Flyfocals: Vision and Vectors Help Hunting Robber Flies

Image credit: Thomas Shahan

Robber flies (Asilidae family) are not your typical house flies. They are small, predatory insects that feed on a vast array of other arthropods. While they are small in size (10 times smaller than a dragonfly), these guys are serious hunters. For example, Mallophora omboides is known as the “Florida bee killer” for its taste for honey bees. Other robber flies hunt down wasps, dragonflies, spiders, or grasshoppers, just to name a few. Perhaps almost as impressive as the types of prey is how they are subdued. Typically, robber flies will perch out in an open sunny place and wait, seizing their prey in flight and injecting it with neurotoxic or proteolytic enzymes that both immobilizes it and liquefies its insides.

A recent study in Current Biology took a closer look at the robber fly’s “aerial attack strategy.” The authors focused on the genus Holcocephala, a group native to the Americas. Let’s start by going over something you know about but probably never realized had an actual term: constant bearing angle (CBA) strategy. Initially, I tried to describe this just using text, but it is really best visualized with the help of a supplemental graphic from the paper.

Figure S1 from Wardill et al. (2017). Diagram showing how the constant bearing angle strategy (CBA) and proportional navigation can be used to intercept targets. It looks like an eye, but you are actually looking down on the "Human" and seeing the top of the head (black) and shoulders (white). 

Visualize this: You are walking along and ahead of you a ball is rolling along the ground from your left. But you decide that you want to get to the ball before it would intercept your path. If you want to catch the ball you can’t run straight for where you see it or it will have rolled past that spot before you get there. If you want to intercept that ball while it is still on your left, you will technically have to turn to backward, changing your “bearing angle.” You must anticipate where it will be and run in a straight line to that spot. This line is a “parallel range vector.” There are several of these vectors, depending on when you choose to change course.

The study considered whether the flies were using this CBA strategy to catch their prey. To do this, the researchers went out to a field and hung up a big white sheet as a backdrop for their high-speed video cameras. Next, they set up their “fly teaser,” a custom made plastic frame that housed a stepper motor and several pulleys to move taut fishing line. This allowed for precise, computer controlled movements of the beads they attached to the fishing line. When a robber fly perched on a blade of grass in their study area, they “teased” it with a bead (a.k.a. dummy prey item for the fly). They included several variations including bead size and direction. They recorded the fly with two synchronized cameras running at 1000 fps to get a 3D view of the attack. For each attack video, they analyzed the frame at which the flies started to take off and until it began a terminal deceleration on final approach of the target. Then: measure, measure, measure, math, math, math.

They found that the flies were fairly consistent with the CBA model. If they decelerated or reversed the bead during the attack, the robber flies compensated, actively keeping the range vectors parallel. One unexpected finding occurred in cases where the bead moved in front of the fly and it took off with a head-on collision course. They found that the fly still intercepted the bead while flying at a backward angle, meaning that the latter part of its trajectory was distinctly curved. When they took a closer look, the found the results to reflect a “lock-on” process “during which the fly has a new heading and the speed is fixed to a value slightly higher than that of the prey.” This lock-on strategy has not been described in any other flying animal. The flies were able to compensate for unexpected changes in the target’s velocity and uncertainties in the location, size, and speed of the target.

Adapted from paper's graphical abstract

This type of hunting relies very heavily on vision. So each robber fly was captured for later, high detailed analysis of the head and eyes. It is important to remember that insects have compound eyes. Repeating units (the ommatidia, which have hexagonal faces called facets) that make up the eyes function as separate receptors that, when put together, assemble view of the environment. The researchers measured several parts and angles within the eyes, and once again math, math, math. This revealed the ommatidia in the front, center portion of each eye (colored red in the picture) to be nearly double the size of those in other areas, have extended focal lengths and smaller receptors. This means that the flies can reduce diffraction, focus incident light, and optimize resolution in this area. This results in a frontal fovea, or area within the eye that provides greater visual acuity than the rest of the eye. Sort of like the embedded lens of bifocal glasses; while that is an incredibly simplified way to look at it, it tells you a lot about how the flies might strategize prey capture. Also, they could be judging distance using stereopsis. This is when they use both eyes in combination to depth and 3D structure. The authors sum things up nicely, so I'll leave it in their words: "[It is kind of amazing the] accurate performance that a miniature brain can achieve in highly demanding sensorimotor tasks."

Interested in more details? Here’s a video summary put together by the researchers:

Wardill, T., Fabian, S., Pettigrew, A., Stavenga, D., Nordström, K., & Gonzalez-Bellido, P. (2017). A Novel Interception Strategy in a Miniature Robber Fly with Extreme Visual Acuity Current Biology, 27 (6), 854-859 DOI: 10.1016/j.cub.2017.01.050


Read more about robber flies at University of Florida's Featured Creatures page.