I 



356 



integument of the leg immediately surrounding the bristle. 

 From each of these cells a process is given off backwards to the 

 nerve of the leg. It seems that this minute process is merely 

 a neurolemma (since it stains feebly with haematoxylin, which 

 the nerve fibre will not do), and this neurolemma protects 

 an even more delicate nerve fibre. This nerve fibre can 

 actually be seen leaving the outer part of the mesodermal cell 

 and communicating with the bristld cell, within the collar 

 developed from the former (fig. 19). 



I have not been able to determine exactly the function of 

 the whole armature of bristles, which are so numerous on the 

 legs; that they aid the insect in clinging to objects is un- 

 doubtedly true, but it seems quite possible that a very large 

 proportion of them have also a definite tactile function. In 

 the case of the first tarsal joint this is certain; scattered 

 bristles on other parts of the tarsi also have nerves connected 

 with them, but the structures dealt with are so minute, that 

 I cannot definitely say whether similar nerve-endings are pre- 

 sent on all of them. 



Lowne has shown that the general integument of a fly 

 is sensitive to touch. Tlie same author describes bipolar 

 ganglion cells lying in close contact with the tactile bristles; 

 I suspect, however, that they are really either ectodermal 

 "receptor" cells, which have produced the bristle, or meso- 

 dermal cells acting as a neurolemma, which encloses the delicate 

 nerve fibre that terminates on the bristle. Bipolar nerve cells 

 in this position are quite absent in Nasonia; but the meso- 

 dermal cells which lie beneath the tactile bristles closely 

 resemble such cells, the enclosed nerve fibre being very diffi- 

 cult, or often impossible to detect, partly on account of its 

 feeble staining capacity. But when the development of tlie 

 tactile organs is followed out, the nature of these cells becomes 

 quite clear. In the eye they are ectodermal in origin (see 

 "Organs of Vision") ; in the legs and antennae they are meso- 

 dermal, and the nerves which grow towards them from the 

 brain or ventral nerve cord are quite devoid of cell nuclei, 

 consisting merely of nerve fibres, whose nuclei remain in the 

 central nervous system. 



As Lowne did not observe the development of the tactile 

 sense organs, he naturally put the more obvious interpretation 

 on his observations. Indeed, it seems to be the generally 

 accepted view that the cell lying below the tactile hair is a 

 nerve cell ; an examination of the embryology of the structures 

 concerned will, however, show this view to be erroneous. I 

 shall refer to this again later. 



Leucocytes, containing large amounts of debris from the 

 histolysed tissues, enter the legs in an early stage in their 



