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 cox* 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¬ 
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 cox* 
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 spinipes (nat. size). 
