Why is this ant doing a face-plant?

According to a new study by Olivier Roux et al in PLoS One, she is spreading pheromones from a previously unknown gland:

Abstract: In Oecophylla, an ant genus comprising two territorially dominant arboreal species, workers are known to (1) use anal spots to mark their territories, (2) drag their gaster along the substrate to deposit short-range recruitment trails, and (3) drag the extruded rectal gland along the substrate to deposit the trails used in long-range recruitment. Here we study an overlooked but important marking behavior in which O. longinoda workers first rub the underside of their mandibles onto the substrate, and thenâin a surprising postureâtilt their head and also rub the upper side of their mandibles. We demonstrate that this behavior is used to recruit nestmates. Its frequency varies with the rate at which a new territory, a sugary food source, a prey item, or an alien ant are discovered. Microscopy analyses showed that both the upper side and the underside of the mandibles possess pores linked to secretory glands. So, by rubbing their mandibles onto the substrate, the workers probably spread a secretion from these glands that is involved in nestmate recruitment.

What's remarkable here is that Oecophylla is one of the most conspicuous and well-studied African ants, and yet researchers are still finding entirely new behaviors and entirely new morphological structures.  Imagine what discoveries are hiding among the hordes of lesser-known species.

photo from Roux et al, Figure 1.