Kleptoparasitic flies
Thu, Mar 19, 2009 at 2:37 AM
Hi guys,
I got this photo of tiny flies trying to get to the ant captured by this jumping spider. Apparently they are Milichiidae (Diptera, Schizophora) some of which are kleptoparasitic of spiders, some specialising in ant snacks such as this one. The spider is a female Salticid, Zenodorus orbiculatus known locally as ant hunters. She is about 7mm long so you can see how tiny those flies are.
aussietrev
Queensland, Australia

Freeloader Flies share Ant Hunter's prey

Hi Trevor,
Though you have a long history of providing our site with awesome images of Australian fauna, this image is, in our opinion, one of the most fascinating. The fact that you captured this nuanced example of Kleptoparasitism is phenomenal. One animal stealing food or prey from another is common in the animal kingdom, and it is easily observed in our own brand new aquarium, but to photograph these minuscule creatures evolutionarily adapted to this activity is nothing short of fantastic. These Freeloader Flies, as they are called on one website, in the family Milichiidae, are described by Irina Brake on the Introduction to Milichiidae website: “Thu, 2009-02-12 13:48 — Irina Brake
The Milichiidae (Diptera, Schizophora) are small, mostly black acalyptrate flies. The family contains about 240 described species in 19 genera and is worldwide in distribution.
The behavior of several species of Milichiidae is very specialized. For example, in some species the adults are myrmecophilous (= ant-loving), whilst in some others they are kleptoparasitic, feeding on the prey of spiders or predaceous insects.
The habitats of Milichiidae are diverse. Adults can be collected in open landscapes, such as steppes or meadows, in wadis, at the edges of forests, inside forests, in the forest canopy, in stables or houses, or even in caves. However, they do not seem to be attracted to coastal habitats or to other places near water.
The Milichiidae are divided into three subfamilies, Madizinae, Milichiinae, and Phyllomyzinae.
Common names
Common names are only rarely cited for Milichiidae and seem to be more of an invention of the author than a commonly used name. The English term “filth flies”, for example, which is sometimes used for Milichiidae, was introduced by Sabrosky (1959) in the title of a paper about the genus Meoneura , which now belongs to the family Carnidae. Sabrosky probably used the general expression “filth fly” to describe the biology rather than intending the term to be a common name for the family Milichiidae. The term “filth flies” is generally used for several different taxa associated with ‘filth’.
Since people keep stumbling over the name ‘Milichiidae, I herewith introduce a new english common name: “freeloader flies”. The name refers to the biology of Milichiidae. Definitions for ‘freeloader’ are: ‘ someone who takes advantage of the generosity of others’ ( wordnet.princeton.edu/perl/webwn ) or ‘ one who depends on another for support without reciprocating’ ( http://www.answers.com ). ”
BugGuide also has information on the family Milichiidae. The Geocities website has some nice images of the Ant Eater Spider or Ant Hunter Spider, Zenodorus orbiculatus.

Correction: Mon Mar 23, 2009  7:08:13 AM America/Los_Angeles
Dear Daniel,
thanks for alerting me to your photo and citing my webpage. However, I
discussed it with a collegue of mine and we both think that your flies
are Chloropidae, not Milichiidae. Michael von Tschirnhaus is a
Chloropidae specialist and has more experience with actually watching
the live flies than I have. He wrote to me that from the habitus the
flies are certainly Chloropidae. There are several species who are
kleptoparasitic on spiders. He doesn’t know all Australian genera, so he
can’t tell you which genus it is. Many species of different genera
develop in spider cocons and stay with the spider for a longer period of
time. They can wait endless in the spider net.
Best wishes,
Irina