RESUMO
Mythimna (Pseudaletia) sequax Franclemont, 1951 (Lepidoptera: Noctuidade) is the most important armyworm in the American tropics and subtropics. In this study, we describe the external morphology of the immatures and adults, based on larvae fed on quicuio grass (Pennisetum clandestinum Hochst. ex Chiov - Poaceae). The external morphology of the eggs, larvae, pupae, and adults is described and illustrated. Important taxonomic structures of the larvae are described, including hypopharynx, spinneret, and mandible. Adult structures that allow the differentiation of the species, such as male genitalia and scent brushes, are also described. The results are compared and discussed based on morph functionality and with publications on the congeneric species.