Oryctes gnu is one of the largest species of
Oryctes and is usually larger than 50 mm. In his taxonomic review of
Oryctes species,
Sebö Endrödi gives the length as 54–65 mm for 9 examined specimens.
O. gnu can be confused with
Oryctes rhinoceros, a species with a similar distribution range. Both species belong to the subgenus
Rykanes which is characterized by the end of the hind tibia having only 2 fixed spikes. This similarity has been already noted in the original description of
O. gnu as well as that of
O. trituberculatus, a synonym of
O. gnu, and by various other authors. However,
O. rhinoceros is smaller, usually less than 50 mm long, narrower and males of both species can be readily separated by the shape of the pronotum. In
O. gnu the depression (concavity) on the pronotum is very large, broad and has 3 knobs (tubercles) on its hind margin. The depression is smaller and narrower in
O. rhinoceros with only 2 knobs, see the arrows in the illustration on the right side. The head of
Oryctes gnu has a long horn (longer than in
O. rhinoceros) which is strongly curved backwards at the tip. In the diameter, the horn is compressed laterally, but in
O. rhinoceros it is compressed in the anterior-posterior direction. The clypeus of
O. gnu is very broad with a central deep indent that reaches the horn. Its corners are narrow, almost thorn-like. The elytra are longer than their combined width (about 35:30). Their surface is covered with many pits which also form several double rows. In males, the last abdominal segment, the pygidium, is rounded with no hair on its dorsal side and few, fine hairs on its ventral side. In females, the pygidium is more pointed and covered with short hair. ==Biology==