Suastus gremius has brown upperwings, the forewings are decorated with pale yellow hyaline spots at the ends of the cells and spaces 1b, 2, 3, 6, 7 and 8. Cilia in the tornal area are whitish. The undersides of the wings are grayish-brown with a number of fairly large and distinct black spots.
Larva: larvae of Suastus gremius are cylindrical, body color and head capsule are bright orange red with long setae in the posterior segment. At the next instar, the body of the caterpillar turns pale yellow and the head capsule is brownish with two white stripes.
Pupa: Suastus gremius pupa dark green on the thorax and wing pads, yellowish brown on the abdomen. The length of the pupa is about 19-20 mm.