506 



the "retinula" and pigment cells; from the lower the rhab- 

 dome cells. The cells arrange themselves in little groups 

 (ommatidia) each consisting of one central rhabdome cell, 

 surrounded by seven sheath (''retinula") cells (of which on© 

 later disappears), which are, in turn, surrounded by four 

 pigment cells. At the outer end are formed four vitreous and 

 two lens cells. The latter surround the former 'and their 

 upper ends secrete the lens. The rhabdome cell chitinises. 

 An ingrowth from the ectoderm which surrounds the eye forms 

 a membrai^.e beneath the developing eye (''perioptic mem- 

 brane"). Its cells serve as neurolemmae for the cells of the 

 optic ganglion. The remarkable process of the development 

 of the eye innervation in connection with the perioptic mem- 

 brane is fully described. 



The ocelli are similarly modifications of the integument 

 only, and the nerve reaches them, quite independently, from 

 the brain. 



C . Respiratory System. 



The larva has a pair of longitudinal tracheal trunks, 

 connected by transverse trunks in front and behind. The 

 longitudinal trunks open to the exterior by four short stig- 

 matic trunks, which increase to nine in the second larval 

 instar. Tlie air is carried to the tissues by extraordinary 

 tracheoles, of a kind not hitherto, apparently, described. Each 

 branching system, of which there are two to five in a segment, 

 is simply a highly branched hollow cell, formed from a greatly 

 modified cell of the tracheal epithelium (giant tracheoloblast) . 

 Increase of complexity during larval life is produced by 

 growth of the size of these cells, and by further branching. 



The whole tracheal system degenerates at metainorphosis, 

 partly dissolving in the blood, partly removed by leucocytes 

 after degenerating. A simultaneous regeneration from 

 embryonic cells which form imaginal ''nests" at the bases of 

 all the stigmatic trunks, prevents any discontinuity in the 

 tracheal system occurring. The embryonic cells grow over the 

 dead larval cells of the main trunks; growing out in nlaces 

 they now form the true branching multicellular tracheal 

 vessels in the head, alitrunk, and abdomen. Some of them 

 are modified into the great thoracic air sacs. Tlie "spiral" 

 intima of the tracheae is formed as a chitinisation of ridges 

 formed on the inside of the cells which compose the tracheal 

 epithelium. 



D. The Muscular System. 



The larval muscular system begins to degenerate in places 

 before defaecation, e.g., in the thorax, embryonic cells 

 (myoblasts) begin to crawl over certain dorsal longitudinal 



