Spiders and Their Near Relatives 



II —THE CHARACTERS OF THE ARACHNIDA 



At first sight, the Arachnida appear to be distinguished from 

 all other air-breathing arthropods by the absence of antennae; 

 for they have no feelers projecting forward from the head. But 

 it has been found that a pair of nippers, the chelicera?, with 

 which the head is armed, correspond to the second antennas 

 of Crustacea. Vestiges of the first antennae have been observed 

 in the embryos of certain arachnids; but these appendages either 

 disappear completely before birth or, according to the views 

 of some writers, become consolidated to form the upper lip or 

 rostrum. We can say, therefore, that in the Arachnida the first 

 pair of antennae are wanting as distinct appendages, and 

 the second antennae are modified so as to form prehensile 

 organs. 



Most Arachnida differ from all other air-breathing arthropods 

 in having the segments of which the head and thorax are composed 

 consolidated so as to form a single region, the cephalothorax ; 

 but in certain generalized forms the thoracic segments are more 

 or less distinct. In most cases the cephalothorax and abdomen 

 are distinct; but in the mites the entire body forms a single region. 



In the scorpions and in some other forms the abdomen i, 

 divided into two portions; a broad preabdomen, and a slenderer 

 tail-like division, the postabdomen. 



Perhaps the most distinctive characteristic of the Arachnida 

 is the fact that in this class the combined head and thorax bears 

 only six pairs of appendages; this, however, is also true of the 

 Palaeostracha, which some writers class with the Arachnida. 

 The nearest approach to this condition among air-breathim* 

 arthropods is found in the insects, where the head and thorax 

 bear seven pairs of appendages. 



Another striking characteristic of the Arachnida, which, 

 however, is also possessed by the Palaeostracha, is the absence 

 of true jaws. In other arthropods one or more pairs of append- 

 ages are jaw-like in form and are used exclusively as jaws; but 

 in the Arachnida the prey is crushed either by the prehensile 

 antennae alone (as in Kanenia) or by these organs and other 

 more or less leg-like appendages. The arachnids suck the blood 

 of their victims by means of a sucking stomach; they crush 







