300 



NATUEAL HISTORY. 



.X(^ 



5 10. The class of 

 Arachnida is divided 

 into two groups. In 

 the first group the 

 respiratory organs 

 are different from 

 those of Insects, Ii> 

 stead of passages ev- 

 Fig. 233.— The Scorpion. gry where for air, 



there are some sac or bag like cavities in the abdomen, 

 and in these are thin membranous plates arranged like 

 the leaves of a book. The air goes in among these, and 

 acts on the blood in the vessels spread out on them. 

 This group includes the Spiders and Scorpions. In the 

 second group the respiratory apparatus is like that of 

 Insects. This includes Mites of various kinds, Father- 

 loDg-legs, the minute red Spiders of green-houses, etc. 



511. Most of the true Spiders are great spinners. They 

 do not spin for themselves a cocoon as the caterpillars 

 do, for they undergo no metamorphosis. They spin chief- 

 ly for two purposes — to construct a dwelling for them- 

 selves, and to construct traps to catch their prey. Some 

 also, like some of the caterpillars, spin as they drop to 

 escape their enemies, and thus save themselves from a 

 fall. Some throw out a long thread into the air from 

 their spinning machine, and let it, when it is of sufficient 

 length, bear them aloft like a balloon. And some spin a 

 cocoon in which they deposit their eggs. I found one 

 of these cocoons the past summer fastened to the bark 

 of a tree. I opened it, and it was all a moving mass 

 within. On looking at it with a pocket microscope, I 

 found that it was full of little Spiders, which probably 

 had just been hatched from the eggs, but were not yet 

 ready to come out. The manner in which the cocoon is 

 formed and filled with eggs is curious. The Spider first 

 spins the lower half of it, and into this silken cup it drops 

 the eggs. It not only fills it, but piles up eggs on top 



