132 AN INTRODUCTION TO ENTOMOLOGY 



seta,, and a bipolar nerve-cell. These organs are of the type known as 

 sensillum trichodeum referred to in the preceding section of this 

 chapter. 



According to the observations of Hilton ('02) the terminal pro- 

 longation of the nerve-cell enters the hair and ends on one side of it at 

 some distance from its base (Fig. 148). The proximal part of this 

 nerve-cell is connected with the peripheral sensory nervous system, as 

 already described (page 128). 



The presence of this nervous connection is believed to distinguish 

 tactile hairs from those termed clothing hairs, and from the scales 

 that are modified seta?. If this distinction is a good one, it is quite 

 probable that many hairs and scales that are now regarded as merely 

 clothing will be found to be sense-organs, when studied by improved 

 histological methods. In fact Guenther ('01) and others have shown 

 that some of the scales on the wings of Lepidoptera, especially those 

 on the veins of the wings, are supplied with nerves ; but the function 

 of these scales is unknown. 



Hilton states that he ' 'found no evidence to indicate nerves ending 

 in gland cells or trichogen cells by such branches as have been described 

 and figured by Blanc ('90), but in every case the very fine nerve 

 termination could be traced up past the hypodermal cell layer with 

 no branches." Many figures of unbranched nerve fibers ending in 

 sense-hairs are also given by O. vom Rath ('96). 



A very different form of nerve-endings in sense-hairs is given by 

 Berlese ('09, a). This author represents the nerve extending to a 

 sense-hair as dividing into many bipolar nerve-endings. 



XL THE ORGANS OF TASTE AND OF SMELL 

 (The chemical sense-organs) 



It is necessary to discuss together the organs of taste and of smell, 

 as no morphological distinction between them has been discovered. 

 If a chemical sense-organ is so located that it comes in contact with 

 the food of the insect, it is commonly regarded as an organ of taste, if 

 not so situated, it is thought to be an organ of smell. In the present 

 state of our knowledge, this is the only distinction that can be made 

 between these two kinds of organs. 



Many experiments have been made to determine the function of 

 the various chemical sense-organs but the results are, as yet, far from 

 conclusive. The problem is made difficult by the fact that these 



