INSTINCTS AND CLASSIFICATION OF ARACHNIDA. 221 



fastens the end of one of these threads, of which the opposite ex- 

 tremity is still inclosed in the secreting apparatus, and of which it 

 can consequently increase the length at pleasure. The colour and 

 thickness of these threads vary greatly ; a Mexican Spider forms 

 a web composed of red, yellow, and black threads interwoven 

 with astonishing skill ; and it has been calculated that ten thou- 

 sand threads from the pores of a single spinneret of some of our 

 common Spiders do not equal in thickness one of our own hairs ; 

 whilst among other species inhabiting warmer climates, they form 

 such strong webs, that they are sufficient to stop small birds, and 

 even Man has to use some effort to break them. The manner 

 in which Spiders employ their skill in working, is subject to no 

 less variation ; some of them are satisfied with spreading their 

 threads irregularly ; others weave a web, whose meshes are of 

 extreme regularity. Sometimes we see them motionless in the 

 middle of their web, watching for their prey ; at other times 

 they conceal themselves in a retreat, which they construct very 

 near, and which has in some instances the form of a silky tube, 

 in others that of a small cup. 



822. The ARACHNIDA may be divided into two orders, from 

 differences in the organs of respiration and circulation. 



I. The Pulmonary Arachnida ; which are principally charac- 

 terised by the existence of pulmonary cavities, and of a vascular 

 apparatus ; they have six, eight, or even more eyes, and we also 

 find two, four, or eight stigmata on the under part of the abdomen. 

 The general form of these animals varies ; some, as the Spider, 

 have a globular body, with spinnerets at its extremity, and the 

 palpi small ; whilst in other instances, as the Scorpions, the body 

 is lengthened, and composed of several rings, the palpi large, ex- 

 tended like arms, and armed with pincers ; and the abdomen is 

 not terminated by spinnerets, but by the poisonous apparatus. 



II. The Tracheary Arachnida ; which have no pulmonary sacs, 

 but breathe by tracheae like Insects, and appear to have no com- 

 plete vascular apparatus for the circulation of the blood. Some of 

 them have no eyes ; and amongst those which possess these or- 

 gans, we never find more than two or four. Some of these animals 

 very much resemble Spiders; others have the mouth formed 

 for sucking, and constitute the family of Acaridte, or Mites. 



VOL. II. R 



