CHAPTER IV 
ACCIDENTAL OR FACULTATIVE PARASITES 
In addition to the many species of Arthropods which are normally 
parasitic on man and animals, there is a considerable number of those 
which may be classed as accidental or facultative parasites. 
Accidental or facultative parasites are species which are normally 
free-living, but which are able to exist as parasites when accidentally 
introduced into the body of man or other animal. A wide range of 
forms is included under this grouping. 
Acarina 
A considerable number of mites have been reported as accidental 
or even normal, endoparasites of man, but the authentic cases are 
comparatively few. 
In considering such reports it is well to keep in mind von Siebold’s 
warning that in view of the universal distribution of mites one should 
be on his guard. In vessels in which animal and other organic 
fluids and moist substances gradually dry out, mites are very abund¬ 
antly found. If such vessels are used without very careful prelimi¬ 
nary cleaning, for the reception of evacuations of the sick, or for the 
reception of parts removed from the body, such things may be readily 
contaminated by mites, which have no other relation whatever to 
them. 
Nevertheless, there is no doubt but that certain mites, normally 
free-living, have occurred as accidental parasites of man. Of these 
the most commonly met with is Tyroglyphus siro, the cheese-mite. 
Tyroglyphus siro is a small mite of a whitish color. The male 
measures about 500^ long by 250^ wide, the female slightly larger. 
They live in cheese of almost any kind, especially such as is a little 
decayed. “The individuals gather together in winter in groups or 
heaps in the hollows and chinks of the cheese and there remain 
motionless. As soon as the temperature rises a little, they gnaw 
away at the cheese and reduce it to a powder. The powder is com¬ 
posed of excrement having the appearance of little grayish microscopic 
balls; eggs, old and new, cracked and empty; larvae, nymphs, and 
perfect mites, cast skins and fragments of cheese, to which must be 
added numerous spores of microscopic fungi.” — Murray. 
