The Internal Anatomy of Spiders 



d. THE LYRIFORM ORGANS 



There are found in several of the orders of the Arachnida organs 

 of a peculiar type, the function of which has not been determined, 

 but which have been supposed to be organs of hearing. They 

 occur in spiders, in the Phalangida, the Pedipalpida, and in the 

 Pseudoscorpionida, and are known as the lyriform organs, 



These organs vary greatly in form but agree in their essential 

 features. The distinctly characteristic external feature is a 

 slit in the cuticula in which there is a wider, oval, central portion, 

 and, on each side of this, a long, narrow portion, which may or 

 may not taper to a point. At the bottom of this slit there is a 



thin layer of the cuticula, so that 

 the slit does not reach the body- 



, _ V*** cavity. These slits occur in the 



sternum and in the cuticula of most 



y~* of the segments of the legs, except 



fhVr : the tarsi, and of the mouth-parts. 



aJUiIjW They are extremely small; but are 



very constant in their form and in 



their position. 



In the sternum the slits may 

 occur singly or in groups. But the 

 more characteristic form of these 

 organs, and the one that suggested 

 the name lyriform, is found on the 

 appendages, where they usually occur 

 near the distal end of a segment; but 

 Fig. 171. lyriform organs sometimes they are found near the 



middle of a segment. In this type 

 there are several slits grouped side by side; and the cuticula sur- 

 rounding the group of slits, as well as that of the spaces between 

 them is greatly thickened (Fig. 171). The spaces between the 

 slits suggest the cords, and the outer rim of the organ, the frame 

 of a lyre, hence the name lyriform. In Fig. 171, a represents 

 the lyriform organ in the metatarsus of the fourth leg of Argy- 

 rodcs irigonus; and b that found in the same position in Theridion 

 frondeum. 



As to the internal structure of the lyriform organs, it has 

 been found that there is beneath each slit a nerve-end-cell. This 



168 



