Though the proboscis is fairly long in most species, this is not obvious because it commonly is geniculate, having a knee-like fold in the middle that holds it inconspicuously beneath the head when the animal is not feeding.

When it is looking for a place to feed on the prey of a spider or the like, the proboscis is extended, giving an impression of licking, as shown in the accompanying video.

[6] The adults of some species linger close to invertebrate predators, where they act as kleptoparasites, feeding on bodily fluids of the prey.

[6] The "jackal" habit has been widely documented, with many pictures and references to Millichiidae assembling on the prey of spiders (especially Nephilinae, Oxyopidae, and Thomisidae).

This would be compatible with the idea that predators that commonly hunt prey that release a dramatic and characteristic olfactory signal, thereby create a niche for suitable kleptoparasites.

For one thing, though they may be the most conspicuous of such partakers, the jackal habit is not unique to the Milichiidae, nor do all species in the family necessarily indulge in it, nor for all the same reasons.

Certainly careful inspection of photographs of spiders' prey often will show a few specimens from other families of small flies present among the Milichiidae.

However, this impression might be regional; reports on field work suggest that in parts of Florida for example, the predominant kleptoparasites were in the family Cecidomyiidae.

[14] Another activity observed in some species of Michiliidae, shows them to be serving a function analogous to that of cleaner wrasse and cleaner shrimp; they literally scavenge around the chelicerae and anal openings of large spiders, such as species of Araneus and Nephila, that cooperatively spread their wet and sticky chelicerae thus allowing the flies to feed actively all over the bases, fangs and mouth.