MITES. 99 



The sexes are separate in the Araohnida, and may usually be distinguished by the 

 modifications of the palpi, pincers, or some of the legs. Some, as the scorpions, are 

 viviparous, but the majority do not hatch from the egg until after oviposition. The 

 eggs may either be deserted by the parent or carried around with her. We shall refer 

 to the development when treating of the different groups. 



The Arachuidae are usually divided into three orders: Acarina, Araneina, and 

 Arthrogastra. 



Obdee I. — ACARINA. 



This, the lowest order of the Arachnida, embraces the forms familiarly known as 

 mites and ticks. In these the hinder segments of the body are usually distinctly sepa- 

 rated from each other, but there is no constriction marking the division of the body 

 into cephalothorax and abdomen, such as is so evident in the true spiders. The mouth 

 parts (uj)per lip, chelioerae, and palj)i) are usually united to a greater or less extent, 

 forming a sucking tube, which in some forms acquires a considerable development. 

 Respiration is usually effected by tracheas, which commonly arise from two stigmata, 

 but, as we have just said, sometimes all respiratory organs are entirely absent. With 

 the exception of the Oribatidse, which bring forth living young, all are viviparous, and 

 the young, when they leave the egg, almost invariably have three pairs of feet, resem- 

 bling, in this respect, the hexapod insects. The fourth pair are added with growth. 



The different forms present many differences in habits ; many are parasites ; some 

 live in the water, both salt and fresh, some in the earth, while others pass the greater 

 portion of their life parasitic upon animals or plants. Directly they enter but slightly 

 into the affairs of man, but indirectly many of them play an important part in human 

 interests, some destroying injurious insects, while others, on the other hand, attack 

 many objects which are of use or value to mankind. 



The Acarina are divided into seven families, but it must be borne in mind that at 

 least the minor divisions cannot all be regarded as firmly established ; for there exists 

 among many members of the order a marked polymoi'phism, the same species, at differ- 

 ent ages or under varying environment, assuming characters so different as to lead 

 naturalists to assign the different forms to distinct genera, or in some cases even to 

 different famihes, and the errors thus introduced have only been discovered after long 

 and careful study of the life-history of the form in question. So far as is known, the 

 mites first appear in the pliocene division of the tertiary age, specimens of Sydrachna 

 and Trombidiitm being found in the amber of Pomerania. 



The first family to be considered is the Acaeid^. These have a soft skin, the body 

 oval or elongate, ocelli absent, and the feet frequently terminated with an adhesive 

 vesicle. The chelicerse are chelate or needle-shaped. Though minute, these forms are 

 by no means to be despised, as they all come under the head of noxious insects, and 

 besides injuring and destroying many human products they even produce disease in 

 mankind. Space will allow us to mention but a few prominent representatives of the 

 family. 



In 1841 Henle described a peculiar parasite found in the hair-follicles of the human 

 body, and a little later Simon published a careful account of the same form. From 

 this time to the present this form has attracted considerable attention, and has been 

 described under five generic names, that of Demodex having priority. The number of 

 species is uncertain, it not being known whether the forms found in different animals 



