(A-D) The Drosophila melanogaster, a holometabolous insect, (A) adult. The adult eye compound eye of Drosophila develops from (B) larval eye imaginal disc. (C, D) The SEM images of the Drosophila adult compound eye comprising of 600-800 unit eyes. The inter-ommatidial bristles are present in the Drosophila eye. In Drosophila adult fly, eye is present in a socket on the head where eye is posterior and antenna is positioned anteriorly. There is no distinct proximo-distal (PD) axis. (E) Stalk-eyed fly, C. whitei (F) larval eye-antennal imaginal disc, which metamorphose into (G) the adult compound eye. (F) The eye-antennal imaginal disc of stalk-eyed fly has a distinct lateral extension, which connects the eye with the antennal field (arrowhead). The compound eye of stalk-eyed fly is present on a lateral extension from the adult head, which exhibits a prominent proximo-distal (PD) axis. Antenna is located posterior to the eye on the inner side of the stalk. (H) The compound eye comprises of many unit eyes, which lack the inter-ommatidial bristles. (The orientation of all imaginal discs in the figure is posterior to left and dorsal up. [AN: antenna].