ARTICULATA. 87 
organic matter which would otherwise pollute the water by its decay, and 
affording food for fish. 
Arachnida.—The Arachnida are commonly regarded as intermediate 
between Crustaceans and Insects. They have generally eight legs ; 
although some, like insects, have only six. None of them have 
wings. The higher kinds—spiders and scorpions—breathe by means of 
pulmonary cavities ; the lower—mites and ticks—by air-tubes. All of 
them have twe or more eyes; many have eight. Some of the lowest 
Arachnida are parasitic upon insects, and a few live on decaying animal 
or vegetable substances, of which we have an example in the cheese- 
mite. In this and other mites and ticks, the mouth is a mere proboscis 
formed for suction; but spiders and scorpions have a mouth fitted 
for tearing and masticating their prey. Scorpions are remarkable for 
the sting at the tip of the tail, the tail itself being a prolongation of the 
abdomen. Spiders also subdue their prey by means of poison ; but it is 
emitted through the mandibles. Both spiders and scorpions prey chiefly 
on, insects; but there are very large spiders in the tropical parts of 
South America which occasionally prey on small birds. Many spiders 
catch their prey by stealthily approaching it, and suddenly springing 
upon it; but others employ webs for this purpose, the spider lurking in 
a corner of the web till the vibrations of its threads announce that 
an insect is entangled in its meshes. Spiders’ webs are formed of a 
substance exuding from small protuberances called spinnerets, at the 
extremity of the abdomen. This substance is at first glutinous, but dries 
into thread as soon as it comes into contact with the air. 
Insecta.—Insects,! so named from the extremely marked division of the 
three segments which form the body, are the most important and numer- 
ous class of Articulata. The segments are often so deeply divided that 
the slenderness to which the body is reduced between them cannot be 
contemplated without admiration, as in the neck of a wasp, or in the 
link which connects its abdomen with its thorax. The first segment 
is the head, which may be regarded as formed of several rings, modified 
and condensed together till their character as distinct rings is lost. 
The second segment is the thorax, which is always formed of three rings, 
closely combined, but easily distinguishable. The third segment is the 
abdomen, usually consisting of nine rings. All insects in their adult 
state have six legs; some are destitute of wings; others have two, and 
others four; but no insect, and indeed no animal, has any other 
number, and the only animals having four wings are insects. The 
external covering of the body of insects is of a horn-like substance, in most 
parts hard, but more or less flexible. It is the principal framework of 
the body, and to it the muscles are attached. Respiration is extremely 
N 1 Latin insecta, cut into, from in, into, and seco, to cut. 
v 
