■6o 
Parasitic Arthropods 
The Trombidiidae, or Harvest Mites 
In many parts of this country it is impossible for a visitor to go 
into the fields and, particularly, into berry patches and among tall 
weeds and grass in the summer or early fall without being affected by 
an intolerable itching, which is followed, later, by a breaking out of 
wheals, or papules, surrounded by a bright red or violaceous aureola, 
(fig. 43). It is often regarded as a urticaria or eczema, produced by 
change of climate, an error in diet, or some condition of general health. 
Sooner or later, the victim finds that it is due to none of these, but 
to the attacks of an almost microscopic red mite, usually called 
"“jigger” or “chigger” in this country. As the term “chigger” is 
applied to one of the true fleas, Dermatophilus penetrans, of the tropics, 
these forms are more 
correctly known as 
“harvest mites.” 
N atives of an infested 
region may be so 
immune or accus¬ 
tomed to its attacks 
as to be unaware of 
its presence, though 
such immunity is by 
no means possessed 
by all who have been 
44. Harvest mites. (Larvae of Trombidium). After C. V. , " , . .. 
Riley. long exposed to the 
annoyance. 
The harvest mites, or chiggers, attacking man are larval forms, 
possessing three pairs of legs (fig. 44). Their systematic position was 
at first unknown and they were classed under a special genus Leptus, 
a name which is very commonly still retained in the medical literature. 
It is now known that they are the larval forms of various species of 
the genus Trombidium, a group of predaceous forms, the adults of 
which feed primarily on insects and their eggs. In this country the 
species best known are those to be found late in summer, as larvae 
at the base of the wings of houseflies or grasshoppers. 
There is much uncertainty as to the species of the larvae attacking 
man but it is clear that several are implicated. Bruvant has shown 
that in France the larvae Trombidium inapinatum and Trombidium 
holosericeum are those most frequently found. The habit of attacking 
man is abnormal and the larvae die after entering the skin. Normally 
they are parasitic on various insects. 
