SOME INSECTS CLASSIFIED. 63 
By comparing the notes we have already made we 
find that the insects so far studied have Jd2lateral 
symmetry, jointed bodies, and jointed appendages, as 
legs and antenne. They have three parts to the body; 
head, thorax, and abdomen. All have six jointed 
legs, and in the adult stage one or both sexes are 
usually provided with wings. Commonly two pairs of 
wings and a pair of compound eyes are present. 
An examination of the internal structure of insects 
shows a series of ganglia connected by nerves situated 
along the ventral portion of the body. Above this is 
found the digestive cavity, consisting of a tube more 
or less branched extending lengthwise of the body, 
from the mouth to the anus. Near the dorsal part of 
the body is found a large blood-vessel which performs 
the function of a heart. 
Breathing is carried on by means of spiracles con- 
nected with trachee. Naturalists have agreed to call 
animals having these characteristics ‘‘ Insecta ’’. 
Differences. But the class Insecta contains so many 
individuals that we readily see the necessity of classify- 
ing them in some way. The divisions of classes are 
called orders. If we can divide the class Insecta into 
orders we make our future study more systematic and 
more satisfactory in many ways. Such a classification 
must depend on differences as well as resemblances. 
If we should study the mouth-parts alone of the insects 
we have already examined, we could easily put them 
into two orders or subclasses, one comprising those 
insects which have biting mouth-parts and the other 
those which have sucking mouth-parts. This division 
is sometimes used, but for our purpose it will be 
better to find some differences which will give us a 
greater number of orders, and so greater convenience. 
A mode of separation based on peculiarities of the 
wings has been much used, and the common names 
for some of the orders as now most often written 
attempt to describe peculiarities of wing-structure. 
