THE PLACE OF SPIDERS IN NATURE 27 



pair has been lost, and in only a few are there vestiges of the anterior 

 lateral spinnerets. Thus the four spinnerets of the tarantulas and 

 most of their allies represent the single, small, posterior median pair 

 and the longer, posterior, lateral, segmented pair. The loss of the 

 spinning function seemingly has preceded the degeneration and 

 obliteration of the spinning organs. 



Most true spiders have retained the eight spinnerets in one form 

 or another, and in only a few instances have they reduced their 

 number below three pairs. In all the cribellate spiders the anterior 

 median pair is still retained as the cribellum, a flat spinning field 

 which is used in conjunction with a comb of hairs on the fourth 

 metatarsus, the calamistrum, to produce characteristic threads. The 

 cribellum probably existed before the anterior median spinnerets 

 had lost their spinning function, and became greatly changed and 

 important because of its special function. Whether the cribellum 

 is a new development from the ancient anterior median spinnerets, 

 or represents the ancestral condition of all true spiders, is still a 

 debatable question. At any rate, in almost all higher spiders a vestige 

 of variable size evidences the former presence of an anterior median 

 pair of spinnerets. In some it is a fingerlike colulus; in others, a 

 pair of flat plates, connate plates, or a single sclerotized plate, all 

 set with covering hairs; and in still others, a tiny point or blister 

 bearing one or two erect setae. In some groups of true spiders the 

 hind spinnerets are greatly reduced in size and become obsolete to 

 a considerable extent, but their former location is marked by some 

 sort of vestige. 



The ordinary true spider has three pairs of well-developed spin- 

 nerets set closely together in a single group. The anterior pair is 

 two-segmented, and the apical segment is bountifully supplied with 

 many spools and a fewer number of spigots 'on the spinning field. 

 The posterior pair is likewise segmented, most commonly with two 

 but frequently with three or even more segments, and is also well 

 supplied with spinning equipment. Between the latter are the me- 

 dian spinnerets, each of a single segment and ordinarily less well 

 provided with spinning openings. 



In the sedentary orb weavers and comb-footed spiders, which 

 are the finest spinners, the spinnerets are relatively short, with small 

 apical segments, and are set closely together in a small field. In 

 many other spiders whose spinning is less noted the spinnerets are 

 sometimes long and conspicuous, frequently many-segmented, and 

 arranged in different ways. 



