22 



INTRODUCTION. 



b.m 



l.p 



frequenting beetles (as Anillus), are blind, and only possess quite 

 rudimentary organs of vision. 



The compound facetted eye is one of the most intricate 

 and wonderful structures in the whole animal kingdom. Each 



facet is the outside covering 

 or cornea of an elongate and 

 complex apparatus called an om- 

 matidium (fig. 16), each consisting 

 of a corneal or crystalline lens (cor.), 

 under which comes the crystalline 

 cone {con.), which is borne on a rod- 

 like structure or rhabdom (r.) ;, 

 underneath the rhabdoms is found 

 the basal or fenestrated membrane 

 (6.m.), which is backed by a mass of 

 nerves ; these latter penetrate the 

 membrane and run up into the space 

 between the rhabdoms. According 

 to Sharp and others the penetrating- 

 nerve have their distal extremities 

 connected with the delicate sheaths, 

 by one of which each rhabdom is sur- 

 rounded, the combination of sheath 

 and nerves forming a retinula. 

 Two zones or layers of pigment are 

 present, one, in which the pigment 

 cells are shorter, enclosing the chief 

 part of the crystalline cone, called 

 the iris-pigment, and the other, in 

 which the pigment cells are longer, 

 surrounding the lower part of the 

 retinula and called the ret ina - 

 pigment ; the nerve-fibres are branches of the optic nerve. 

 Tracheae or air passages also pass through the fenestrated 

 membrane. 



The ommatidia vary in number very greatly, and in some beetles 

 (e. g. Mordella) the eye is said to contain as many as 25,000. In 

 some families and tribes they vary in different species, and this 

 variation (of finely or coarsely facetted eyes) has been made use of 

 occasionally (as in the Languriin^e) as a generic character. 



The structure of the ocelli or simple eyes is very different. 

 They consist of a cornea, lens, nerve-fibres, and a retina, together 

 with pigment cells ; they are the ordinary organs of vision of 

 Coleopterous larvae, but are very rarely found in the perfect beetles. 

 The function of the oeelh has been much disputed, but according 

 to Lubbock and Fore), followed by Packard, Folsom, and others 

 they are useful in dark places and for near vision. The last named 

 writer (Entomology, with special reference to its Biological and 

 Economic Aspects, 1906, p. Ill) says : " Since the form of the lens 

 is fixed and also the distance between the lens and the retina 



Fig. 16. — Two ommatidia from 

 the eye of Colymbetes fusci/s. 

 cor., cornea; con., crystalline 

 cone ; v., rhabdom ; b.m., basal 

 membrane, with nerve struc- 

 tures beneath; i.p., iris-pig- 

 ment ; r.p., retina-pigment. 

 (After Exner.) 



