366 



projected beyond the general surface of the antenna. This 

 process is made possible by the curious shrinking, already 

 referred to, which is seen in the developing appendages of the 

 early pupal period. The shrinking is probably due to a closer 

 packing of the integumentary cells, which transforms the 

 ungainly appendages of the early pupa into the exquisitely 

 moulded structures of the imago. Twenty-four hours later 

 the process of bristle formation has become completed over 

 the whole antenna, and the secretion of new chitin begins. 



The integumentary cells at this stage are rather long and 

 columnar, and leucocytes, in various stages of disintegration, 

 may be seen lying amongst them. The leucocytes of the lumen 

 of the antenna are also undergoing slow disintegration, by a 

 process which will be referred to later. 



At thirty-six hours after pupation the process of chitini- 

 sation has become marked; it continues for a long time, but 

 differs in a rather important respect from what we see in the 

 chitinisation of the general body integument. The cells do 

 not undergo complete chitinisation, but remain partly as living 

 cells (receptor cells), with which the antennal nerve fibres com- 

 municate. Thus only the distal portion of the bristle-forming 

 cells chitinises ; the proximal portion remains below and in close 

 contact w^th the bristle which it has secreted. Even the cells 

 which form the general integument of the antenna do not 

 chitinise completely. 



Meanwhile the mesoderm has been undergoing remark- 

 able changes. The fibres of the network already referred to 

 have increased in length ; the cells have increased considerably 

 in number and also in size, and they now become so disposed 

 as to occupy a position below and in close contact with the 

 bristle secreting cells; a very delicate connection can actually 

 be seen, joining the mesodermal cell to the bristle cell above it; 

 so intimate, indeed, is the communication between the two 

 that it gives the appearance of a large binucleate cell (fig. 49). 



Meanwhile the nerve of the antenna has grown in size, 

 and extended as a great "tendon-like" axis right through the 

 antenna. Covering it is a thin layer of cells, the splanchno- 

 p leu re of the brain. 



In each segment of the antenna this great nerve (fig. 15) 

 gives off fibres, so^ that at the tip of the distal segment, it is 

 represented only by a loose outwardly radiating bunch of 

 fibres; and in good preparations it is frequently possible to 

 trace a single nerve fibre from the great antennal nerve 

 through the padding tissue into one of the mesodermal cells 

 which lie beneath the bristle cells; that portion of the nerve 

 fibre between the antennal nerve and the nucleus of the 



