366 Insect Architecture 



. CHAPTER XVIII. 



STRUCTURES OF SPIDERS. 



MODEEN naturalists do not rank spiders among insects, 

 because they have no antennae, and no division between 

 the head and the shoulders. They breathe by leaf-shaped gills, 

 situated under the belly, instead of spiracles in the sides ; 

 have a heart connected with these ; have eight legs instead 

 of six ; and eight fixed eyes. But as spiders are popularly 

 considered insects, it will sufficiently suit our purpose to 

 introduce them here as such. 



The apparatus by which spiders construct their ingenious 

 fabrics is much more complicated than that which we have 

 described as common to the various species of caterpillars. 

 Caterpillars have only two reservoirs for the materials of 

 their silk; but spiders, according to the dissections of 

 M. Treviranus, have four principal vessels, two larger and 

 two smaller, with a number of minute ones at their base. 

 Several small tubes branch towards the reservoirs, for carry- 

 ing to them, no doubt, a supply of the secreted material. 

 Swammerdam describes them as twisted into many coils of 

 an agate colour.* We do not find them coiled, but nearly 

 straight, and of a deep-yellow colour. From these, when 

 broken, threads can be drawn out like those spun by the 

 spider, though we cannot draw them so fine by many degrees. 



From these little flasks or bags of gum, situated near the 

 apex of the abdomen, and not at the mouth, as in caterpillars, 

 a tube originates, and terminates in the external spinnerets, 

 which may be seen by the naked eye in the larger spiders, in 

 the form of five little teats surrounded by a circle, as repre- 

 sented in the following figure. 



We have seen that the silken thread of a caterpillar is com- 



* Hill's Swammerdam, part i. p. 23. 



