Phorid Flies

Phorid flies are known to parasitize ants. The common means of parasitism, and until recently thought to be the only method, is as follows. A gravid female phorid fly seeks out foraging ants to parasitize, and most flies appear to specialize on specific species of ants or at least a group of congeners. Once found the female attacks host workers and injects eggs into their head. The egg hatches, feeds from the ant that is hosting it and eventually pupates. In many cases the fly emerges from its pupal case in such a way that it causes the ant's head to pop off.

Recently it was discovered there is another method of ant decapitation. Some adult phorids were observed sawing the heads off of injured Odontamachus ants. Presumably the ant heads are used for food by the adult flies.

Impact on Foraging Activity
Braganca et al. (1998): Females of the parasitic phorid Neodohrniphora sp. were collected in the field and released singly inside an observation chamber placed between a laboratory colony of Atta sexdens and its foraging arena. The number and speed of loaded and unloaded ants returning to the nest, the weight of foragers and their loads, the number of leaf fragments abandoned by ants, and the number of small workers ‘hitchhiking’ on leaf fragments were measured before phorids were released, while they were in the observation chamber, and after they were removed. Relatively fewants were attacked by Neodohrniphora sp., but the presence of flies prompted outbound ants to return to the nest and caused a significant reduction on the number and mass of foragers. Additionally, the weight of leaf fragments transported by ants was reduced and the number of abandoned fragments increased in response to Neodohrniphora sp. Presence of the parasitoid caused no significant changes in the number of hitchhiking ants. The regular ants’ traffic was resumed after phorids were removed, but foraging activity remained below normal for up to three hours. In the field A. sexdens forages mostly at night, but colonies undergo periods of diurnal foraging during which ants are subject to parasitism from several species of phorid flies. Considering that daytime foraging may be necessary for nutritional or metabolical needs, phorids may have a significant impact on their hosts by altering their foraging behavior regardless of the numerical values of parasitism.

Impact on Colony Growth
Kaspari (2000) points out that the risk of parasitism may slow the growth rate of some ant colonies. Recent work on the interactions between phorid flies and ants exemplifies this phenomenon. Phorids are tiny parasitic flies, many of which specialize on a single ant species or genus (Brown and Feener 1991a, 1991b; Brown 1993). Phorid flies search for their host ants (often following the odor plume of the ants themselves), hover, then zoom in to lay an egg somewhere on the worker ant's body (Porter et al. 1995a, 1995b; Feener et al. 1996). The worker falls over, stunned, then eventually returns to the colony and dies when the egg hatches and the ant serves as food for the developing maggot.

However, phorid flies first must catch worker ants. Host ants often run and hide in the presence of their phorid parasite (Porter et al. 1995c). This reaction is so profound as to interfere with foraging, and perhaps swing the competitive balance away from the host ant to its phorid-free competitor (Feener 1981).

Pseudacteon
Pereira et al. (2015) - The genus Pseudacteon has a worldwide distribution and comprises parasitic myrmecophilous species that decapitate host ants. Seventy three species are known in the genus with 41 of them occurring in the Neotropical Region.