Sexual trimorphism is seen in

Sexual trimorphism is seen in the Stag beetle.
Trimorphism is the property exhibited by certain species of having or occurring in three different forms.
Adult male stag beetles have characteristic antler-like mandibles and can grow up to a whopping 75 mm in length. Females have much smaller mandibles and grow between 30 - 50 mm. Both males and females have shiny black heads and thoraxes and brown chestnut-coloured wing cases.