On 5/5/23, I joined Jonathan Mays to look for robber flies in Gilcrist and Alachua Counties (north Florida). I spotted this Leptogaster navigating among the vegetation, prodding at stems. I was proud of myself that I was able to identify it before it landed — it was fairly large for Leptogastrinae, it had brown wings and a shining red scutum, and the habitat seemed right: Leptogaster obscuripennis.
We followed it hoping it would land. We were about to net it because we wanted to confirm the ID before it flew out of sight, and then it struck a small Argiope spider! The spider and the robber fly were the same size, and while they grappled with each other, it was not clear who would win. The fight lasted several minutes, and it initially seemed like the spider may have the upper hand: the Leptogaster‘s legs seemed to be caught in the web.
Each time it seemed caught, it would free itself. Then the Leptogaster delivered a fatal bite, and the spider eventually slowed. It was like watching National Geographic, but on a very small scale.
After some time, you could see the spider abdomen deflating.
Leptogastrinae hunt in a similar fashion as some damselflies, striking at stationary objects. Some tropical damselflies are specialists at taking spiders, and while I don’t think Leptogaster specialize on spiders, they are known to take more spider prey than any other group of robber flies (Dennis et al., 2012). In the Asilidae predator-prey database (Lavigne, 2003), Leptogastrinae have been recorded to take spiders 208 times, or 60% of all observations of robber flies taking spiders.
References:
Dennis, S. D., Lavigne, R. J., & Dennis, J. G. (2012). Spiders (Araneae) as prey of robber flies (Diptera: Asilidae). Journal of the Entomological Research Society, 14(1), 65-76.
Lavigne, R. J. (2003). Predator-prey database for the family Asilidae (Hexapoda: Diptera). Modified 2005.
Robber Flies of Florida 