The Penicillophorinae (Phengodidae) are a rather morphologically modified and enigmatic group of railroad-worm beetles. Specimens are rarely collected, and nearly all species are only known from their holotypes. A recent trip by AKH to Guatemala resulted in the collection of a specimen of Adendrocera Wittmer, 1976, the second ever known to be collected for this genus. Further studies of that individual indicated that it was a new species. In this paper, we describe Adendrocera carmelita Roza, Hansen, and Ferreira, new species and redescribe Adendrocera flavula Wittmer, 1976, the type species of the genus. We note that Adendrocera is of feminine gender and correct the inflection of the adjective constituting the type species epithet flavulum (neuter) to flavula (feminine). We also discuss the hypothesis that the convergent morphology within this subfamily might be a result of miniaturization and paedomorphic processes.