Spiders and Their Near Relatives 



The legs consist of from five to seven segments. When only 

 five segments are present they are the coxa, trochanter, femur, 

 tibia, and tarsus; the tibia may be divided forming a patella, and 

 sometimes the femur is also divided. In some forms the terminal 

 segments of the legs are divided by more or less distinct false 

 articulations. The tarsus ends in one or two claws; and in many 

 cases an empodium is present; this may be either a cup-shaped 

 sucker or may be claw-like. 



One or more pairs of lateral eyes are usually present; as a 

 rule these are sessile, but sometimes they are elevated on pedicels. 

 Rarely the eyes are situated near the middle line of the body. 



There is great difference in the position of the openings 

 of the abdomen. Normally the genital aperture is on the ventral 

 surface of the abdomen near the base; but in some forms, as the 

 ticks, the genital segment is pushed forward between the legs so 

 far that it is close to the mouth; and in other forms the genital 

 opening is at the extreme tip of the body, and the anus is upon 

 the dorsal aspect of the abdomen. 



Mites breathe either by tubular tracheae or by the general 

 surface of the body. In those forms that possess tracheae there 

 are great differences as to the position of the spiracles; these 

 variations have been used by some writers in limiting the principal 

 divisions of the order. 



In the course of their postembryonic development, mites 

 undergo a metamorphosis. Most species are oviparous, some 

 are ovoviparous and a few are viviparous. From the egg there 

 appears a form which normally has only three pairs of legs; this 

 is known as the larva. The use of the term larva in this con- 

 nection is appropriate; for this form resembles the larva of an 

 insect with a complete metamorphoses in being adaptive, as is 

 shown by the temporary reduction of the number of legs. The 

 larva feeds for a time and then, after a resting stage, the skin 

 is shed and the eight-legged nymph appears. Usually at least, 

 the added pair of legs is the fourth. There may be one or more 

 nymphal moults, and the successive instars may differ in appear- 

 ance, but all lack a completely developed genital orifice. At the 

 close of the nymphal life there is a resting stage during which 

 the nymph transforms to the adult, there being a histolysis and a 

 rebuilding of some of the internal organs as in the pupae of insects. 



Variations from this usual course of transformation occur. 



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