Sand wasps taxonomically grouped under the genus Bembix are solitary, ground-nesting insects with more than 380 species world-wide. Female sand wasps excavate burrows in exposed, sandy soils tunneling into brood chambers where they lay eggs. Following oviposition, females provision the developing larvae with flies they have hunted. These formidable fly hunters, however, can become prey to parasitic flies that exploit the wasps’ nesting behavior.
Here, we’ll examine the co-evolutionary dynamics between these wasps and their fly parasites using footage recorded at the Ocotal beach in the Pacific coast of Costa Rica. Female sand wasps industriously kick large volumes of excavated sand backward using specialized tarsal rakes made up of spines on their front legs. The adaptation of front legs for specialized functions is a fascinating phenomenon evolved independently in a number of insect groups such as the praying mantis and mole crickets.
The deposition pattern of sand residues resulting from tarsal rake action create a narrowing area of contrasting color leading to the nest entrance. This acts much like the nectar guide coloration in flowers that direct pollinators in flight. Tarsal rakes do more than just enhance digging. They can also catapult sand away from the nest entrance, creating an effective visual cue that’s easily recognizable from a distance. Long-range nest orientation is a highly beneficial cognitive trait. It allows females to increase their foraging area and reduces the energy they expend when returning to the nest with prey.
Wasps navigate to their nests by creating a “memory map” of their environment during an initial orientation flight upon leaving the nest. This flight involves flying in a series of expanding arcs, which allow the wasp to view the nest at a range of angles and build a cone of memory snapshots to guide its return. On the return trip, the wasp uses these learned views, combining broad landscape features for distant navigation and ground-level landmarks for precise localization as it gets closer to the nest. Some Bembix wasps continuously feed their growing larvae, a behavior known as progressive provisioning. Others, however, use mass provisioning, stockpiling prey all at once. Remarkably, certain species such as Bembix americana, can even switch between these two provisioning strategies based on environmental conditions.
The female wasp rapidly seals the nest entrance immediately after the larva turns into a pupa. The female expends significant energy to disperse and remove the visual cue associated with the nest entrance. The wasps’ abdominal movements used to compact the sand are quite striking. The dorsal and ventral hard segmental plates of the abdomen are called tergites and sternites. These hard exoskeletal plates have a functional role in the rapid hammering movements used for compaction. This special effort to delete visual cues is crucial because of the many nest parasites that pose a threat. At this final stage, the tarsal rakes once again prove their effectiveness.
Here, you can see a parasitic fly targeting a female wasp as she digs her nest alone. The intense pressure from parasites, has led to the rapid evolution of nesting aggregation behavior in these solitary wasps. The parasitic fly perches near a wasp aggregation, observing their activity, poised to sneak into a nest. When multiple wasps build their nests close together, this can effectively dilute the parasites’ attention. Confused parasites may fail to locate nests with provisions, thereby reducing the chances of any single nest being parasitized. The shift in behavior, from solitary, towards aggregated nesting, is an important move toward the highest level of social organization, known as eusociality.
Parasitic flies are expected to evolve an attention-like cognitive ability, that helps them observe, and, locate nests containing provisions. The fly peripheral visual system primarily relies on compound eyes composed of numerous units called ommatidia. Kleptoparasitic flies, appear to have a greater investment in the peripheral visual system, with higher number of ommatidia and, compound eye surface area. Moreover, in addition to the compound eyes, these flies also have larger simple eyes without retina called ocelli. As we will see, the persecuted host can detect the fly and attack it. Larger ocelli may be advantageous in evading fast approaching predators since they can better detect sudden changes in light, such as a shadow cast by a swooping predator.
The parasitic fly trails a returning host to its nest, through a behavior known as satellite flight. Flies can easily lose attention when their cognitive capacity is overwhelmed by tracking multiple nearby nests, which can have fatal consequences. The parasite risks its life by repeatedly entering nests and exploring chambers to find prey and provisions. The wasp in this view is excavating at least three nests. Although the fly explored the nest only a moment ago, it decided to re-enter the nest.
A) B. occidentalis
B) B. pallidipicta
C) B. belfragei
The underground architecture of these nests can be quite articulated, making the search process much more challenging for the fly. For comparison, this drawing presents generalized cross-sections of three North American Bembix species nests side-by-side. Bembix occidentalis excavates a single tunnel, angled at 45 degrees, which leads to a few false brood chambers. Bembix pallidipicta, however, digs two horizontally connected nest entrances. Deception is crucial in this reproductive arms race: if a female makes a noticeable entrance with prey but secretly exits from another opening, the parasite might hesitate to enter, assuming the host is still inside. Bembix belfragei also digs a nest with a single entrance, but it includes a midway fork that branches into smaller bifurcations leading to false brood chambers, likely to confuse parasites.
The dynamics between the digger wasp and its fly parasite swing constantly from being a host-parasite relationship into that of predator-prey and vice versa. In this frame a fly is demonstrating its attention-like characteristics, appearing to wait for the wasp to leave her nest. When the wasp takes off she executes a recognizable narrow arc flight maneuver. Like a boomerang she comes back to make a pass over the nest, and surprises her fly enemy with a high-speed attack.
The host is now a predator.
Suddenly, a second fly disrupts the attention of the first, and makes a bold dash into the nest while the host is still inside. The first fly appears to have lost its focus. Just as she switches into self-grooming mode, the second fly emerges from the nest and takes off. The first fly drops her guard even further, and turns her back to the nest entrance. After all the commotion, she appears to have forgotten that the host wasp—which is about to become her predator—still hasn’t come out. The emerging wasp immediately recognizes the fly, but the fly also rapidly reacts to this new situation. She realigns herself with the threat and narrowly escapes the attack. Once again in slow motion, the wasp can be observed executing a boomerang maneuver to hunt the fly.
Despite their strictly solitary lifestyle, the way these wasps form nesting aggregations is quite intriguing. Their behavioral diversity may offer clues as to how eusociality could have evolved, shaped by the selective pressures of host-parasite and predator-prey dynamics.
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