Orthodenticle is a homeobox gene involved in Drosophila head development. It produces a homeodomain protein which controls the development of cells in the developing head of embryos. It defines the midline of the head, and is involved in the formation of the top side of the head, including the eyes.
The gene breaks the head down into subdomains; the medial subdomain (contains the ocelli); the mediolaterial ; and the lateral (just above the compound eyes). If orthodenticle is not expressed, structures from the lateral subdomain will be expressed all the way over the head - meaning that ocelli are not produced.
References
Orthodenticle Wikipedia(Text) CC BY-SA