There is also a larger reserve of oxygen available in the body making that these spiders can react quicker and longer than the primitive spiders with only book lungs. Fast running and jumping spiders have a good developed trachea system. Symphyltognathidae are exceptional, they do not posses book lungs but has developed and very well performing trachea system. The size of the heart depends on the size of the developed trachea system. Spiders with a good developed system do not need a large heart because the pumping capacity can be smaller.
Less blood is needed to supply the organ with oxygen.
The hard skin made up of chitin the spider can not grow larger. The spider changes its skin 5 - 7 times in her life. Spider than can grow very old, like the Tarantula that may live for 25 years, the skin is changed every year. This is not because she keeps growing but, like our clothes, the skin becomes worn out. Before the spider changes its skin the color of the spider becomes.