Introduction 15 



in its various layers (Figure 7). The ornamentation of the cuticle is 

 usually constant within species and is used in some groups to dis- 

 tinguish them. 



Many special structures are derived from the cells of the hypoder- 

 mis. There are various glands, setae, dyes, and special sensory organs 

 formed wholly or in part by specialized portions of the hypodermis 

 and its secretions (Figure 8). With the exception of the setae and 

 hypodermal glands integumental derivatives will be considered under 

 the organ systems to which they belong. 



Figure 8 Transverse section through the integument of Trombidiiim holo- 

 sericeiim (Linnaeus), 1758. (After Vitzthum 1940) 



The integument of many acarinids is provided with glands. En- 

 larged cells of the hypodermis pour their secretions directly out on the 

 surface of most ticks. The water mites are well provided with glands 

 that open in specialized areas over the body (Figure 9). These glands 

 are more complicated than those of the ticks. Most of the Sarcopti- 

 formes have a pair of oil glands that produce an oily fluid that is light 

 yellow to deep brown in color. Similar glands have been found in a 

 few of the Mesostigmata, e.g., Eulaelaps stabularis and Euhaemoga- 

 masus horridus. 



The forms of setae are legion. As in most arthropods the setae serve 

 many functions. They are tactile organs in that the nerves are so ar- 

 ranged as to pick up pressure exerted on the setae. Some setae must 

 also serve as organs of chemo-reception. Others actually protect their 

 owners from attacks of predators in the same manner as the quills of 

 a porcupine protect it. In many mites two fundamentally different types 



