454 INVERTEBRATS MORPHOLOGY. 



frequently chelate, but in parasitic forms are reduced to 

 stylets enclosed by the fused basal joints of the pedipalps, a 

 proboscis being thus produced which can pierce the integu- 

 ment and thus render the juices of the host available as food. 

 The pedipalps {Mxp) undergo various modifications, being 

 sometimes long and limblike, sometimes chelate, while their 

 basal joints may or may not be fused. The four pairs of legs 

 are generally adapted for walking, and terminate in ungues or 

 bunches of hairs or, in some parasitic forms, in suctorial 

 disks, while in the Water-mites they are provided with usually 

 long bristles along the sides, serviceable swimming-organs 

 being thus produced. In the genus Demodex the four legs 

 are reduced to short unjointed structures each provided with 

 four ungues, while in the Leaf-mites, Phytoptus, which pro- 

 duce galls on the leaves of various plants, the two pairs of 

 posterior limbs are reduced to wartlike elevations bearing 

 bristles, the two anterior pairs being on the other hand five- 

 jointed. 



The chitinous covering of the body is usually thick and 

 delicately wrinkled. It usually bears numerous setae and 

 occasionally also plates or lateral prolongations, as in Ori- 

 bates and its allies. Dermal glands also frequently occur, 

 producing oily fluids and sometimes odoriferous secretions. 

 Spinning-glands opening on the pedipalps occur in Tetrany- 

 chus, frequently parasitic on the leaves of the Rose, but as a 

 rule they are not developed. 



A pair of stigmata (Fig. 209, st) occurs in many forms, 

 situated usually near the coxae of the last pair of legs, but 

 not unfrequently they are much further forward, lying near 

 the basal joints of the pedipalps or even of the chelicerse. 

 They open into tracheae which branch once, bunches of lateral 

 tracheae being situated at intervals upon the two branches. 

 Frequently, however, especially in parasitic and aquatic forms, 

 both tracheae and stigmata are wanting, as is usually also the 

 heart. When present {Gamasus, Ixodes) this latter structure 

 is small, with but a single pair of ostia, and is prolonged an- 

 teriorly into a slender aorta. 



The digestive tract is frequently provided with glands 

 opening into its anterior portion and supposed to be salivary. 



