THE APPENDAGES OF THE HEAD. 25 



We will first describe the ocelli, which are theoretically the 

 most anterior organs of the head, ending with the basal appen- 

 dages, the labium (second maxilla?) being the hindermost. 



The simple eye, Ocellus, or Stemma, is the simplest form of 

 the eye. Its most elementary form (seen in the larva of the 

 Bot-fly and the Cecidomyian larva of Miastor) is that of a brown 

 spot, or group of pigment-cells lodged under the skin and 

 against which a nerve-filament impinges. Over this spot New- 

 port states that the tegument is transparent and convex, 

 resembling a true cornea, or eye-lens. A well-developed 

 ocellus consists, according to Newport, of a "very convex, 

 smooth, single cornea, beneath which is a spherical crystalline 

 lens, resting upon the plano-convex surface of the expanded 

 vitreous humor, the analogue of the transparent cones of the 

 compound eyes." Miiller believes that the function of the ocelli 

 is the perception of nearer objects, while that of the compound 

 eyes is to sec more distant objects. The ocelli constitute the 

 only visual organs in the Myriapods (except Cermatia), the 

 Arachnida, and the larvae of many Six-footed Insects ; they 

 are usually from one to six on a side. In adult insects 

 they are generally three in number, and 

 are generally present except in the large 

 majority of Coleoptera. Their normal site 

 is in front of the eyes, but they are usually Fig. 33. 



thrown back, during the growth of the insect, behind the eyes, 

 on the vertex, or topmost part of the head (Fig. 33). 



The Compound Eyes are a congeries of simple eyes. During 

 the growth of the insect the simple eyes of the larva increase 

 in number, and finally coalesce to form the compound 

 ej-e, or compound cornea, the surface of which is 

 Fig. 34. very convex and protuberant in the predaceous insects, 

 or those requiring an extended field of vision. 



The number of facets, or corneas, vary from fifty (in the Ant) 

 to 3,650, the latter number being counted by Geoffrey in the 

 eye of a Butterfly. These facets are usually hexagonal, as in 

 the Dragon-fly (Fig. 34), or, rarely, quadrangular. 



FIG. 33. Ocelli of three species of Sand- wasps, Pompilus. From Cresson. 

 FIG. 34. Three hexagonal facets of the compound eye of a fossil Dragon-fly, 

 greatly magnified. From Dawson. 



