Why do only some ants sting?

Although these two ants in northern Argentina look like they're ignoring each other, they are in fact doing just the opposite. This end-to-end confrontation is an intense chemical duel. What's particularly interesting about the image is the juxtaposition of two different defense systems.

At left is Forelius nigriventris, a speedy little insect armed with a nozzle at the tip of the abdomen that releases a chemical cocktail into the air. The Forelius colony discovered a breach in the mound of a neighboring fire ant Solenopsis invicta and has recruited hundreds of workers to pilfer the tasty and protein-packed fire ant larvae.

The fire ants respond with their standard weaponry, extruding a volatile venom on the stinger (visible as a little droplet) and waving it about, attempting to flick or smear the venom on the attackers if they approach closely. In contrast, Forelius lacks a sting altogether.

Why do some ants have a stinger, and others not?

If we map the stinger to an ant phylogeny (simplified from Brady et al 2006 using the program Mesquite 2.5) we see that all the early lineages have one. Of course, this is a sensible observation given that ants evolved from within the wasps. The Ur-Ant possessed a waspy sting, and the device is carried through to many extant species.

We can also see that many lineages have lost the sting. The loss is repeated in several recent groups, suggesting a strong trend towards stingless ants. It is as if stingers are more burden than benefit.

While we do not know for certain why the stinger has become a liability among so many modern ants, the ecology of species with differing sting morphologies suggests that the loss of the sting is related to some significant changes in lifestyle over the course of ant evolution. As no one has yet done the requisite work to properly test hypotheses of sting evolution, permit me to speculate about what I think is driving the observed pattern.

The early-diverging ant lineages- the ones colored in black at the top of the phylogeny- are nearly all predators. Most forage alone, without the help of nestmates, and their prey is not killed but paralyzed. The prey remain alive until consumed, a nifty trick for ants with small colonies. This habit of catching and maintaining live prey is not unlike the strategy of predatory wasps that capture spiders or caterpillars for their young. Only, ants are social and carry their quarry back to communal nests.

Below is an example of one such species, the western dracula ant Amblyopone oregonensis. This insect specializes on geophilomorph centipedes in the old growth forests of western North America. As we can see, the stinger is well-developed and is used primarily to inject a blend of paralyzing enzymes into the centipedes. The hapless myriapod is then fed to the voracious ant larvae.

A further example, from a different subfamily, is the Indian Jumping Ant Harpegnathos saltator. Here, one has paralyzed a cricket:

Many modern ants have since moved away from this ancestral habit of solitary hunting. The more diverse lineages retrieve prey in groups and get much of their sustenance from hemipteran honeydew or scavenged detritus. It is in these lineages that the sting often is lost or reduced. Where present, the function usually switches from prey capture to defense.

The infamous Solenopsis fire ants display an intermediate condition. They retain a functional stinger primarily as a weapon against vertebrates. Here's one valiantly defending her nest against the intrusive photographer:

While fire ants may also use their sting to subdue prey, much of what they eat is either plant-based (via hemipteran honeydew) or scavenged. Consequently they are not as dependent on the organ as more ancient ants like Amblyopone and Harpegnathos.

And a stinger is not always so useful against ant competitors. In combat with other ants, fire ants may sting. But just as often they resort to waving volatiles about, or smearing venom, instead of trying to pierce the chitinous armor of their enemies. For example, notice the fire ant's raised stinger in this skirmish with Argentine ants (Linepithema humile):

And again, in the aforementioned conflict with Forelius nigriventris:

A further step in stinger reduction is displayed in the Crematogaster acrobat ant. She does have a stinger, but unlike the fire ant's it is thin, weak, and floppy. Instead of impaling her enemies, she uses her sting as a brush for smearing a toxic slurry. Here C. depilis from southern California demonstrates the chemistry:

Assaulted by an armored and sting-resistant Podomyrma, some Australian Crematogaster fight back with a noxious frothy smear.

Two of the largest subfamilies, the dolichoderinae and the formicinae, have dispensed with the sting altogether. Species in these groups tend to have large colonies, use group prey retrieval, and are heavily subsidized by honeydew and nectar. Some predatory functions of the sting are replaced by concerted group action, as shown by these African weaver ants:

Other defensive functions are replaced by airborne volatiles that in some species can be aimed and sprayed at a distance:

In general, then, what seems to favor a sting is either the need for foraging ants to paralyze prey- the ancestral state for the ants- or the need to defend against vertebrate enemies. Ants that do not require live prey, or ants whose lives involve a lot of fighting against other ants, have often arrived at more effective tools.

As to the outcome of the Forelius vs. fire ant battle, the opposing sides maintained a standstill for the entire morning I watched them. The Forelius never penetrated deep enough into the fire ant nest to plunder any significant amounts of brood, but the fire ants also failed to repel their attackers.

A direct hit by Forelius:

Competing chemicals:

Ferocious fire ants:

Although the long term evolutionary trend might seem to favor the airborne chemical warfare of Forelius, for the morning both weapons systems remained locked in an equilibrium. What an equivalent battle might look like in another 50 million years is anyone's guess.