THE ARACHNIDA. 42/ 



times before adult age and all its powers are perfected. The 

 moulting may, therefore, be considered in the light of an incom- 

 plete metamorphosis. A great deal of careful observation is still 

 required before any decided assertions can be made satisfactorily 

 concerning even the habitual moultings of all the spiders that do 

 not undergo metamorphosis. Some genera do not even appear 

 to moult, and the incompleteness and deficiency of any metamor- 

 phoses have been stated in others, so that our present knowledge 

 permits us to say that the Arachnida very closely allied by habits, 

 instincts, and structural peculiarities, differ materially in their 

 method of evolution after leaving the zg^ ; moulting and metamor- 

 phosis being apparent or absent in spiders which resemble each 

 other very much. 



The relation of the metamorphoses to the classification of the 

 spiders is, therefore, very interesting to the philosophical naturalist, 

 but unsatisfactory to the species maker and to those who only 

 classify. 



The Arachnida are divided into two great orders. In the first, 

 which includes the true spiders and scorpions, the respiration is 

 carried on by means of sac-like depressions in the body, and in the 

 second the creatures breathe like the true insects and Myriapoda, 

 by means of tracheae. The true insects are thus connected with 

 the second order of tracheary spiders by a similarity of construc- 

 tion ; and the Crustacea or crab tribe, to be noticed hereafter, are 

 allied to the pulmonary Arachnida with the respiratory sacs, and 

 by the general arrangement of the segments of the body and limbs. 

 The Arachnida thus form a link between the insects and the 

 Crustacea. 



How the Arachnida are to be distinguished from these classes 

 may be understood by examining their peculiar conformation. Thus, 

 the Arachnida may be said to have the body divided into a chest 

 or thorax and an abdomen. Wings are never present, and not 

 even their rudiments. The legs are eight in number in the adult ; 

 the eyes are smooth ; and the head is soldered into and continuous 

 with the chest. 



If the Mygale fodiens — which is represented in the middle of its 

 curious nest in the accompanying engraving — be examined, it will 

 be found to consist of a swollen abdomen, which is attached to the 



