194 THE ENTOMOLOGIST. 



the four cells being usually indicated, in the imago, by the 

 persistence of four nuclei which lie immediately behind the 

 facet of the cornea. 3. The recipient rod, connected more 

 or less intimately with the inner extremity of the crystal 

 cone. These bodies, as is well known, converge towards the 

 optic ganglia, and form the greater part of the radiating 

 structure of the eyes. The author proposes the terms 

 retinula and rhabdom to designate the parts of this structure, 

 which exhibits a very different condition in the diff'erent 

 forms of compound eyes. 4. The pigment cells which sur- 

 round the crystal cone and the recipient rod. 



1. Aconic Eyes. — In these the primitive cells of the crystal 

 cone remain distinct throughout life, so that this organ 

 cannot be said to be developed. These cells are so arranged 

 that they form a funnel-shaped body, narrow at its inner 

 extremity, which is closely surrounded by pigment, so that 

 only a very small clear space is left at its apex opposite the 

 centre of the corresponding facet. The retinula consists of 

 seven cells, which are parallel or nearly parallel, except at 

 their outer extremities, where they curve towards each other, 

 so that the rods which they have imbedded in their substance 

 approach the opening in the pigment. The axial cell of the 

 retinula is most strongly developed, and appears alone to be 

 connected with the optic nerve by a single nerve fibre. 

 The rods which are contained in these cells are enlarged at 

 their outer extremities, and terminate in points behind. The 

 nematocerous Diptera, the Cimicidcc, the Dermaptera, and 

 apparently all the Coleoptera, except the Pentamera^ have 

 aconic eyes. 



2. Pseudoconic Eyes. — Dr. Grenacher describes the eye 

 of Tahdnus bovinus as a typical example ; it is characterised 

 bv the existence of a conical space enclosed in front by the 

 facet of the cornea; behind by four nucleated cells, corre- 

 sponding to those immediately behind the cornea in the 

 aconic eye, and surrounded by two large thin pigment cells. 

 This space is filled during life with a clear fluid, which con- 

 tains but little coagulable substance; it represents the crystal 

 cone of the euconic eye, but it cannot be considered to 

 represent it morphologically, as it lies outside the nucleated 

 crystal cells. The retinula consists as before of seven 

 elongated cells, but these are united so as to form a tube 



