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 afEected 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." 

 Natives of an infested 

 region may be so 

 immune or accus- 

 tomed to its attacks 

 as to be imaware of 

 its presence, though 

 such immunity is by 

 no means possessed 

 by all who have been 

 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 simimer, as larvse 

 at the base of the wings of houseflies or grasshoppers. 



There is much uncertainty as to the species of the larva attacking 

 man but it is clear that several are implicated. Bruyant has shown 

 that in France the larvee 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. 



Harvest mites. (Larva of Trombidium) . After C. V. 

 Riley. 



