ON THK EYES OF ARTHROPODS. 195 



which extends from the crystal body to the optic nerve ; the 

 cells are distinctly nucleated, and the tube which they form 

 contains seven long thread-like rods, which are attached to 

 its inner surface. In Musca iwmitoria the outer extremities 

 of the rods are somewhat thickened, are less highly refractive, 

 and jsass between the cells of the crystal body. These were 

 mistaken by M. Schultze for a fasciculus of fine nerve 

 fibres. Pseudoconic eyes are found in all the true Diptera 

 {Heterocera) . 



3. Euconic Eyes. — Those in which a crystal-like body is 

 found between the retinula and the facet of the cornea formed 

 by the fusion of the four cells already mentioned : the nuclei 

 of these cells are found between this structure and the cornea, 

 at least when they can still be recognized. In the greater 

 nuuiber of these eyes the number of cellular elements which 

 form the retinula is still seven. There are, however, many 

 deviations from this number. In bees and hornets there are 

 eight cells, as there are also in a great exotic species of 

 Cicada. In Orthoptera and in Geodephagous and Hydra- 

 dephagous Coleoptera tliere are four ; in some of the latter, 

 however, there are certainly more than four, but only four 

 take any part in the formation of the rods. Jn the diurnal 

 Lepidoptera the estimation of the number of component 

 cells is so difficult, owiug to their intimate fusion, that the 

 author states that he can say nothing certain about it. In 

 the euconic eye the cells of the retinula are united into a 

 tube which closely surrounds the rhabdom, an angular rod 

 consisting of the united rods of the retinal elements. In 

 some pentamerous Coleoptera, the Orthoplera, Hymenoplera, 

 Cicadte, dragonflies, and dnnwdX Lepidoptera, the retinula is 

 of nearly equal thickness, except that it is slightly narrowed 

 within on account of the radial arrangement of these organs. 

 In these insects the rhabdom is not remarkably modified in 

 any part of its course. In the CrepusciiUiria and in the 

 Nocturna, in some Coleoptera and in the PlcryganidcB, it 

 usually exhibits two swellings — a smaller conical swelling 

 immediately behind the crystal cone, and a more considerable 

 enlargement at its inner extremity. The outer swelling con- 

 tains the nuclei of the retinula ; and the posterior is remark- 

 able from the manner in which the rhabdom sends broad 

 plate-like processes between the constituent cells, so that in 



