1911.] NATURAL SCIENCES OF PHILADELPHIA. 377 



more or less parallel with each other. This group of slits is generally 

 surrounded by a border (fig. 536) 1 , which is nothing more than a 

 difference in pigmentation, that gives the lyre-shape to the organ. 

 Inside the border the pigmentation is usually much lighter than out- 

 side, but sometimes the reverse occurs. Hence with considerable 

 magnification these organs appear as light or dark spots while the 

 slits inside appear almost transparent. The superficial appearance 

 of a slit reminds one of a long slightly bent spindle which has an 

 opening or dilatation (di.) at the centre or nearer one end than the 

 other (fig. 53). 



Lyriform organs may be divided into compound and simple organs. 

 A compound organ contains four or more single slits, which may 

 either be enclosed by a common border or each slit may be surrounded 

 by its own border; in the latter case if the borders are not joined with 

 each other, then the structure may be called a diffused organ; some- 

 times the border is entirely absent. A compound organ never exceeds 

 more than thirty slits and the average for Theridium tepidariorum 

 is ten, but the number containing four, five or six is comparatively 

 small. A simple organ contains either two or three single slits, around 

 all of which there is a common definite border. 



A cross-section of a lyriform organ shows that it may sometimes be 

 located on a slight elevation or at other times in a slight depression, 

 but in either case the cuticula at this place is considerably thicker 

 and sometimes twice as thick as at other places. Fig. 1 shows this 

 very well. This drawing represents about one-third of the entire 

 cross-section of the trochanter of a mature male Agalena ncevia just 

 moulted. The cuticula (cu.) is only diagrammatic as taken from 

 various species, but here its thickness represents its greatest develop- 

 ment at any time in this species. The nerve (N.) is drawn in the 

 location it holds when its branch (N. b.) penetrates the hypodermal 

 basement membrane (b. m.), 220 microns in front of this locality 

 where the nerve branch just begins to leave the nerve the latter is 

 much nearer the centre of the leg. The sense cells (s. c.) are shown in 

 their natural positions but they have been reconstructed from nine 

 sections just in front of where the nerve branch enters the hypodermis. 

 All the other parts are taken from only one section in front of the organ. 

 In this drawing one notices that some of the slits pass entirely through 

 the cuticula, some two-thirds through, and others show only a slight 

 indentation. This means that the section passed exactly through 



'The figures are numbered consecutively on Plates XXX-XXXIII. 

 25 



