98 ENTOMOLOGICAL NEWS [April, '20 



definite in their application; that facet be used for the 

 external surface and that ommatidium be used for the en" 

 tire element including the cornea and consequently the facet. 

 This provides a taxonomic term, facet, and a morphological 

 or histological term, ommatidium. It is not practical to ex- 

 clude the facet from inclusion with the ommatidium since the 

 facet is only the outer surface of the cornea'which must be 

 considered as a part of each ommatidium. Compound eyes 

 are present in both nymphs and, adults of ametabolous and 

 exometabolous insects, but only in the adults of entometabo- 

 lous insects. The developing compound eyes of the adult can 

 frequently be seen in the pupa of entometabolous insects and 

 rarely in their larvae, but these stages are never provided with 

 functional compound eyes. 



Many adult insects have a group of simple eyes located on 

 the dorsal or cephalic aspect of the head. While there are 

 usually three of these simple eyes arranged in the form of a 

 triangle with the apex of the triangle directed toward the 

 mouth, there are never more than three of these simple eyes, 

 sometimes there are only two, rarely only one, and frequently 

 all are wanting. The pair of cephalic discs described by Kochi 

 as probably representing a primitive divided ocellus appears 

 to be only discs for the attachment of muscles. Each of 

 these simple eyes is universally known as an ocellus, plural 

 ocelli. The ocellus forming the apex of the triangle is known 

 as the median ocellus and each of the others as a lateral ocel- 

 lus. When only two ocelli are present, it is the median ocel- 

 lus that is wanting and when only a single ocellus is present, 

 it is the lateral ocelli that are wanting. The ocelli are also 

 known as stemmata, singular stemma. Ocelli are never pres- 

 ent in ametabolous insects and only in the adults of exometa- 

 bolous and entometabolous insects. They can soriietimes be 

 seen as black spots through the transparent cuticle of some 

 nymphs and they have been described as ocelli, but functional 

 ocelli are never present in nymphs, larvae, or pupae. 



Ametabolous insects and those with exometabolous meta- 

 morphosis, as the nymphs and adults of Collembola, Mallo- 

 phaga, etc., and the larvae of insects with an indirect meta- 



