78 NATURAL HISTORY. [n. ZOOL. GAL* 
lower half of the upper wing is like parchment or leather, 
and the other half membranaceous, like the lower pair, are 
in the 11th Case, as the Shield-bugs, Fire-flies ( Fulgora ), 
Sc c. 
The Apterous, or Wingless Insects, generally have 
four or five pairs of legs, and sometimes many more. They 
are always destitute of wings ; and the chief alteration 
that takes place in their approach towards maturity, is, 
that they gain additional legs; this change sometimes takes 
place at a very early age, which has caused some natu¬ 
ralists to believe that they do not undergo any meta¬ 
morphosis. 
The Arachnida (Cases 11 and 12) differ from the true 
insects in the head and thorax being united into a single 
mass, as well as in having more numerous legs and no wings. 
Some have pulmonary sacs, which fulfil the functions of 
lungs, with a heart, veins, and very distinct vessels ; these 
have six or eight simple eyes, and from the manner in 
which they respire, are called Pulmonarice , as the various 
kinds of true Spiders. By the side of the kind of Spider 
that forms it, is placed the tubular case of one of the earth 
spiders, which forms a door to its tube that falls down 
to close the cavity when the animal is within. The other 
Arachnida respire by tracheae, or air tubes, which traverse 
their body, as is the case with insects in general. The 
organs of circulation are wanting or very incomplete. 
They have two, or at most but four, simple eyes, and have 
been called Trachearice . Among these are the Tarantulae , 
Scorpions, &c. 
The Myriapodes, or Centipedes, are so called on ac¬ 
count of the number of their legs. They have gene¬ 
rally twenty-four or more, arranged along the entire 
length of the body, on a series of rings, each bearing one 
or two pairs. The first, and in several even the second 
pair of legs seem to form a part of the mouth. 
These animals bear some resemblance to little snakes 
and Nereides, from their feet being so closely applied to 
each other through the whole extent of the body. The 
number of the rings and the feet increases with age, a 
character which distinguishes them from the true Insects, 
which always have the same number of rings from their 
being first hatched to their adult state; all the true legs, 
