Fig. 2.

Development of beetle horns. Life cycle shown for the dung beetle Onthophagus taurus. After hatching, beetles pass through three larval instars before molting first into a pupa and then into an adult. Black arrows indicate feeding periods; gray arrows indicate nonfeeding periods. Arrow thickness approximates overall animal body size, and gaps between arrows indicate molting events. The final (third) larval instar can be divided into a feeding period and a nonfeeding prepupal period. Drawings inside the arrows illustrate egg, first through third larval instars, prepupa, pupa, and adult. Horn development can be divided into two stages: a period of horn growth when horn cell proliferation occurs, and a period of horn remodeling. The top box shows horn growth. Front view of thoracic (green) and head (blue) horn discs are shown, along with two profile views of the prepupal head and thorax during this stage. The side box shows horn remodeling. The drawings illustrate the profile of a large male just after pupation and the head and thorax of the same male at two later stages during the pupal period. The head horns are remodeled slightly, to form a pair of curved and slender adult horns. The pupal thoracic horn is removed completely, and is not present in adults. Close-up profiles of prepupa and pupa are adapted from figure 2 of ref. 48.