28 THE INSECT WORLD. 
If, in addition to these characteristics, one considers that these 
animals are not provided with interior skeletons—that their 
nervous system is formed of a double cord, swelling at intervals, 
and placed along the underside of the body, with the exception of 
the first swellings or ganglions which are under the head—that they 
are not provided with a complete circulating system—that they 
breathe by particular organs, termed trachee, extending parallel 
to each other along each side of the body, and communicating 
with the exterior air by lateral openings termed spiracles—that 
their sexes are distinct—that they are reproduced from eggs—and, 
in conclusion, that the different parts we have mentioned are not 
complete until the creature has passed through several successive 
changes, called metamorphoses, a general idea may be formed of 
what is meant in zoology by the word “insect.” 
Insects, whose general organisation we have briefly traced, have 
been classed by naturalists as follows :— 
. ApTERA (Fleas and Lice). 
. Divrera (Gnats, Flies, etc.). 
Hemiptera (Bugs, etc.). 
. LeprpoprTera (Butterflies and Moths). 
. OnTHoprERA (Grasshoppers, Crickets, Cockroaches, etc.). 
. Hymernortera (Bees, Wasps, etc.). 
. NevrortEra (Libellula, or Dragon-fly ; Ephemera, or las fly ; 
Phryganea, or Alder-fly). 
8. CoLzoprEeRA (Beetles). 
NOOO FP WON 
We shall commence the history of the various orders, by 
examining the Aptera. 
