142 



Mr. B. T. Lowne. On the 



[Apr. 12, 



Lenticulus as a segment of the Dioptron. In many insects, especially 

 in the larvae, each segment has a distinct Retinula, consisting of a 

 small bundle of Bacilla, which is connected with the ganglion by a 

 distinct nerve enclosed in a separate pigmented sheath. I have 

 named this form of retina Segregate. In other insects the retina is 

 continuous over the inner surface of the Basilar membrane, but is 

 connected with the deeper structures by a number of separate nerve 

 bundles ; whilst in the most highly developed Insects, a single 

 decussating nerve connects a continuous retina with the ganglion. 

 The ganglion consists of several nuclear and molecular layers, which 

 are extremely like the corresponding layers of the retina of a 

 Vertebrate. 



All the structures of the Dioptron are developed from the cellular 

 Hypoderm, whilst all the structures of the Neuron are formed from a 

 solid papilla, or from a number of papillae which are outgrowths 

 from the Cephalic Ganglia, so that in this respect there is ground for 

 a morphological comparison of the Dioptron with the dioptric struc- 

 tures, and of the Neuron with the nervous structures of the eye of a 

 Vertebrate. 



The Compound Ocellus of the larval insect is merely a single 

 segment of a compound eye, with all the apparatus of the Dioptron 

 and Neuron. I have used the term compound in relation to the 

 refractive apparatus. The Neuron consists of a single bundle of 

 Bacilla connected with the ganglion by a separate nerve bundle. 



For several years I sought in vain for an explanation of the 

 manner in which the compound eye could serve the purpose of 

 vision. I discarded all the theories hitherto advanced, as being 

 defective, and incapable of explaining the phenomena, consistently 

 with the structure of the Great Rods. 



Two years ago, whilst examining the recent eye of a small moth 

 (Pterophorus) , I was surprised to observe that the structure of the Great 

 Rods was very different to anything with which I had previously been 

 acquainted. The inner extremities of the Great Rods have been 

 named Spindles, and are well known to present a very remarkable 

 structure. 



I first observed, in this moth, that the Spindles are, during life, 

 large ovoid bodies, filled with transparent highly refractive fluid ; the 

 slightest injury gave rise to the escape of the fluid and left the 

 Spindles in a shrivelled condition, the usual appearance of these 

 bodies. 



A further investigation has shown me that all compound eyes when 

 uninjured have similar ovoid Spindles. These organs appear to act 

 as magnifying and erecting lenses. Their anterior foci correspond 

 to the position of the subcorneal images, and the posterior foci with 

 the Bacillar layer of the retina. 



