96 ARTHROPOD A. 



Order III. ACARIDEA. 



MONOMEROSOMATA. 



Head, thorax, and abdomen united. Eight legs, six in the 

 young. Eespiration tracheal or dermal. With or without eyes. 

 Mouth either masticatory or suctorial. 



The bases of the chelicerae and of the pedipalpi sometimes 

 coalesce with the labrum and give rise to a suctorial rostrum. In 

 some mites certain pairs of legs are terminated by suckers, in 

 others by setse. In a common form, probably the earlier state of 

 a Trombidium (Acarus libellulce of Carus), of a bright scarlet 

 colour, found attached to the wings of various insects, there are 

 no legs or other appendages. Phytoptus (supposed by some to be 

 a larval form) does not appear to have more than four legs. 



Acariclea are mostly oviparous ; some are subject to a kind of 

 metamorphosis, being worm-like when hatched. They are gene- 

 rally parasitic, but many are also found in clung, decaying matter, 

 and on plants. Some are marine, and a few are found in fresh 

 water. Acarus- domesticus is the cheese-mite. Leptus autumnalis 

 is the harvest-bug. Gamuts coleopterorum occurs abundantly 

 on dung-beetles. Atax lives in the branchiae of Mytilus, Unio, 

 and Anodonta ; and Halarachne halichari in the nostrils of the 

 seal. Demodex folliciilorum is found in the sebaceous follicles of 

 man ; in the dog it causes the mange. It is a very minute ani- 

 mal, footless, and without a mouth ; after two or three changes of 

 skin it acquires its adult condition. The itch is caused by Sar- 

 cojrfes scabiei, an eyeless form, burrowing through the skin. 



Various species of Pkyfopfas are very injurious to vegetation, 

 either through the bud or through the leaves ; they browse on 

 the surface of these " until they have flayed it to the quick." 

 Galls are frequently caused by them ; but these are not true galls, 

 as " they have always an opening leading into them " (Murray). 

 The "witch-knot" found on the birch, and resembling a great 

 mass of twigs like a bird's-nest, is an abnormal growth of some 

 years caused by Phytopti. Some of these galls were formerly 

 supposed to be Fungi, and received from botanists such names as 

 Erineum, Phyllarium, &c. 



Tetranychus telarius (the red spicier) spins silky webs on the 

 leaves of various plants. Its colour is very variable. 



The young Acariclea with six feet were placed by Latreille in 

 his family " Microphthires." Hermann combined this order with 

 Phalangidea under the name of " Holetra." 



