The larva of Syssphinx tamaulipasiana (Brechlin and Meister) is described and compared with the three congeners with which it co-occurs in the Lower Rio Grande Valley of Texas: S. albolineata (Grote & Robinson), S. blanchardi (Ferguson), and S. heiligbrodti (Harvey). All four species feed exclusively on mimosid trees, but have nearly non-overlapping host use. We illustrate six instars of S. tamaulipasiana from a lab-reared cohort and wild last instars of all four moths, and offer a key to their last instars. Three of the four species (all but S. heiligbrodti) are restricted to the Lower Rio Grande Valley, where they are threatened by urbanization and insularization.