SOME HUMAN PARASITES 293 



microscope; it is quite impossible to see them with the 

 naked eye. 



The structure of the adult is clearly shown in fig. 94. 

 Its short legs, striated, worm-like abdomen, and its mouth 

 parts arranged for sucking, render it so curious an object 

 that it cannot be confounded with any other mite. The 

 female mite lays heart-shaped or fusiform eggs, from which 

 legless larvae emerge ; they later become six-legged, then 

 they transform into eight-legged nymphs, and after two 

 moults become adult. Very similar mites have been found 

 in the dog, cat, pig, and ox. The human species is cosmo- 

 politan and possibly pathogenic ; it has been considered as 

 a probable agent in the spread of cancer, leprosy, and a 

 tropical skin disease known as Lichen spinulosus. 



Annoying pests in this country during the autumn 

 months are the harvest mites, Leptus autumnalis (fig. 95), 

 first described by Shaw in 1790 ; there are two American 

 species, known as Leptus irritans and Leptus americanus. 

 These six-legged insects are the larvae, or the immature 

 forms, of eight-legged members of the family Trombidiidce ; 

 the life-history of the British species is not fully known, 

 and, as a consequence, its identity is uncertain. In size 

 these creatures are exceedingly small ; in colour they are 

 blood red. They belong to the same class as the ticks and 

 spiders, so they are not true insects. 



Their attacks are most frequent on those parts of the 

 body nearest the ground, so that the ankles and legs, just 

 below the knee, suffer most. According to some authorities, 

 these minute creatures penetrate the sweat glands ; in any 

 case, they cause intense irritation by burrowing beneath 

 the skin of their human host. The life-history of an 

 American species is as follows : Minute, brown, spherical 

 eggs, to the number of three hundred, are laid in or on the 

 ground by each female. Almost immediately they are laid 

 the outer skin splits, exposing a pale membrane within. 

 The larvae are six-legged, circular or ovoid creatures, and 



