TYPE ARACHNID A. 



453 



7. Order Acarina. 



The Acarina are for the most part srnall forms, many 

 being almost microscopic, while the largest, the Ticks (lxodes\ 

 do not when at their greatest size exceed a centimeter in 

 length, the males being much smaller. Some forms, such as 

 Onbates and Nothrus, live among moss and in similar situa- 

 tions, while others, such as Hydrachna and Atax, are aquatic. 

 Many forms are, however, parasitic either upon plants (Te- 

 tranyclius and Pliytoptus) or on animals, the genus Sarcoptes 

 being the cause of the disease termed the Itch in man, the 

 symptoms being produced by the Mites burrowing beneath 

 the skin. Other forms affect various animals and birds, the 

 genera Dermaleichus, Analges, etc., feeding upon the feathers 



FIG. 208. A, Sarcoptes scabiei ; B, Demodex phylloides (after CSOKOR from 



WRIGHT). 



of various birds, while others, such as Demodex (Fig. 208, B\ 

 live ia the hair-follicles or sebaceous glands of the skin, pro- 

 ducing acnelike pustules. The larvse of many forms which 

 are non-parasitic in adult life have a parasitic habit, as for 

 instance the larvae of many of the Water-mites and of the 

 Harvest-mites (Trombidiuni), while other forms live upon 

 organic matter of various kinds, as does the Cheese-mite, 

 Tyroglyphus. 



A distinguishing characteristic of the Acarina is the ab- 

 sence of any segmentation and the fusion of head-thorax and 

 abdomen to a single mass (Fig. 208, A). The form of the 

 appendages varies greatly in different genera according to the 

 use to which they are put. The cheliceree (Fig. 209, Md) are 



