Optic Organs of Tribolium confusum and T. castaneum and Their Usefulness in Age Determination (Coleoptera: Tenebrionidae)

Two pairs of stemmata are present in all larval instars and in the early pupa of T. confusum Jacquelin du Val and T. castaneum (Herbst). They degenerate in the pupa as the compound eyes begin to develop. Larval instars are separable by determining the distance between the two pairs of stemmata. The stemmata are located near the antennae from the first throughout the early part of the last larval instar. With each successive moult the distance between the stemmata increases. During the latter part of the last larval instar, however, the interstemmatal distance decreases as the stemmata migrate to a position near the vertex. The degree of stemmatal migration can be used as an index for determining the ages of last larval instars. The orientation of the stemmata in the latter part of the last larval instar also differs from that seen in other instars. The pupal age can be estimated by counting the number of rows and considering the degree of development of the ommatidia of the compound eyes.