_Pediculoides ventricosus_ (fig. 52 and 53) is, of all the Tarsonemidæ reported, the one which has proved most troublesome to man. It is a predaceous species which attacks a large number of insects but which has most commonly been met with by man through its fondness for certain grain-infesting insects, notably the Angoumois grain moth, _Sitotroga cerealella_, and the wheat straw-worm, _Isosoma grande_. In recent years it has attracted much attention in the United States and its distribution and habits have been the object of detailed study by Webster (1901). [Illustration: 54. Pediculoides ventricosus, male. After Braun.] There is a very striking sexual dimorphism in this species. The non-gravid female is elongate, about 200µ by 70µ (fig. 52), with the abdomen slightly striated longitudinally. The gravid female (fig. 53) has the abdomen enormously swollen, so that it is from twenty to a hundred times greater than the rest of the body. The species is viviparous and the larvæ undergo their entire growth in the body of the mother. They emerge as sexually mature males and females which soon pair. The male (fig. 54) is much smaller, reaching a length of only 320µ but is relatively broad, 80µ, and angular. Its abdomen is very greatly reduced. As far back as 1850 it was noted as causing serious outbreaks of peculiar dermatitis among men handling infested grain. For some time the true source of the difficulty was unknown and it was even believed that the grain had been poisoned. Webster has shown that in this country (and probably in Europe as well) its attacks have been mistaken for those of the red bugs or "chiggers" (larval Trombiidæ). More recently a number of outbreaks of a mysterious "skin disease" were traced to the use of straw mattresses, which were found to be swarming with these almost microscopic forms which had turned their attentions to the occupants of the beds. Other cases cited were those of farmers running wheat through a fanning mill, and of thrashers engaged in feeding unthrashed grain into the cylinder of the machine. [Illustration: 55. Lesions produced by the attacks of Pediculoides ventricosus. After Webster.]