INSECT VARIETY. 



99 



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[g) which pass to the optic ganglion. The faceted eyes 

 are classed by Di-. Grenacher according to the development of 

 the crystal-like cone, as follows : Aconic eyes, in which the 

 cellular structure of the crystal-like cone is distinct — Nemato- 

 cerous Flies, Cimicidae, Earwigs, and Coleoptera, with excep- 

 tion of the Pentamera; Pseudaconic eyes, where the crystal- 

 like cone is represented by a conical space behind the facet of 

 the cornea filled with a clear fluid, and by four nucleated cells 

 posterior, as heteroceral flies, Tahanus, and Micsca ; Euconic eyes in 

 which the cellular structure of the crystal-like body is alone trace- 

 able posterior to the cornea 

 — Pentamerous Beetles, 

 Crickets, and Grasshoppers, 

 Bees, Cicadse, Phryganidae, 

 Dragon Flies, and Butter- 

 flies. These are thought 

 to have the most perfect 

 organs of vision.^ In 

 Crustaceans the structure 

 of the faceted eyes is in 

 the main that of the euconic 

 in Insecta. 



The faceted eyes are, of 

 course, immobile, and form 

 two distinct lateral images, and thus, if deficient in 

 perception, they have great range of vision. They vary 

 considerably in direction. In aquatic beetles and the 

 Leucanidce they are somewhat triangular, divided by an 

 angular lap of the integument, and directed downwards and 

 upwards. In Flies and Dragon-flies they are often large 

 and conical, directed upwards and forwards, as in the Cattle 

 and House Flies. In Orthoptera they are ellipsoid, vertical, 

 and directed sideways. In Longicorn Beetles they are directed 

 backwards and lunate, encompassing the tactile antennae, which 

 anteriorly supplant their office. In some Flies and Neuroptera 

 they occupy projections of the head. The maximum perfection 

 of the compound eye, according to Dr. Lowne, is attained in the 



* The ramifications of the trachct surround the oculus and commingle M'ith 

 its nerves. 



H 2 



V7 



FACETED EYE (oCULUS). 



