30U NATURAL HISTORY. 
oN gi RSE 510. The class of 
ree vies Arachnida is divided 
into two groups. In 
the first group the 
respiratory organs 
are different from 
those of Insects. In- 
eZ stead of passages ey- 
Fig. 233.—The Scorpion. ery where for air, 
uuere 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- 
long-legs, the minute red Spiders of green-houses, ete. 
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 spmning 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 
