

ANATOMY AND PHYSIOLOGY 



other terrestrial beetles of the same family. Superficially, a compound 

 eye is divided into minute areas, or facets, which though circular in the 

 agglomerate type of eye (Fig. -3 7) are commonly more or less -hexagonal 

 (Fig. 38), as the result of mutual pressure. These facets are not 

 necessarily equal in size, for in dragon flies the dorsal facets are fre- 

 quently larger than the ventral. In diame- 

 ter the facets range from .016 mm. (Lyc&na) 

 to .094 mm. (Cerambyx). Their number is 

 often enormous; thus the house fly (Musca 

 domestica) has 4,000 to each eye, a butterfly 

 (Papilio) 17,000, a beetle (Mordella) 25,000 

 and a sphingidmoth 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 dor- 

 sal in position. Lateral ocelli (Fig. 39) 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, arranged in a triangle, as in 

 Odonata, Diptera (Fig. 40) and Hymenoptera (Fig. 41) as well as many 

 Orthoptera and Hemiptera. Few beetles have ocelli and almost no 



FIG. 39. Head of a cater- 

 pillar Samia cecropia, to show 

 lateral ocelli. 



FIG. 40. Ocelli and compound eyes of a fly, Phormia regina. A, male; B, female. 



butterflies (Lerema accius 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 perceive 

 form and movements and the ocelli to form images of objects at close 

 range or simply to distinguish between light and darkness. 



Sexual Differences in Eyes. In most Diptera (Fig. 40) and in 

 Hymenoptera (Fig. 41) and Ephemeridae as well, the eyes of the male are 



