SENSORY SETiE OF TARANTULA AND SOME OF 



ITS RELATIVES 



WILLIAM A. HILTON 



The general sensory terminations of insects are found in liairs. These 

 seta^ are of a])()ut the same type throughout the group and consist of hollow 

 extensions of eliitin which may be microscopic or nuich larger. The base of 

 the hair is usually movably, articulated in a chitinous ring. Below the cuticle 

 and level with the hypodermal cells, there is at the base of each hair a larger 

 "trichogen" or hair-producing cell. Sensory structures similar to seta; in 

 insects have been described and figured in the Crustacea and some of these 

 have been found with the bases of the hairs solid so that nerve fibres cannot 

 penetrate very far into the hairs. Judging from methylene blue preparations 



Figure 254 



Sketch of a section through a body sense hair of an insect, showing the penetration 

 of a nerve tibre into the base of the hair. 



studied in section (Hilton '02), the nerve fibres of insects go only to the 

 basal portions of the hairs and there terminate on one side (Figure 254). 



In a study of the hairs of insects, spiders and other arthropods, it becomes 

 necessary to distinguish between the hollow sensory seta^ and the solid eleva- 

 tions of the cuticle which are either very mucii smaller or very much larger 

 and heavier. These minute hair-like elevations and the larger spines or 

 projections seem to be largely for protection. They are undoubtedly also for 



