The dilapidated remnants of Chironde, an old hunting camp near the town of Inhamitanga in Mozambique, don’t make for a particularly enticing location for visitors and most of the year the camp sits empty, looked after by a lone guard whose most exciting part of the day used to be cooking a pot of rice under a leaky roof of a small outdoor kitchen. But after years of neglect the roof collapsed entirely, and the guard had to find another place. The now abandoned structure was quickly colonized by various organisms, including a large colony of carpenter ants (Camponotus importunus), who found the cracked concrete floor of the kitchen absolutely perfect for building an extensive colony.
I have been coming to Chironde frequently over the years with researchers and students since the surrounding pristine miombo woodland provides a fantastic environment for biological exploration and a couple of weeks ago I went there again with two visiting scientists. During the night it started to rain, and the water must have seeped into the underground ant colony because the following morning we were met with a sight of hundreds of ants frantically pulling their larvae and pupae to the surface in an apparent attempt to dry them. As we watched the ants, suddenly a few large flies appeared and started wresting with the ants over their precious cargo. Although the ants were quite large, in almost all cases the flies managed to overpower them and fly away with the larva or pupa in their grabby legs. The flies would land a few meters away and start sucking the content of their prey, within minutes leaving nothing but a shriveled shell. I recognized them instantly as members of the Old World genus Bengalia (Calliphoridae: Bengalinae), aptly known as highwayman flies.
It wasn’t my first encounter with highwayman flies. A few years earlier I was looking for inquilines of termites, animals that share their underground living spaces, and broke off a small section of the termite mound, exposing a group of pale and soft workers to the light. Within seconds highwayman flies showed up out of nowhere and started kidnapping the confused creatures. The flies would alight on a bush a few meters away, suck the poor termite dry, and come for more. At that time I had no idea of Bengalia’s existence and so I took a few photos and started searching entomological literature, looking for clues. It didn’t take me long to identify the flies but was quite surprised to discover how little was known about their biology, despite multiple studies on the flies’ taxonomy. In fact, most of what I found were old, anecdotal observations from SE Asia of the flies hunting termites or stealing larvae and pupae of ants, although recently a study in China shed additional light on the interactions between highwayman flies and ants.
Highwayman flies truly behave like their human namesakes. They target travelers, either termites marching towards their food source or ants carrying their brood or food from one place to another. On rare occasions they depart from their kleptoparasitic tendencies and attack small solitary creatures, such as nymphs of blattodeans, but they really prefer to rob or hunt social insects. It is not clear how highwayman flies locate ants carrying their brood or exposed termite workers but it is likely that they use a combination of vision and scent. There is some indication that they are attracted to the smell of fungal gardens grown underground by termites, a smell that usually indicates a damage to the mound and the possibility of exposed workers.
A peculiar aspect of the flies’ behavior is that they truly prefer to steal rather than earn an honest living. If offered the exact same prey item but without the ants present, an ant larva or pupa lying alone, they will simply ignore it. But the moment an ant picks it up, then the game is on. It’s almost as if they were unable to make their own judgement as to what is worth taking without someone telling them. They will even steal breadcrumbs from ants, something that they probably would never consider eating, if they see the insects carrying them.
The development of Bengalia is nearly completely unknown. A larva of this fly (hatched from an egg laid under laboratory conditions) was described for the first time in 2016 and there is a single observation of a female lying eggs in the soil near an incipient colony of termites. Larvae of a closely related genus Verticia (Bengalinae) are parasitoids of termites, developing inside the head of termite soldiers, over time filling it completely, and eventually leaving the host by crawling through its neck, thorax, and abdomen, only to emerge at the posterior end (while the host is still alive). It is therefore quite likely that Bengalia larvae do something equally horrendous to termites. On a few occasions I found what looked like fly larvae in the termite colonies. I am now tempted to look for them again and attempt to rear them in hope of unraveling the mystery of Bengalia’s development. If I have any luck, you will be the first to know.
