ANATOMY AND PHYSIOLOGY 



size, for in dragon flies the dorsal facets are frequently larger than the 

 ventral. In diameter 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 butterfly (Papilio) 17,000, a 

 beetle (M or delta) 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, supplemen- 

 tary 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. 39) and Hymenoptera (Fig. 40) 

 as well as many Orthoptera and Hemiptera. 

 Few beetles have ocelli and almost no butter- 

 flies (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. 



FIG. 38. Head of a cater- 

 pillar, Samia cecropia, to show 

 lateral ocelli. 



FIG. 39. Ocelli and compound eyes of a fly, Phormia regina. A, male; B, female. 



Sexual Differences in Eyes. In most Diptera (Fig. 39) and in 

 Hymenoptera (Fig. 40) and Ephemeridae as well, the eyes of the male are 

 larger and closer together (holoptic) than those of the female (dichoptic). 

 This difference is attributed to the fact that the male is more active than 

 the female, especially in the matter of seeking out the opposite sex. 

 Among ants of the same species the different forms may differ greatly 



