. Members of this order are obligate commensals or mutualists, inhabiting the digestive tracts of their hosts and helping break down nutrients. They reproduce sporangiospores and often attach to the gut lining via specialized holdfasts. Harpellales are ecologically significant because they influence the nutrition, development, and population dynamics of their insect hosts, and they serve as interesting models for studying host–microbe interactions.