The External Anatomy of Spiders 



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the lower, and the median, these terms referring to their relative 

 positions when the spider is hanging suspended by a thread; 

 but as very many spiders never assume this position, it is better 

 to designate the three pairs of spinnerets as the fore, middle, and 

 hind respectively. 



The hind spinnerets usually consist each of two segments, 

 but sometimes of three or even four segments; the middle spin- 

 nerets are not seg- 

 mented; the fore 

 spinnerets consist 

 almost always of 

 two segments each. 

 The sides of 

 the spinnerets are 

 comparatively firm, 

 but the terminal 

 portion is mem- 

 branous; this mem- 

 branous terminal 

 portion constitutes 

 the spinning field. 

 The spinning field is 

 always surrounded 

 by hairs, some of which are simple and some are barbed. These 

 are movable and appear to have some function in the spinning; 

 it is probable that some of them at least are tactile. 



Sometimes, at least, the spinning field is surrounded by a chit- 

 inous ring, to one side of which is attached the tendon of a flexor 

 muscle (Fig. 137), which probably moves the spinneret as a whole.* 



The spinnerets represent the fourth and fifth pairs of abdominal legs (the 

 other abdominal legs are lost in the course of the development of the embryo). 

 Each of these legs acquires, at an early embryonic period, a biramose form, like 

 the primitive appendages of the Crustacea, consisting of an axis to which is at- 

 tached an inner endopodite and an outer exopodite. Spinning glands may develop 

 on both rays, and it is thus suggested that the primitive number of spinnerets was 

 eight. No existing spiders are known in which these eight spinnerets are fully 

 developed as functional organs; in Liphistius, however, (which also shows the 

 generalized character of a segmented abdomen) the full number are present, but 

 only the four external ones (exopodites) possess functional spinning glands. In 

 the true spiders, we find present six functional spinnerets, the four large ones 

 being derived from the exopodites of the fourth and fifth pairs of abdominal 

 appendages, while the middle spinnerets are developed from the endopodites of 

 the fifth pair, the endopodites of the fourth pair being altogether wanting or else 

 concerned in the formation of the cribellum or colulus. (Korschelt and Heider, 

 111., p. 79.) 



134 



Fig. 137. FORE SPINNERET OF ARANEA 



a, chitinous ring around the spinning-field b, tendon of flexor 



muscle c , outlet of cylindrical gland (after Bucholz and Landois) 



