HARVEST SPIDERS. 



239 



one another and to the carapace to form a single large plate, its separate elements 

 being merely defined by shallow grooves. The lower surface of the carapace is 

 either almost wholly covered by a forward prolongation of the sterna of the 

 anterior abdominal segments, or by the ingrowth of the coxae of the appendages. 

 The mandibles are composed of three segments, and are always pincer-like, and 

 sometimes very powerfully developed. The appendages of the second pair 

 (maxilla and palpus) consist of six segments, and are never chelate, although in 

 some species they are armed with spines, and the claw is much enlarged and capable 

 of being folded back upon the tarsus. In these species the appendage is used as an 

 organ of attack and 

 defence. The four 

 pairs of legs are alike 

 in form and function, 

 being used for loco- 



O 



motion. In addition 

 to the mandibles and 

 maxillae, there are 

 often accessory 

 mouth - parts, taking 

 the form of masti- 

 cating lobes on the 

 maxillae and the coxae 

 of the first and second 

 pairs of legs ; while 

 above the mouth 

 there is frequently a 

 labrum, or upper lip, 

 and above this a 

 second piece, or 

 clypeus. As in false 

 scorpions, breathing 

 is effected by means 

 of tracheal tubes, 

 opening by a pair of 

 orifices situated on 

 the sternal plate of 

 the abdomen, immediately behind the coxae of the first pair of legs. In addition 

 to these stigmata, there is one on each side of the cephalothorax lying below the 

 edge of the carapace and above the coxae of the first pair of legs. These were 

 originally regarded as the apertures of breathing - organs, but it is now known 

 that they lead into glands, probably secreting an odorous and repellent fluid. In 

 some species of harvest spiders, the males and females are almost exactly alike ; 

 but usually the two sexes are recognisable by sharply marked characters. In the 

 males, for instance, the body is smaller and often more brightly coloured, while the 

 legs are both longer and more strongly spined, some of their segments being often 

 modified in shape. The greatest modification, however, is found in the mandibles, 



SOUTH AMERICAN HARVEST SPIDER, Gonyleptes S2nnipes (nat. size). 



