AKACHNIDA. 641 



than before, the whole animal assuming an embryo-like appear- 

 ance, and moving about like a rounded mass in its enclosure. 

 Indeed is this process not (though Claparede does not say so) 

 a histolysis of the former larval tissues, and the formation of a 

 new body, as in the change of the six-footed insect beneath the 

 larva skin, where the pupa is formed ? A new set of limbs 

 grow out, this time there being four instead of three pairs of 

 legs, while the old larval skin is still embraced within the 

 membrane containing the second larval rounded mass. Soon 

 the body is perfected, and the pupa, as we may properly call it, 

 slips out of the larval membrane. 



The "second larva" after some time undergoes another 

 change ; the limbs grow much shorter and are folded beneath 

 the body, the animal being immovable, while the whole body 

 assumes a broadly ovate form, and looks like an embryo just 

 before hatching, but still lying within the egg. This may also 

 be comparable with the formation of the adult fly within the 

 puparium. (Compare Weismann's account of this process in 

 Musca, pp. 63, 64.) This period seems to be an exact repeti- 

 tion of the histolysis, and the formation of new tissues for the 

 building up of a new body which preceded the pupal stage, 

 while the adult mite slips out of its pupal membrane just as 

 the pupa threw off its larval membrane. This process, again, 

 may be compared to an adult butterfly, or fly, emerging from 

 its pupal membrane. 



Thus the mites, at least several species, pass through a series 

 of metamorphoses similar to those of such insects as have a 

 complete metamorphosis (except that the Acarian pup'a is 

 active), while the absence of such a metamorphosis in the 

 spiders is paralleled by the incomplete metamorphosis of the 

 Orthoptera and many Neuroptera, which reach adult life by 

 simple moultings of the skin. 



In the genus Myobia there is not only a deutovum, besides 

 the original egg, but also a tritovum-stage. The eggs of this 

 mite are long, oval and conical at the posterior end. The em- 

 bryo, with the rudiments of limbs, is represented by Fig. 5 of 

 Plate 11. The little tubercles md and mx, represent the man- 

 dibles and maxillae, while the three pair of legs, p l -p s , bud 

 out from the middle of the body ; Ic represents the head-plate. 

 41 



