126 THE FOOD OF ANIMALS 



a thread-like appendage, looking like an attenuated representative 

 of the Scorpion's tail, or, in some members of the group, there is 

 nothing but a little knob to represent the tail. The tailed species 

 have both small and large pincers, as in the ordinary Scorpions, 

 but the latter are not so well developed. Those appendages of 

 the tailless forms which are equivalent to the large pincers are 

 of considerable size, but do not end in nippers, having instead a 

 spiny end-joint, which can be bent back on the equally spiny 

 joint by which it is borne to form an efficient grasping organ. 

 The arrangement may be compared to that found in the fore- 

 legs of the Praying Mantis (p. 117), and it is a common device 

 in many groups of animals which require a simple holding- 

 apparatus. It has already been pointed out that in Insects the 

 slender feelers or antennae are of great use in giving the animal 

 a means of exploring to the front, and that in Scorpions the large 

 pincers are used for this purpose. The same end is attained in 

 the Whip-Scorpions by diverting the front pair of legs from their 

 usual locomotor function, while at the same time they are modi- 

 fied into forwardly-directed organs of touch. The specialization 

 has not gone far in the tailed members of this group, but in the 

 tailless ones it is extreme. We have here an excellent example 

 of the principle of " change of function ", of which instances have 

 already been given in other connections, and the comparison of 

 Insect, Scorpion, and Whip- Scorpion, just instituted, also illustrates 

 another important biological principle, that of analogy, which lays 

 it down as an axiom that organs performing equivalent functions 

 are not necessarily equivalent as regards their origin and structure. 



SPIDERS (ARANEID^:) 



Spiders (Araneidce) are highly rapacious forms which con- 

 stitute a large and exceedingly interesting order, distinguished 

 from the other groups of Arachnida by many peculiarities. There 

 are no large grasping organs as in the forms so far described, 

 the appendages corresponding to them being short feelers with 

 bases that are used as jaws. Poison-glands are present, however, 

 but these, instead of being lodged in a posterior sting as in a 

 scorpion, are contained within the first jaws (chelicerce), upon 

 the tips of which they open. These jaws are not in the form 

 of nippers, but present the arrangement just described for the 

 seizing organs of whip-scorpions, the claw-like end-joint moving 



