6^ Miscellaneous, 



appear to be constantly present in all species of caterpillars. They are 

 found to the number of six on each side of the head, immediately 

 above the articulation of the mandibles. The cornea of each of them 

 strikes one directly, according to M. Landois, by its division into 

 three segments. From the centre of the cornea three lines diverge 

 at angles of 120° towards the margin, and divide this organ into 

 three equal parts, each of which presents a curvature of its own. 



Directly beneath the hypoderm (chitinogenous stratum) of the 

 cornea, there are three crystallines, corresponding to the three parts 

 of the cornea, and each formed of striated and nucleated fibres, 

 arranged concentrically around the centre of the organ. Beneath 

 this triple crystalline is an organ which M. Landois regards as an 

 iris. It is a kind of diaphragm formed by about thirty-five fibres 

 directed like rays from the periphery to the centre of the iris, and 

 strongly pigmented. These fibres are very contractile, and probably 

 of a muscular nature. The centre of the iris is pierced by a tri- 

 angular aperture with rounded angles ; and from each angle issues 

 a yellow appendage, to which the author gives the name of the loop 

 of the iris. 



Beneath the iris comes that part of the visual apparatus which, in 

 facetted eyes, has generally received the name of the crystalline body. 

 According to M. Landois, this body is not perfectly limited, except 

 by the action of reagents, and originally it was in continuity of tissue 

 with the nervous branch which follows. In this case M. Ley dig's 

 previsions would be confirmed, and it would be necessary to regard 

 J;he supposed crystalline body as a terminal nervous inflation of the 

 optic fibres. This inflation is divided, like the cornea, the crystal- 

 line, and the iris, into three parts, each of which is continued into a 

 nervous fibre, which may be traced, attached to the others, as far as 

 the optic nerve. The fibres and dilatations are protected by a 

 neurilemma. 



The nervous portions of the eye are protected by three masses with 

 violet pigment, which the author calls enveloping bodies; in fact 

 they form an envelope round the optic nerve and the nervous infla- 

 tion, only leaving between them a small aperture for the passage of 

 the loops of the iris, which apply themselves to the nervous inflation. 

 Lastly, the whole eye is protected by a muscular layer and by a 

 double membrane which envelopes it. 



M. Landois considers that the eyes of caterpillars are intermediate 

 between simple and facetted eyes, and proposes for them the name of 

 compound ocelli. In reality each of these ocelli is the complete 

 homologue of an isolated element of a facetted eye. The author 

 indicates in detail the homologies of these two forms of eyes — 

 homologies which no one can miss seeing. A single surprising fact 

 remains. In the facetted eyes, as has been demonstrated by M. 

 ClaparMe, the whole organization is based upon the number four ; 

 in those of caterpillars, on the contrary, as M. Landois has just shown, 

 it is based upon the number three. It will be interesting to ascertain 

 by the study of chrysalids how the typical form of the eyes of the 

 perfect insect succeeds that of the caterpillar. 



