AEANEAE 437 



anus is at the binder end of the body; the genital pore lies in front 

 of it. 



The internal structure is characterized by its compactness. The 

 digestive tract is well developed. Two Malpighian tubules, if any, are 

 present. Many forms have no special respiratory system; others have 

 tracheae, which open to the outside through a pair of spiracles situated 

 either at the base of the mandibles or near the hind legs. 



The nerve ganglia are all united into a single mass, which is pierced 

 by the oesophagus. Mites are unisexual animals. The young animal is 

 usually bom as a larva with six legs (Fig. 713, C) ; after feeding awhile 

 it passes into a resting stage, from which it emerges as a so-called 

 nymph, which has eight legs but no genital orifice; at the end of the 

 nymphal stage it again becomes quiescent and develops into the adult. 

 In some forms the development is abbreviated and the young animal is 

 born as a nymph, or even as an adult. The six-legged larval stage has 

 been observed in certain species to be preceded by an eight-legged embry- 

 onic stage, which seems to indicate that the former condition is not a 

 primitive one. 



Most mites are land animals and about half the species are parasitic, 

 many, as the itch and mange mites and the ticks, being among the most 

 troublesome and even dangerous parasites infecting man and his domestic 

 animals. Many are parasitic on insects, especially on beetles and ants. 

 A few are entoparasites, Halarachne, living in the trachea of seals, and 

 Pneumonyssus, in the lungs of a monkey. The non-parasitic forms eat 

 small animals, including each other, and also decaying plant or animal 

 matter, and are found on plants and the ground, under bark or dead 

 leaves, while some form galls on plants. The Hydrachnidae and Halaca- 

 ridae are aquatic, the former living in fresh and the latter in salt water. 

 They are, however, very little modified for swimming, and possess no gills. 

 The order contains about 3,000 species, about 500 being known in this 

 country. These are grouped in about 26 families. 



Key to the families of Acarina here described : 



Ot Body elongate and worm-like, the hinder part ringed ; animals minute. 



6 t Gall mites ; but 4 legs present 1. ERIOPHYIDAE 



6 2 Eight legs present ; mammalian parasites 2. DEMODICIDAE 



a a Body not worm-like. 



&! No spiracles or trachea present ; minute mites. 

 Cj On mammals or birds, 

 dj Itch and mange mites ; parasites in the skin of mammals. .3. SARCOPTIDAE 



d 2 Bird mites ; among the feathers of birds 4. ANALGESIDAE 



c 2 In fruit, grain, cheese, etc., and in certain plants 5. TYROGLYPHIDAE 



6 2 Spiracles and tracheae usually present. 

 c a Horny mites ; a pair of usually club-shaped bristles on cephalothorax. 



6. OBIBATIDAE 

 c a No such bristles. 



