A Tale of Deception and Murder

Ants really like sugar. This fact is obvious to anyone who has encountered a discarded lollipop on a summer sidewalk. Of course, this sweet tooth makes sense, as a diet with a high energy content is necessary for insects that spend the majority of their life on the move. However, ants don’t always wait around for upset toddlers to throw candy on the ground; no, many ants feel the need to take matters of sweetness into their own hands tarsi.

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Ants in the genus Lasius “farming” dock aphids, Aphis rumicis Linnaeus, 1758, for honeydew. (photo copyright InfluentialPoints)

A fairly common behavior documented in many ant species is the farming of honeydew, a sugary substance excreted by aphids, scale insects, and other homopteran insects. Honeydew is produced as a waste product of sap consumption, essentially containing excess sugar not needed by the insects’ metabolism. This unusual relationship is a good example of mutualistic symbiosis: the ants benefit by possessing a captive renewable food source, while the aphids benefit by protection, with ants purposefully defending their herds from predators such as ladybugs or parasitic wasps. However, the relationship is slightly skewed in favor of the ants. Interesting research published in Oliver (2009) found that chemicals released by the feet of ants can limit aphid activity, allowing ants to herd their aphids into specific boundaries. It is also not uncommon for ants to eat some of their aphids if conditions in the colony have become particularly desperate.

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Ptilocerus ochraceus Montandon, 1907 photographed in Gunung Leuser National Park, Sumatra. (photo copyright Paul Bertner)

My primary focus for this post is not to discuss formicid honeydew farming, although that tangent was admittedly long, but instead to describe the remarkable predatory habits of an obscure southeast Asian insect that takes advantage of ants’ unquenchable thirst for honeydew. The Reduviidae is a large and highly diverse family of true bugs commonly called “assassin bugs” due to their stealthy hunting behavior towards other arthropods, often using camouflage to remain hidden until just the right moment. The subfamily Holoptilinae is relatively small and morphologically conservative, containing only about 80 species that generally specialize in ant predation and may be referred to rather awesomely as “ant wolves.” The Indonesian species Ptilocerus ochraceus Montandon, 1907 exhibits a particularly interesting hunting strategy that was documented in detail by Jacobson (1911). The behavior is now known to be pretty typical of holoptilin holoptilines, but was altogether unknown prior to Jacobson’s observations in Semarang, Java in the summer of 1909.

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Pinned specimen of a black ant, Dolichoderus thoracicus (Smith, 1860), photographed by Andrea Walker. (photo copyright California Academy of Sciences)

P. ochraceus is only known to attack one species of ant, the black ant, Dolichoderus thoracicus (Smith, 1860), a species that has been used as a biological agent to control pests on cocoa trees, Theobroma cacao L. The assassin bug positions itself directly in the path of the worker ants and, when approached, raises the front of its body into the air to expose a strange tuft of yellow hairs on its underside. This tuft, known as a “trichome,” secretes a sweet substance that is highly attractive to the ants, who cannot resist but to stop and stroke the trichome with their mandibles, the same behavior used to “milk” honeydew from their aphid herds. The ant may drink the substance from the assassin bug for many minutes while the bug remains motionless: or, as Jacobson puts it in my personal favorite quote from the original description,

“It is surprising to see how the bug can restrain its murderous intention as if it was knowing that the right moment had not yet arrived.” (Jacobson, 1911)

This is because the assassin does not need to act, the sweet substance itself is doing all the work. Shortly after finishing its meal, the ant only has time to move a few inches away from the bug before it begins to be paralyzed, losing all motility and curling its legs under itself. At this point, the assassin bug springs into action, seizes its prey and pierces it with its fearsome beak to finish the job and liquefy the ant’s internal organs. Jacobson made a point of observing this process many times to confirm that the bug did not harm the ant during the “milking,” but that the source of paralysis really was the substance itself. He also noted that many more ants killed themselves on the enticing treat than could ever be eaten by a single bug, a testament to the extremely high population densities of this ant on the island of Java. This incredible behavior, practiced by both adults and nymphs, is an amazing example of coevolution and offensive mimicry designed to take advantage of ants’ insatiable sweet tooth.

REFERENCES

Jacobson, E. (1911). Biological notes on the hemipteron Ptilocerus ochraceus. Tijdschrift voor entomologie54, 175-179.

Oliver, T. H. (2009). The Ecology and Evolution of Ant-Aphid Interactions.

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