Green Lacewings lay their eggs in clusters with each egg attached to a long slippery stalk. Hatchlings are big-headed larvae with long, piercing mandibles. They also possess curved spines, called setae, that protrude from the sides of the body, projections that help secure debris that larvae hoist onto their back. In fact, the first order of business of the young larvae that we observed was to adorn their body with empty egg cases and stalks. Is it because egg stalks are coated with an ant repellent?
Soon after hatching, many larvae of Green Lacewings seek out aphid colonies for food, hence their common name "aphid lions". Along with killing their prey, the larvae cover their bodies with the shriveled remains of their victims. But for what purpose? Are larvae attempting to build a disguise or perhaps a shield that will protect them from even larger predators? When an aphid lion encounters an ant, for example, it instinctively lowers its head while simultaneously arching its back. Such maneuvers, especially when the back is covered with debris, helps cover vulnerable parts of a larva’s body, as effectively as a box turtle enclosed within its shell. Apparently even a tiny amount of debris can discourage ants from probing very young aphid lions.
Gradually the amount of debris carried by an aphid lion increases. This doesn’t mean that larvae must kill an aphid before its skin is available. To the contrary, molted skins make the best accessories. Furthermore, a lion’s first meal is unlikely to be a live aphid. Instead, young lacewing larvae feed on the sweet honeydew secreted by aphids, the same concoction sought after by ants. Ironically, the quest for honeydew is perhaps the reason why aphid lions bother to build a protective shield.
Consider the following facts: Aphids are docile creatures that spend most of their time feeding on plants; they have not evolved spines, formidable jaws, or toxic bodily fluids to discourage predators. Yet aphids are not entirely defenseless. Along with living in colonies where there is safety in numbers, aphids solicit ants as bodyguards. In effect, aphids and ants have established a mutualistic relationship whereby ants, in return for repelling predators, are rewarded with an ample supply of sweet honeydew.
Take for example the predacious lacewing larvae. Unlike their namesake, aphid lions do not coordinate members of a pride when attacking. They are solitary predators that use stealth when approaching prey—not, however, to avoid scattering aphids when a lion approaches the colony, but to ward off retaliatory responses from attending ants.
Once alerted the best that ants can do in defense of the colony is force an aphid lion to retreat, either by ripping away the intruder’s shield or shooting the attacker with a stream of formic acid. In the episode we observed both defensive strategies were used, although the ants didn’t discharge acid until the lion moved away from the colony. Why the defender’s response was delayed isn’t clear. Perhaps a very strong scent of the alarm pheromone is required to motivate ants to spray; alternatively, we can’t rule out the possibility that the ants were responding to a second threat.
It wasn’t until viewing the videotape that we noticed the presence of a Syrphid Fly maggot, a type of hoverfly, feeding on an aphid. More surprisingly, the ants ignored the predator. Watch closely and you’ll see the maggot turn and strike the aphid lion, an action that could have caused the intruder to retreat sooner than it normally would have. Why don’t aphids emit pheromones when attacked by the maggot? Perhaps they do, but for some reason the ants are unable to identify the source of the threat.
According to research on hoverflies, maggots living among aphids acquire the chemical signature or scent of residents of the colony. In effect, the maggots are chemically camouflaged and are thus identified as aphids by tending ants. Such deception is also used by other species of hoverflies whose larvae feed not on aphids, but on ant pupae. In this context, a hoverfly’s maggot acquires the smell of the ant colony in which it is raised.
For most of us, predator-prey interactions involve lions chasing herds of wildebeests, wolves pursuing reindeer, or perhaps, eagles and bears snatching fish out of a river. Such dramatic conflicts, however, are few compared to the bizarre arrangements and subterfuge that normally unfolds between predators and their prey. In the world of insects, in particular, there are often layers of complexity involving multiple players trying to eat and avoid being eaten. Identifying the roles of all players makes interpreting the natural world exciting and challenging.
With regard to lacewings, a complete picture of their habits must include all stages of their metamorphoses from egg to adult. The third stage begins when a larva spins a spherical cocoon from which a fully formed adult with wings emerges. The cocoons that our aphid lions spun were covered with the same debris that the larvae carried on their back which raises the question: Does such debris protect the developing adult inside a cocoon from predators or parasitic wasps? As always, insightful comments from viewers are welcomed on our Facebook page. Video
