58 Parasitic Arthropods 



body, are able to exist for a greater or less period of time in their 

 unusual environment. These are generally called in the medical 

 literature "pseudoparasites" but the term is an unfortunate one. 



We shall now take up the different groups of arthropods, discussing 

 the more important of the parasitic forms attacking man. The 

 systematic relationship of these forms, and key for determining 

 important species will be found in Chapter XII. 



AcAEiNA OB Mites 



The Acarina, or mites, form a fairly natural group of arachnids, 

 characterized, in general, by a sac-like, unsegmented body which is 

 generally fused with the cephalothorax. The mouth-parts have been 

 united to from a beak or rostrum. 



The representatives of this group undergo a marked metamor- 

 phosis. Commonly, the larvag on hatching from the egg, possess but 

 three pairs of legs, and hence are called hexapod larvcB. After a molt, 

 they transform into n3Tnphs which, like the adtilt, have four pairs of 

 legs and are called octopod nymphs. These after a period of growth, 

 molt one or more times and, acquiring external sexual organs, become 

 adult. 



Most of the mites are free-living, but there are many parasitic 

 species and as these have originated in widely separated families, the 

 Acarina form an especially favorable group for study of the origin of 

 parasitism. Such a study has been made by Ewing (191 1), who has 

 reached the following conclusions: 



"We have strong evidence indicating that the parasitic habit has 

 originated independently at least eleven times in the phylogeny of the 

 Ararina. Among the zoophagous parasites, the parasitic habit has 

 been developed from three different types of free-living Acarina: 

 (a) predaceous forms, (b) scavengers, (c) forms living upon the jtiices 

 of plants." 



Ewing also showed that among the living forms of Acarina we can 

 trace out all the stages of advancing parasitism, semiparasitism, 

 facultative parasitism, even to the fixed and permanent type, and 

 finally to endoparasitism. 



Of the many parasitic forms, there are several species which are 

 serious parasites of man and we shall consider the more important of 

 these. Infestation by mites is technically known as acariasis. 



