ENTOMOLOGY 



FIG. 38. 



eter the facets range from .016 mm. (Lyccena) to .094 mm. 

 (Cerambyx). Their number is often enormous; thus the 

 house fly (Musca domestica) has 4,000 to each eye, a butter- 

 fly (Papilio) 17,000, a beetle (Mor- 

 della) 25,000 and a sphingid moth 

 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 larvae of 

 most holometabolous insects and in 

 parasitic forms. Dorsal ocelli, sup- 

 plementary to the compound eyes, 

 occur on or near the vertex, and are 

 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 



FIG. 39. 



Head of a caterpillar, 

 Samia cecropia, to show 

 lateral ocelli. 



Ocelli and compound eyes of a fly, Phormia regina. A, male; B, female. 



(Lerema acelus with its one ocellus being the only exception 

 known), though not a few moths have two ocelli. 



As explained beyond, the compound eyes are adapted to per- 

 ceive form and movements and the ocelli to form images of 



