Last week we had a chance to see the amorous side of some of our six-legged friends. Or as one viewer commented, maybe it was just insect sex. This week we visit one of our regular guests, odorous house ants, doing what they do best in nature instead of in our kitchen. And then we will escape the polar vortex in Maryland and head to Costa Rica to meet a member of the carpenter ant clan tending a very attractive caterpillar.
Here: Odorous house ants have a storied history of home invasions as they forage for sweets, usually in the kitchen. But let’s face it, millions of years ago when these ants evolved the world was not chockfull of cupboards laden with sugar spills and drops of pancake syrup. Nature’s cupboard of carbohydrates was supplied in part by a bounty of sucking insects such as aphids, scale insects, mealybugs, whiteflies, treehoppers and their kin, which produce copious amounts of sweet honeydew as a waste product of the nutrient rich plant sap they consume. In addition to carbohydrates, honeydew of different insects may contain amino acids, proteins, minerals, and vitamins.
Stroking aphids with antennae and legs often elicits a drop of honeydew for these hungry ants.
Baltic amber provides clues that the association between ants and their honeydew producers dates back some 50 million years. Insects that depend on ants during part or all of their lives are called myrmecophiles. The relationship between ants and sucking insects is a special kind of mutualism and these sucking insects have been termed trophobionts, literally nourishment organisms. As all good biologists know, for a relationship to be a mutualism both partners must benefit. Clearly ants gain nutrients from the association with these sucking insects, but what do the suckers get from the deal?
Clever experiments involving the removal of ants from these associations demonstrate that ant shepherds provide protection from predators and parasites intent on turning suckers into a meal or an unwilling food source for their offspring. Ants may also remove excess honeydew, thereby maintaining the hygiene of the aphid colony. In other cases, ants are known to build shelters for aphids that may provide protection from harsh weather. In a floodplain near my home in Maryland, herbaceous plants send slender stems skyward in spring. Aphids lining the stem are tended by odorous house ants, their bellies swollen with honeydew as they keep vigil over their flock, all the while stroking the aphids to obtain the next sweet drop. Nymphs and hatchet-shaped adult treehoppers on leaves of my sunflower are similarly tended by ants each year.
The watchful eyes of odorous house ants protect a newly molted Entylia treehopper and its siblings on a leaf of my sunflower.
There: Let’s jump 2,000 miles south and warm up to 80ᵒ Fahrenheit in the lowland rainforest of Costa Rica where a species of carpenter ant engages in a fascinating relationship with a member of a different clan. Butterflies in the families Lycaenidae and Riodinidae form mutualistic relationships with many species of ants. Caterpillars of some lycaenids and riodinids have specialized glandular structures called pore cupolas scattered over the surface of their bodies. These glands produce compounds highly attractive to ants. In the case of myrmecophilous lycaenid caterpillars, additional glands called Newcomer’s glands produce sugar-rich secretions consumed by attending ants. Evidence of a mutualistic relationship between riodinid caterpillars and Camponotus ants comes from the observation that caterpillars lacking their Camponotus bodyguards are preyed upon by other species of ants. Furthermore, species of myrmecophilous lycaenid butterflies favor plants with ants as sites to deposit eggs. Upon hatching, tiny caterpillars can eat and grow under the watchful compound eyes of six-legged bodyguards.
While wandering along a rainforest trail I stumbled on a small sapling missing several leaves and part of a stem. Several ants milled around the terminal of one twig and upon closer inspection I spied a small cryptic caterpillar at the center of the ant’s attention. The ants moved freely over the caterpillar’s body stopping near the head and one of the posterior abdominal segments to taste an unknown substance on the surface of the larva. Mutualisms between ants and butterfly larvae are not unique to the tropics and with some good fortune, when spring returns one might catch a glimpse of caterpillars and their bodyguards in the meadows of Maryland.
This lucky riodinid caterpillar in the genus Synargis has the rapt attention of several Camponotus ants in the Costa Rican rainforest. If you are wondering, head is to the right.
The wonderful reference “The Ants” by Bert Hölldobler and E. O. Wilson was used to prepare this episode. We thank Don Harvey, Ted Schultz, and Jack Longino of the National Museum of Natural History, Smithsonian Institution and University of Utah, respectively, for help in identifying the riodinid larva and carpenter ant.