152 



DIFFERENT FORMS OF EYES. 



The last form differs principally from the two first 

 in that the elements which constitute the crystalline 

 cone and the retinula have become completely coalesced 

 and solidified. The differences are, no doubt, im- 

 portant, but I need not enter into them at length here. 



Even the eucone eyes differ considerably, as may be 

 seen from the following figures, representing (Fig. 103) 

 an eyelet from the eye of a cockroach (Periplaneta), and 

 (Fig. 104) one from that of a cockchafer (Melolontha), 

 both taken from Grenacher. 



•t2 



Fig. 103.— Eyelet of cockroach (after 

 Grenacher). If, Cornea ; kk, crys- 

 talline cone ; pg', pigment cell ; rl, 

 retinula ; rm, rhabdom. 



Fig. 104.— Eyelet of cockchafer 

 (after Grenacher). If, Cor- 

 nea; kk, crystalline cone; 

 PO^ Pg\ pigment cells ; rl, 

 retinula ; rV, rhabdom. 



With some few exceptions (Corethra, Libellula, etc.), 

 the larvae of insects do not possess faceted eyes; indeed, 

 as a general rule their powers of vision are very limited, 

 or they are altogether blind. Most caterpillars have 



