LESSON 91.] 



THE EYE IN INSECTS. 



293 



they appear to be of great size ; they have consequently been re- 

 duced in the figure given of them (Fig. 416). 



1323. In the perfectly fresh state in which they were seen, each 

 optic filament formed a very beautiful sight, possessing, in every in- 

 stance, an axis-cylinder ; the white substance was too transparent to 

 be visible. 



1324. The cones of the Caterpillar, above referred to, are still 

 larger (Fig. 416), and these have been preserved, fortunately, in a 

 saline solution, but they are opaque, and, whether the nervous fila- 

 ment enters the cone or not, cannot now be determined ; the bases 

 of these cones are much flatter than those of Musca. 



1325. The mode of connecting the several lenses of the compound 

 eye of an Insect with the FlG 417 



brain, is shown in a figure 

 copied from a preparation 

 of the brain of Blatta 

 Americana (Fig. 417). 



1326. The inferior por- 

 tion of the brain, or iufra- 

 oesophageal ganglion, can- 

 not be seen in this Insect, 

 in viewing the brain from 

 the upper surface, because 

 it lies immediately below 

 the supra-03sophageal gan- 

 glion, and, being much 

 smaller, is concealed by 

 it, and can only be seen 

 from the under surface. 



1327. There is one fact in connection with the compound eyes 

 of Musca carnaria, that has escaped the observation of the authori- 

 ties, namely, that all the important elements 



of a visual organ rest upon and are supported 

 by an aggregated arrangement of fat lobules, 

 of exquisite beauty, and, as might be expected, 

 existing in a state of the most perfect analysis. 

 A figure of these lobules is given (Fig. 418). 



1328. When the plan of connection of the Fat i obu ies of the eye, 

 compound, or facetted eye, with the brain is 



considered, it is not difficult to understand its action ; each individ- 

 ual organ transmits to the optic lobe a picture of what it sees, and 



Brain, Blatta Americana. 



a. The Cerebrum, or supra-oesophageal ganglion. 

 &. The optic nerves, terminating in the optic lobes. 



c. Optic filaments, one of which is iu connection with. 



each individual eye. 



d. Facetted external cornea. 



e. Antenneal nerves. 



/. Nerves connecting the brain with the first thoracic 

 ganglion. 



FIG. 418. 



