70 



Parasitic Arthropods 



There is a very striking sexual dimorphism in this species. The 

 non-gravid female is elongate, about 200(x by 70(x (fig. 52), with the 

 abdomen sHghtly 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 larvae 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 3 2 o [x but 

 is relatively broad, 

 ScfA, and angular. Its 

 abdomenis very great- 

 ly reduced. 



As far back as 1850 

 it was noted as caus- 

 ing 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 Trombiidse). 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. 



The medical aspects of the question have been studied especially 

 by Schamberg and Goldberger and from the latter' s summary (19 10) 

 we derive the following data. Within twe ve to sixteen hours after 

 exposure, itching appears and in severe cases, especially where expo- 



54. Pediculoides ventricosus, male. After Braun. 



