Aphids excrete a liquid residue from their food, honeydew, composed essentially of sugars and amino acids. The droplets produced (see video) and ejected through the anus are sometimes deposited abundantly on plant foliage. Many insects come along, day and night, taking advantage of this sugar resource by licking or sucking (see video). Some of them, such as ants, have learned to attract aphids actively with their antennae to palpate the abdomen of these Homoptera. This action stimulates the emission of a drop of honeydew, which is absorbed immediately.

This behaviour has long been known in ants, but was observed only recently in a few Diptera of the genus Fannia (F. serana and F.armata) and to a lesser extent in Bibionidae (Dilophus febrilis) in the Netherlands on maple aphids (J.A. van Erkelens).

In France this phenomenon has been seen two years running on a willow aphid (Tuberolachnus salignus) (see video). The dipteran concerned was Fannia manicata (Identification M. Martinez). Although the genus Dilophus was present, the stimulation behaviour has not been observed in these Diptera.

In the example of Fannia manicata, it is the fore-legs that are used to palpate the aphid to elicit honeydew emission. Nevertheless, contrary to those ants that have developed a mutualist relationship with aphids, the relation these dipterans form with them is only opportunist.(cleptotrophobiosis).