xliv INTRODUCTION. 



The appearance of an iris in some insects is often 

 due to the presence of a highly coloured pigment 

 disposed behind the circumference of the lens. In the 

 Cicadinse the colour of this pigment is red, deep brown, 

 or black. The facets of the oculi are hexagonal, like 

 those of most other Homoptera, each facet having its 

 own cone lined with pigment. 



By dissection the bundle of filaments which com- 

 poses the optic nerve may be made visible, and each 

 cylinder shown to have its proper connection with the 

 cephalic ganglion. I have nothing more to add to these 

 details, but reference may be made to John Mailer's 

 treatise on the Eyes of Insects (p. 355 et scq.). His 

 investigations have been the authority for much of 

 what is here stated. 



The compound eyes of the Cicadinffi are very large. 

 It is difficult to account for the disproportionate size of 

 these organs in insects. We might predicate that the 

 apparatus which collects the largest amount of light 

 for the optic nerves would be found in crepuscular 

 insects, and those which make their homes in dark 

 places. In crepuscular Vertebrata, such as the cats 

 and the owls, we notice the large eye-balls and the 

 dilateable irides, but amongst light-loving insects we 

 find the elaborate visual organs of the Libcllula, the 

 gorgeous eyes of the Tabamis, and the broad hemi- 

 spherical compound systems of the Diptera — all these, 

 like the sun-loving Cicada, sport themselves in the hot 

 sunshine and glare of day. 



Doubtless large eyes, as a rule, are connected with 

 active habits ; and, apposite to this, naturalists are 

 familiar with the circumstance that some animals with 

 eyes adapted for broad daylight show tendencies to 

 lose them when bred in dark caves. 



Dr. Forel's late observations on the deep-water Fauna 

 of the Lakes of Switzerland have an interesting bearing 

 on this point. He shows that at depths below fifty 

 fathoms, where light does not penetrate, the eyes of 

 Gijrator are absent, and that at such great depths other 



