The metathoracic scent gland efferent system (MTSG) is one of the apomorphic and diagnostic structures in true bugs (Heteroptera), developed only in adults. External structures of MTSG, such as ostiole, vestibular scar, peritreme, and evaporatorium (terminology by Kment & Vilímová 2010: Zootaxa 2706: 1–77) provide useful characters for systematics of Heteroptera, as was documented e.g. for Reduvioidea and Coreoidea. In the superfamily Pyrrhocoroidea (firebugs, cotton stainers) including two families (Largidae, Pyrrhocoridae) with 72 genera and more than 660 species, the structure of MTSG had been only poorly known so far. We have examined 16 genera of Largidae and 41 genera of Pyrrhocoridae mostly by scanning electron microscopy. We have detected two basic types of external structures of MTSG: 1. peritremal disc in Largidae with two subtypes differing between subfamilies Larginae and Physopeltinae; 2. auricle in Pyrrhocoridae with various modifications and reductions. The situation in Largidae supports the present division into two subfamilies which was based on other morphological features. In the Pyrrhocoridae, there are several cases of modifications of the peritreme and the evaporatorium, which may indicate at least two groups of genera and several subgroups; these groupings, however, still need to be tested cladistically using a larger set of morphological characters, not only MTSG. Notably, some genera in Pyrrhocoridae (Callibaphus, Cenaeus, Dermatinus, Gromierus, Jourdainana, Paradindymus, Probergrothius, Pyrrhocoris, Pyrrhopeplus, Scantius, and Sicnatus) are distinct in sharing a more or less reduced auricle, which may represent a synapomorphy or convergence. The study is a part of an ongoing dissertation focusing on the morphology and phylogeny of the Pyrrhocoroidea.