32 ENTOMOLOGY 
eter the facets range from .o16 mm. (Lyc@na) to .og4 mm. 
(Cerambyx). Their number is often enormous; thus the 
house fly (Wusca domestica) has 4,000 to each eye, a butter- 
fly (Papilio) 17,000, a beetle (Mor- 
della) 25,000 and a sphingid moth 
Eres 38. 
27,000; on the other hand, ants have 
from 400 down, the worker ant of 
Eciton having at most a single facet 
on each side of the head. 
Ocelli.—The simple eyes, or ocelli, 
appear as small polished lenses, either 
lateral or dorsal in position. Lateral 
ocelli (Fig. 38) occur in the larve of 
most holometabolous insects and in 
parasitic forms. Dorsal ocelli, sup- 
plementary to the compound eyes, 
Head of a caterpillar, 
Samia cecropia, to show occur on or near the vertex, and are 
lateral ocelli. . 
more commonly three in number, ar- 
ranged in a triangle, as in Odonata, Diptera (Fig. 39) and 
Hymenoptera (Fig. 40) as well as many Orthoptera and He- 
miptera. Few beetles have ocelli and almost no butterflies 
ErGs30: 
Ocelli and compound eyes of a fly, Phormia regina. A, male; B, female. 
(Lerema accius with its one ocellus being the only exception 
known), though not a few moths have two ocell1. 
As explained beyond, the compound eyes are adapted to per- 
ceive form and movements and the ocelli to form images of 
