Platyhelminthes

The phylum Platyhelminthes includes two classes of worms that involve ants: the cestodes (tapeworms) and the trematodes (flukes and blood flukes). These two groups have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts, including ants.

Cestodes
All cestodes are parasites of vertebrates; many have complex life histories, including a stage in a definitive (main, vertebrate) host in which the adults grow and reproduce, often for years, and one or two intermediate stages in which the larvae develop in other hosts (typically arthropods or other vertebrates). The adults live in the digestive tracts of vertebrates, while the larvae often live in the bodies of other animals.

Trematodes
Trematodes are primarily internal parasites of molluscs and vertebrates. Most trematodes have a complex life cycle with at least two hosts. The primary host, where the flukes sexually reproduce, is a vertebrate. The intermediate host, in which asexual reproduction occurs, is usually a snail, but includes several species of ants as well.

The best studied example is Dicrocoelium dendriticum, the lancet liver fluke. Adults of Dicrocoelium dendriticum are found in the bile ducts of sheep, cattle, pigs, deer, cottontail rabbits, and woodchucks, while earlier life stages inhabit snails and ants. Fluke eggs are eliminated in the feces of their vertebrate hosts and are ingested by terrestrial snails. Cercaria (the free-swimming larval stage) develop in the snails, are secreted in the snail’s mucus or slime ball, and are ingested by ants. Once within the ant the parasites develop in two areas of the ant’s body, the brain and the abdomen. Those in the brain cause the ant to disperse to the tips of grass blades or other vegetation at dusk, lock their mandibles to the vegetation, and remain there until dawn (this behaviour is called summiting behavior). In this position they are most likely to be eaten by herbivores (the fluke's primary host) in the early morning. This behaviour will be repeated every night until the ant is eaten by a vertebrate host in which the parasite can fully mature and complete its life cycle.

Summiting Behaviour
Botnevik et al. (2016) note that when searching for infected ants they sampled between 1400 and 1700 hr. This time period may not be optimal in terms of achieving the greatest number of ants, which are more common in the morning and evening hours (Spindler et al., 1986). However, it was selected to avoid collecting ants infected by the entomopathogenic fungus Pandora formicae, which induces the same behavior as D. dendriticum (Malagocka, 2016). Ants infected with P. formicae are almost exclusively found in tetany in the evening after which the fungus rapidly kills the ant, produces conidiophores, and spreads its conidia spores during the night and early morning hours (Malagocka, 2016).