HYPOPID^. 243 



Next, these eggs produce young Tyroglyphi, exactly after the 

 type of their parents, but with only six legs. See first figure on 

 p. 22,^^. This io the usual first stage of development in all 

 acarids. 



It is not until the young animal has passed from the six-legged 

 into the second or eight-legged unsexual stage that either Claparede 

 or Megnin begin to see any other form inside either Tyroglyph 

 or Hypopus. The figures they give of such incidents all repre- 

 sent both outer and inner animal as eight-legged. 



It is only when it approaches the nymph stage (towards the end 

 of the first sexual eight-footed stage, which is the third stage of 

 development) that the internal transformation is first seen. At 

 that stage Claparede saw and gives the figure of a young eight- 

 footed Tyroglyph with another Tyroglyph inside, which we have 

 reproduced (second figure on p. 238) ; and Megnin saw and gives 

 a figure of a Hypopus with a Tyroglyph inside, and another figure 

 of a Tyroglyph bursting its way out from the shell of a Hypopus, 

 both eight-footed, which we have copied, p. 242. Both agree that 

 it is a Tyroglyphus that is within, and that comes out; but Claparede 

 says it is inside a Tyroglyphus, and Megnin, inside a Hypopus, 

 and Dujardin that it is a Gamasus inside a Hypopus. Have these 

 gentlemen been looking at the same thing, or do their figures 

 represent different events happening at different stages in the life 

 of the same individual ? If the former, then it seems clear that 

 it can only be determined by further experiments made by some 

 other equally competent observers. If we take both as happening, 

 but happening at different stages, then it is clear that by doing 

 so we diminish the probability of one of them ; for changes by 

 development are only accomplished at the periods of moulting, 

 and as these are limited in number, each additional change intro- 

 duced reduces the opportunities of effecting it. We presume that 

 it will never be possible to ascertain, by actual observation, 

 the exact number of times that these mites change their skin ; but 

 the number of times is said to be four, and at all events there are 



Q 2 



