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 egg 
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 Arachnida 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 concerning 
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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