550 THE SENSES AND SENSORY ORGANS. 



The fact that each rhabdome has only a single nucleus on 

 its surface which projects from it, is very manifest at this and 

 in all subsequent stages in the nymph ; such rhabdomes are 

 figured by \Veismann [2 (Fig. 55, F.)] with only a single nucleus. 



The epithelial elements at this stage are four sub-corneal 

 nuclei, the five iris cells and five cells at the inner extremity of 

 each rhabdome. 



In Immature Diptera, Eristalis and Musca even some hours 

 after their escape from the pupa, the rhabdomes are seen to be 

 connected in some specimens at their inner extremities with 

 large stellate cells (PI. XXXIX.) which lie in great numbers 

 between the outer surface of the retina and the inner surface 

 of the dioptron in the pupa. I formerly figured this connection 

 [219, Figs. 9, 12, and 13], and mistook them for ganglion cells ; 

 these cells gradually disappear as the retina comes up to the 

 basilar membrane. I have recently examined a number of 

 specimens prepared by my friend Brigade-Surgeon Scriven, 

 in which the connection of these cells with the rhabdomes is 

 very distinctly seen in an immature imago, although the retina 

 is close to the inner end of the rhabdomes, and the cells in 

 question are flattened. These cells surround the trachaea of 

 the sub -dioptric space, but are distinctly separated from 

 the retinal end organs by the flattened pre-retinal cells 

 (PI. XXXIX./>r). 



The Morphology of the Rhabdome. It appears to me that the 

 rhabdome must be regarded as a mesoblastic, or parablastic 

 structure, developed from the same kind of cells as the tracheal 

 vessels. It is worthy of remark that the formation of a 

 cuticular sac within a single: cell only occurs in the tracheae and 

 the rhabdomes ; and when the tracheal intima is first developed 

 within the cells from which the smaller tracheae originate, this 

 sac is filled with fluid, which is only subsequently replaced by air. 

 That the parablastic elements of the tracheae penetrate the 

 dioptron is undoubted, and in many of my preparations the 

 gelatinous ovoid body from which the rhabdome is developed 

 is undoubtedly connected with the branching cells from which 

 the tracheae originate. In its mesoblastic origin the rhabdome 



