248 



HARDWICKE'S SCIENCE-GOSSIP. 



Together with compound eyes, many insects are 

 furnished also with simple eyes, usually three in 

 number, and disposed in a triangle on the forehead. 

 The while fenestra, which in the cockroach lie 

 internal to the antennary sockets, may represent two 

 simple eyes which have lost their diojitric apparatus. 

 In many larvoe only simple eyes are found, and the 



gonal. In many insects the deep layer of each facet 

 is separable, and forms a concavo-convex layer of 

 different texture from the superficial and biconvex 

 lens. The facets taken together, are often described 

 as the cornea ; they represent the chitinous cuticle of 

 the integument. The subdivision of the cornea into 

 two layers of slightly different texture suggests an 



/;, / 



1 



.^ III u 



•pj^Jlp^^A 



f/i -—u 



111 h. 



^ til I 



Fig. 150. — Diagrammatic oullines of sections of the brain of a Cockroach. Only one side of the brain is here represented. The 

 numbers indicate the position in the series of thirty-four sections into which this brain was cut. al, antennary lobe ; nth, mush- 

 room bodies (r«//a'j), with their cellul.ir covering, c, and their stems [J>cduncles], st ; a, anterior nervous mass (.c<!«//i:«/«^) ; 

 111, median nervous mass {trabecula). 



compound eye is restricted to the adult form ; in 

 larval cockroaches, however, tlie compound eye is 

 large and functional. 



Each facet of the compound eye is the outermost 

 element of a series of parts, some dioptric and some 

 sensory, which forms one of a mass of radiating 

 rods or fibres. The facets are transparent, biconvex 

 and polygonal, often, Ijut not quite regularly, he\a- 



achromatic correction, and it is quite possible, though 

 unproved, that the two sets of prisms have different 

 dispersive powers. Beneath the cornea we find a 

 layer of crystalline cones, each of which rests by its 

 Ijase upon the inner surface of a facet, while its apex 

 is directed inwards towards the brain. The crystal- 

 line cones are transparent, refractive, and coated with 

 dark pigment ; in the cockroach they are compara- 



