THE AC ARID A. 433 



many times, and even after that period it is still subject to 

 moults, which occur every year — in the spring and after the &^^g 

 laying.* There is no phase in this progressive development like 

 a metamorphosis, but nevertheless the unfavourable condition of 

 the lately-born spider is very different to that of the adult, and 

 the perfection of all the organs and the strengthening of the 

 tissues keeps pace with the consecutive changes of skin. 



The spiders which undergo metamorphosis, and to which the 

 Acarida or mite tribe, and the Hydrachnidce or water mites belong, 

 breathe by means of tracheae, and there are no segments in and 

 on the abdomen. All these last-mentioned AracJmida undergo 

 metamorphosis, and their larval condition brings them in close 

 relationship with the true insects with six legs. There are some 

 exceptions to the invariable occurrence of metamorphosis, but this 

 should be expected when it is remembered that the phenomenon 

 is not constant even amongst the insects, and that it was probably 

 added to their evolution in consequence of the influence of ex- 

 ternal conditions acting under some law of life. 



The unsegmented condition of the abdomen in the tracheary 

 Arachnida belonging to the genera which undergo metamorphosis 

 is invariable, and this part of the body is united without the inter- 

 vention of a depression even, to the segments which support the 

 legs, and it is often found that the head, thorax, and abdomen are 

 all fused together to form one indivisible mass. The mouth in these 

 is always provided with a lower lip which is spoon or sheath-shaped, 

 and which envelopes for the most part the other portions of the 

 mouth, and does not allow them to be free as in the other Arach- 

 nida. The palpi which are attached to the mandibles vary greatly 

 in size, length, and function, and their peculiar construction and 

 different uses have formed the basis for the classification of these 

 small creatures. 



M. Duges has given some most interesting details concernmg 

 the structures and metamorphoses of the tracheary Arachnida 

 in the " Annales des Sciences Naturelles," 1834. 



He found many small eggs sprinkled over stones in shady 

 places like so many tiny white points, and, on examining them 



♦ Audouin in " Cyclopaedia of Anatomy and Physiology." 



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