THE APPE^'DAOES OF THE HEAD. 25 



We will first describe the ocelli, which are theoretically the 

 most anterior organs of the liead, endinu;' with the basal appen- 

 dages, tiie labium (second maxiUa}) being the hindermost. 



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

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

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

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

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

 port states that the tegument is transparent and convex, 

 resembling a true cornea, or ej'e-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 amilogue 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 see more distant objects. The ocelli constitule the 

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

 Arachnida, and the larvai of many Six-footed Insects ; tliey 

 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 

 thrown l)ack, 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 ej-es of the larva increase 



jj^|jj in munber, and finally coalesce to form the compound 



^10' eye, or com])ouud cornea, the surface of which is 



Fig. 34. very convex and i)i-()tid)erant in the predaccous insects, 



or those requiring an extended Held of vision. 



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

 to 3,G50, the latter number being counted by Geoffroy in the 

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

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



Fig. 3.>. Ocelli of three species of Sand-wasps, Pompilus. — From Cresson. 

 Fig. .'U. Three hexagonal facets of the compound e3'e of a fossil Dragon-fly, 

 (treatly aiaguiJied. — From Dawson. 



