ARACHNOIDEA. 33 



Order ARACHNOIDEA. 



(SCORPIONS, SPIDERS, AND MITES.) 



The system of classification which is adopted for the chief 

 part of this collection, viz. that of beginning with the lower orders 

 and advancing to the higher, has been departed from in the 

 spiders ; first, for the sake of making a natural connection with 

 other orders, and second, for reasons special to Economic Ento- 

 mology, such as keeping together as much as possible those 

 species that attack a common class of objects, and separating 

 those that attack a different class. 



In the present case (the arachnoids), the arrangement has been 

 made from above downwards, commencing with the scorpions 

 and terminating with the mites, instead of the reverse ; this brings 

 the mites into closer connection with the lice, with which it will 

 be found that they have certain points of analogy if not of 

 affinity. 



The arachnoids are generally considered by zoologists as a 

 group apart, belonging neither to the crustaceans nor the insects. 

 Like crustaceans, spiders possess the property of reproducing 

 such limbs as have been detached or mutilated ; and this curious 

 physiological phenomenon is intimately connected with the cast- 

 ing of their skin, for legs, palpi, and spinners, which have been 

 amputated, are observed to be restored, and afterwards to have 

 their dimensions enlarged, at the period of moulting only; but 

 there are much more important grounds for removing them from 

 the crustaceans and classifying them with insects. For example, 

 the crustaceans breathe by gills, the spiders do not. The sepa- 

 ration of the spiders from insects is not nearly so well marked. 

 Insects breathe by tracheae, that is, air-tubes opening on each side 

 of the segments of the abdomen. Spiders breathe by something 

 called pulmonary sacs, which are placed at the base of the abdo^ 

 men. But not only are these pulmonary sacs (although fewer than 



