414 MK. B- THOMPSON LOWNE ON THE COMPOUND VISION 



the inner surface of the memhrana basilaris of the dioptron (figs. 41 & 45) ; so that at 

 the final ecdysis it entirely replaces the partial retinae of the larval eye. 



This complete change of the larval retina at the final ecclysis is undoubtedly a very 

 remarkable phenomenon. When, however, the epiblastic nature of the nervous system 

 is borne in mind, it is not, perhaps, more remarkable that an ecdysis should occur in 

 relation to the nervous than that it should occur in relation to the cutaneous epithelia. 

 It has occurred to me that the kidney-shaped ganglia already alluded to, which appa- 

 rently vary much in number in different stages of development, may be really successive 

 rudimentary ganglia and retinae destined in turn to become functional. 



The extent of the final continuous retina varies much in different species of insects. 

 In Agrion the whole inner surface of the basilar membrane is in contact with the con- 

 tinuous retina in the fully formed imago ; but in the great eyes of JEshnla it only replaces 

 the central retinulse. The same partial replacement of the retinulse is the condition in 

 many insects in which the eye is only functional in the imago. In these it is probable 

 that the central portion of the retina is continuous from its first formation, and that it 

 is, as it were, supplemented by peripheral partial retinre. 



Most of the Lepidoptera apparently exhibit this condition, the continuous retina being 

 most developed in the Diurna. As has been already stated, the larval condition is per- 

 manent in the Crane-flies and Gnats ; whilst in the Diptera with short antennae, Muscidse 

 and Tabanidae at least, where the formation of the pupa almost partakes of metagenesis, 

 no partial retinae are ever formed. It appears as if the developmental processes had 

 been much abbreviated in these insects, and that the final stage is reached by a single 

 and complete metamorphosis. 



IV. On the Morphology of the Eyes or Arthropods. 



Although of late the views of Midler with regard to the relations of the simple and 

 compound eyes of Arthropods have fallen into discredit, I must return to these views. 



The eyes of the Isopod Crustaceans, which he spoke of as aggregate, are undoubtedly 

 really intermediate between a simple and compound eye. The transition from a few 

 widely separated simple eyes, which form the aggregate eye, is so natural that it has only 

 obtained discredit because there is a wide difference in the structure of the stemmata 

 of insects or Arachnids, and of the compound eye. 



The case is, however, otherwise with the simple eyes of larval insects, which are almost 

 identical with a segment of the dioptron of the compound eye. Landois *, describing 

 the eyes of several caterpillars in 1866, remarked that they present a condition inter- 

 mediate between that of the compound eye and the ordinary simple eye, and proposed 

 the term "ocelli comjjosltl" for them. I have figured sections of the eyes of three 

 noctuid. caterpillars (figs. 36-40); these exhibit a lens consisting of three segments placed 

 immediately beneath the very convex corneal facet. 



* Zeitschr. f. w. Zool. Bd. xvi. 



