and smooth or simple Eyes of Insects. 185 



them. This tunic is thin, whereas the varnish of the choroid 

 and the choroid itself are on the contrary thick. We also ob- 

 serve externaliv on the eve the central black point, and several 

 others produced by the same cause with those of the eye of the 

 papilio atakmtn. Only these blackish points are more irregular 

 in their form than those of the papilio alalanla. 



The larvc-e of the iepidopferce have only smooth eyes situated 

 generally on the sides of the head. They are variable in colour 

 and number. Thus we observe eight in the silkworm, and six 

 only in the caterpillar. The former are black, and the latter 

 transparent. As to their internal structure, we shall speak of 

 it elsewhere, inasmuch a§ it exhibits some differences from that 

 of the other sm.ooth eye.?. 



In the neitropterce, the tunic of the cornea is also very much 

 varied, and its colours vary much in various species. In the 

 libelluJa vulgaris, the compound eves exhibit all their upper 

 portion of a reddish brown, and all the lower of a yellowish 

 green. On carefully removing the cornea, it is easily seen that 

 the difference in the two colours is owing to the diversity of 

 colour in the tunic of the cornea. It seems even that this tunic 

 is thicker in the brown jiart of the eye than in the green. We 

 shall also remark that the upper facets of th-3 cornea are greater 

 than the lower; which is rather singular. If we remove the 

 cornea, we distinctly perceive the optic nerves passing through 

 the choroid and its varnish, membranes which are easily distin- 

 guished from their being of a deep black. 



The circular trachea surrounds the retina, and its ramification* 

 pierce with the optic nerves all the other membranes, dispersing 

 themselves ad hjfiiuhim under the cornea. This disposition is 

 so evident, that we mav easily conceive howSwammerdam thought 

 the cornea was formed by the meeting of these tracheae. In 

 other respects the eye of this lihellvla presents nothing parti- 

 cular, with the exception of the enlargement or great extent of 

 the retina. 



I have also dissected the eves of certain larvae of agrion and 

 aeshma, but have observed nothing very different. These larvae 

 seemed to want the circular trachea, the place of which is sup- 

 plied by an infinity of other tracheae, which spread in a great 

 number around the optic nerve, which is very broad and thick 

 in these larvae. I never observed that the eye had any particular 

 conformation in the species which hal)itually frequent the water; 

 and this remark is important, because the case is very different 

 in the other animals. The form of the cornea also presents 

 differences in the species in the acjuatic insects, as well as in 

 those which live habitually in the air. 



If 



