Crithidia is a genus of trypanosomatid Euglenozoa. They are parasites that exclusively parasitise arthropods, mainly insects. They pass from host to host as cysts in infective faeces and typically, the parasites develop in the digestive tracts of insects and interact with the intestinal epithelium using their flagellum. They display very low host-specificity and a single parasite can infect a large range of invertebrate hosts. At different points in its life-cycle, it passes through amastigote, promastigote, and epimastigote phases; the last is particularly characteristic, and similar stages in other trypanosomes are often called crithidial.

Bombus muscorum, commonly known as the large carder bee or moss carder bee, is a species of bumblebee in the family Apidae. The species is found throughout Eurasia in fragmented populations, but is most commonly found in the British Isles. B. muscorum is a eusocial insect. The queen is monandrous, mating with only one male after leaving a mature nest to found its own. Males mate territorially and the species is susceptible to inbreeding and bottlenecks. The species builds its nests on or just under the ground in open grassland and forages very close to the nest. In recent years, populations have significantly declined due to loss of natural habitat. B. muscorum is currently listed as vulnerable in Europe by the European Red List of Bees.