The structure and function of simple eyes of insects

In addition to the compound eyes,most of the insects have some simple eyes called ocelli.Adult and nymphal hemimetabolous insects possess some simple eyes on the head between the compound eyes called dorsal ocelli.Dorsal ocelli vary a lot in number and in structure,it also vary somewhat,but typically it is made up of a corneal lens,a layer of corneagenous cells(which secrets the lens),and the retina(which is composed of up to 1000 photosensitive cells depending on the species).Dorsal ocelli have been shown to be low light sensitive,but are apparently not important in image perception.They may be “the stimulatory organs“ that increase the sensitivity of the compound eyes.Many larvae of holometabolous insects lack both compound eyes and dorsal ocelli,but they possess some simple ocellilike stemmata on each side of the head.The structure of the stemmata is similar to an individual ommatidium of compound eyes.Typically it consists of a cornea,a crystalline body and a number of retinal cells forming a rhabdom.Stemmata are the sole visual organs in holometabolous larvae.In various insects they have been shown to be involved with color,form and distance perception like compound eyes.