Abdomen morphology and venom characteristics of a foaming ant (Pachycondyla sp.) from Cambodia

In most aculeate hymenopterans, venom is delivered via the sting, which plays a central role in predation and defense. Some ants, however, display extreme modifications of the venom apparatus. Pachycondyla rufipes, a little-studied ant found in Southeast Asia, can deliver its venom not just as a liquid through its sting but also as a thick foam emitted when the sting is retracted. Foam production and function has been well studied in other animals, for example the shelters of spittlebug nymphs and egg rafts of frogs, but it is unclear how or why P. rufipes produces its foam venom. To address this gap, we studied the functional morphology of these foaming ants, the chemistry of their venom, and the mechanical properties of the resulting foam. Using a combination of high-speed videography, light and electron microscopy, and X-ray microtomography, we examined the musculoskeletal and glandular morphology of the abdomen to better understand the mechanism of foam delivery. We found that the venom sac is one of the largest structures in the abdomen, extending three-quarters of its length. Large intersegmental muscles telescope the abdominal segments and appear to facilitate aeration of the venom as it leaves the ant with a volumetric flow rate of approximately 35 mm³ sec^-1 of foam. Manual aeration of the venom of non-foaming ants (Odontomachus brunneus and Diacamma sp.) indicated that the chemistry of P. rufipes venom might also contribute to its foaming behavior. Rheology measurements show that foam venom is a viscoelastic solid, exhibiting and an elastic shear modulus of ~100 Pa and a breaking stress in shear of ~30 Pa. The chemical and mechanical properties of the foam provide evidence that the foam may function not as a chemical defense against predators but as a mechanical restraint to trap nest invaders or other predators.