367 



mesodermal cell being protected by a fibre of the mesodermal 

 network already described. 



The mesodermal cells must therefore be regarded as con- 

 stituting a kind of neurolemma for the nerve fibres ; what the 

 actual connection between the nerve and the bristle cell, a 

 connection which must be situated in the delicate connecting 

 piece between the two cells, is, I am unable to say. These 

 remarkable structures are shown in figs. 49 and 51. Very 

 often two mesodermal cells lie in connection with a bristle 

 cell (figs. 51, 53). The apparently erroneous interpretation 

 whdch B. T. Lowne placed on similar cells in Calliphora has 

 already been discussed in connection with the description of 

 the tactile organs on the legs. 



At the tip of the antenna the bristle secreting cells have 

 frequently retreated a considerable distance from the respec- 

 tive bristles, and only a long protoplasmic filament remains 

 to connect them (fig. 50). 



It seems scarcely possible to doubt that the structures 

 here seen are tactile in nature. 



The antenna, however, is the seat of a number of other 

 remarkable sensory structures. 



In the second segment, which is rather longer than those 

 which follow it, there is a number of curious structures, 

 which must, I think, be regarded arS olfactory organs. Of 

 these there are ten, and each is in the form of a long tubular 

 sac, formed by five elongated cells, each with a large nucleus, 

 the ten olfactory sacs hanging in a ring around the antennal 

 nerve, from the distal end of the segment, into its spacious 

 cavity (fig. 52). The lumen of these tube-like sacs is very 

 slender, and appears to be quite devoid of chitin. It com- 

 municates by a short duct with the exterior, the small circular 

 openings lying in a rather deep ring-like depression immedi- 

 ately surrounding the joint between the second and third 

 segments. The masses of padding tissue act as supports for 

 the olfactory sacs. I could not observe the innervation of 

 these organs, a fact which is partly due to the minuteness of 

 the nerves, and to the difficulty of staining them. Nor was 

 I able to follow their whole development — in newly formed 

 pupa-e they do not yet occur; in pupae at the fifty-six hour 

 stage they have already been formed, appearing then as long 

 protoplasmic sacs hanging down from the distal end of the 

 segment, rather gelatinous in consistency, and not so delicate 

 and slender as in the adult condition. They probably begin 

 to develop, then, at about the twelve- to twenty-four hour 

 stage, and there can be little doubt that they are produced 

 simply as invaginations of the developing ectoderm, accom- 

 panied by a great elongation of the cells concerned. 



