Aberrant venom glands in Amblyoponini (Formicidae, Ponerinae): morphology, ultrastructure and histochemistry

All ants studied so far have a convoluted gland portion as part of their venom gland. The venom glands of the amblyoponine genera Amblyopone, Mystrium, Onychomyrmex and Prionopelta, however, are characterized by the absence of a convoluted gland, which makes this ponerine tribe exceptional among ants. The venom gland of Amblyoponini is similar to that of mutillid wasps, which supports a possible tiphiid ancestral form. Ultrastructurally, the cells of the free tubules do not differ from those of most other stinging ants. A separate series of secretory cells, each with its own end apparatus, lines part of the venom reservoir. These few secretory cells near the orifice of the glandular tubule into the reservoir show an end apparatus with wide extracellular spaces and are similar to those lining the venom reservoir in honeybees. Several muscles run parallel with the longitudinal axis of the reservoir, which results in the curved appearance of replenished reservoirs when these muscles contract. The absence of a bourreleted convoluted gland in Amblyoponini, and the presence of only a few lipoidal reservoir cells (histochemical results), when compared with histochemistry and ultrastructure of the convoluted gland in other Hymenoptera investigated, leads us to conclude that the Amblyoponini most likely possess a venom which contains only a fraction of lipoids and/ or pheromones, hence suggesting the presence of a mainly proteinaceous venom used against their prey. This idea is supported by the fact that several other hymenopteran groups, which are not reported to contain significant amounts of pheromones in their venom glands, also lack the convoluted gland tissue.