THE PHOTOGENIC ORGANS OF INSECTS. 379 



by the development over them of folds of the cuticle of the 

 adjacent region of the limb. Two spacious tracheal sacs oc- 

 cupy the greater part of the cavity of the tibia, and a large 

 nerve ends in a ganglion in the remaining space. Upon this 

 ganglion a series of peculiar short rod-like bodies are set. 



The compound eyes of insects differ only in detail from 

 those of the Crustacea. 



In the ocelli, or so-called simple eyes, a sclerotic, a cornea, 

 a lens, a vitreous humor, and a choroid coat, have been dis- 

 tinguished, and the whole organ has been compared to the 

 vertebrate eye. But the "lens " appears to be always a mere 

 thickening: of the cuticle which constitutes the cornea, and 

 the so-called "vitreous humor " is partially or wholly made up 

 of crystalline cones analogous to those which are found in 

 the compound eye. In this respect the ocellus of the insect 

 resembles the simple eye in Arachnida and Crustacea. 1 



Many insects, as the Glow-worm and Lantern-flies, are re- 

 markable for their power of emitting light. 



According to Schulze, 2 the males of Lampyris splendidula 

 possess two photogenic organs, which lie on the sternal aspects 

 of the penultimate and antepenultimate abdominal somites. 

 Each is a thin, whitish plate, one face of which is in contact 

 with the transparent chitinous cuticula, while the other is in 

 relation with the abdominal nerve-cord and the viscera. The 

 sternal gives out much more light than the tergal face. The 

 photogenic plate is distinguishable into two layers, one occu- 

 pying its sternal and the other its tergal half. The former is 

 yellowish and transparent, the latter white and opaque, in 

 consequence of the multitude of strongly refracting granules 

 which it contains. Tracheae and nerves enter the tergal layer, 

 and for the most part traverse it to terminate in the sternal 

 layer, which alone is luminous. Each layer is composed of 

 polygonal nucleated cells. The granules are doubly refrac- 

 tive, contain uric acid, and probably consist of urate of ammo- 

 nia (Kolliker). Hence the cells of the layer which contain 

 them are termed by Schulze the " urate cells," while he calls 

 the others the " parenchyma cells." The branches of the 

 tracheae which ramify among the parenchyma cells end, like 

 those of other parts of the body, in stellate nucleated cor- 



1 Leydig, " Das Auge der Gliederthiere," 18G4. Landois, " Das Raupen- 

 auge " {ZeliscTirift fur wiss. Zoologie, 1866), and " Zur Entwickelungsgeschichte 

 der facettirten Augen von Tenebris molitor" (ibid., 1867). 



2 " Zur Kenntniss der Leuchtorgane von Lampyris splendidula." (" Archiv 

 fur mikr. Anatomie," 1855.) See also Kolliker, " Wurzburg Phvs. Med. Ge- 

 sellschai't," 1857. 



