412 



PEOF. B. T. LOWNE ON THE 



111 most insects tlie outer, highly refractiye ends of the retinal 

 end-organs are imbedded in abundant pigment. The flies are 

 the only exception, and in these the cells surrounding the bacilli 

 are free from pigment. 



The 2V<7c/«eig (Plate XXVII. flg. 3) form a dense network around 

 the inner segments of the retinal end-organs in insects, and 

 branches extend to and perforate the basilar membrane. These 

 fine tracheae are without any spiral markings, and are easily 

 mistaken for fine nerve-twigs. The figure given (fig. 3) shows 

 these tracheae in a moth, and it can be readily seen that they 

 lie between the nerve-end organs, and that they branch 

 dichotomously between the great rods. The aniline stains at 

 once colour the tracheae, whilst they have no efi'ect upon the 

 nerves. These stains, however, attack the nerve- sheaths, but 

 not the outer ends of the retinal end-organs. By the use of 

 aniline stains, especially Spiller's purple, I have been able to 

 trace the finer tracheal vessels, which have been constantly mis- 

 taken for nerves, to the larger tracheal trunks and in one of my 

 photographs this relation is sufficiently evident. 



The illustrations on Plate XXVII. show the large size of 

 the bundles of optic nerve-fibres with their terminations in the 

 retinal end-organs ; they also show that nothing bearing any 

 23roportion to the magnitude of these nerve-cords passes through 

 or even up to the basilar membrane. The basilar membrane is 

 chitinous and has a cellular layer on both its inner and outer 

 surface ; that on its inner surface consists of branching or stellate 

 cells, which are continuous with the sustentacular framework of 

 my retina ; the outer layer consists of pigment-cells, continuous 

 with the pigment-sheaths of the great rods. The perforations in 

 the chitinous layer of the basilar membrane are between and not 

 opposite to the extremities of the great rods, and transmit the 

 tracheal vessels. 



The structure of the great rods has with some been the diffi- 

 culty in accepting my views. The appearance of these structures 

 in many sections is certainly perplexing. The reason is that 

 which I Iiave already insisted upon. In life they are hollow tubes 

 filled and distended with fluid. In bad preparations they appear 

 stellate in transverse sections and present no central cavity ; in 

 radial sections they are separated from each other by wide spaces, 

 often filled by distended tracheal vessels. 



