Visual Organs of Insects and Ci^stacea. 369 



flew only in the direction of that side in which it still saw. 

 In the experiments of Reaumur *, bees no longer flew up- 

 wards when he covered the posterior part of the head with 

 an opaque varnish. 



If these observations are correct, the Neuroptera, which 

 have lateral hemispherical eyes, possess a field of vision cor- 

 responding in extent almost to the entire circle of the horizon. 

 Such is the case with the genera JE'shna^ Libellula, A'grion, 

 Hemerobius. In a new genus of neuropterous insects, Ho- 

 lomma, the eyes exceed, even posteriorly, the boundaries of 

 a hemisphere ; so that not only a circular horizon, but also 

 the posterior part of the body, must fall within their field of 

 vision. To the Neuroptera most distinguished by the extent 

 and precision of their movements may be added the butter^ 

 flies, the J5ombyces, and ASphinges ; then, among the carnivo- 

 rous Coleoptera, the genera Carabus, Calosoma, Cychrus, 

 Brachinus, A^ecydalis. In these latter the eyes are placed 

 lower, because they have no longer any relation to motion by 

 flight. Amongst the other Coleoptera, the genera most dis- 

 tinguished by the perfection of their vision are Lema and 

 JLampyris. The insects whose eyes have a more anterior 

 position, and are only separated from each other by a narrow 

 interval, as the genera Naucoris and Notonecta, move by 

 leaps, which are always directed in front, in accordance with 

 the extent and direction of the visual field. 



The eyes of most of the Hymen6ptera, on the contrary, 

 are only long and narrow segments of spheres, with the 

 longest diameter directed from above downwards ; and such 

 eyes will, of course, have fields of vision equally narrow. 

 The flight of these insects is commonly irregular : leaping, 

 as it were, and only slightly directed laterally ; but, in most 

 cases, rather tending upwards. Travellers have found hy- 

 menopterous insects on the highest mountains. Deluc ob- 

 served insects of the genus Culex at an elevation of 1560 

 toises ; and it appears that they were hymenopterous insects 

 which Humboldt saw flying over Chimborazo, at an altitude 

 of 2850 toises. 



The neuters and females of hymenopterous insects often 

 have eyes much smaller, and these separated by greater in- 

 tervals, than the males : such is the case with the working 

 bees. In most of the orthopterous insects the eyes are seg- 

 ments of very large spheres; they are consequently very 

 flat, and the visual field is small. The movements of these 

 insects, and particularly the apparently timid flight of the 



* Memoires, tom.v. p.287. * 



Vol. IV. — No. 20. b b 



