The ommatidial structure of Archichauliodes, as shown by electron microscopy, agrees with its systematic position as a primitive endopterygote insect. The eight retinula cells form a long column with distally placed nuclei and proximally located rhabdomeres. In the dark-adapted eye four of the retinula cell bodies reach the cone tip but in the light-adapted eye there is a crystalline tract up to 100 $\mu $m long formed by extensions of the cone. Microtubules are abundant in the cells which move on adaptation, i.e. the cone cell cytoplasm and the primary pigment cells which surround the crystalline tract. This paper forms the first of a group on the neuropteran type of compound eye, found also in some Coleoptera and Lepidoptera.