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. 
Acarina or 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 larvae on hatching from the egg, possess but 
three pairs of legs, and hence are called hexapod larvae. After a molt, 
they transform into nymphs which, like the adult, 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 (1911), who has 
reached the following conclusions: 
“We have strong evidence indicating that the parasitic habit has 
originated independently at least eleven times in the phvlogeny 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 juices 
of plants.” 
Ewing also showed that among the living forms of Acarina we can 
trace out ah the stages of advancing parasitism, semiparasitism, 
facultative parasitism, even to the fixed and permanent type, and 
finally to cndoparasitism. 
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. 
