INSTINCTS AND CLASSIFICATION OF ARACHNIDA. 215 



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. 



751. The ARACHNIDA are divided into two orders, according 

 to the structure of the organs of respiration and circulation. 



I. The Pulmonary Arachnida ; which are principally charac- 

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

 apparatus ; but we may also recognise them by other peculiari- 

 ties of structure ; thus the number of their eyes is six, eight, or 

 even still greater, 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 Spiders, 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, extended like arms, and armed 

 with pincers; and the abdomen is not terminated by spinne- 

 rets, but usually 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 organs, 



