CHAPTER XIX 
TRACHEATA 
Cuass III. INSECTA 
CHARACTERISTICS.— Tracheata whose body is divided into three dis- 
tinct regions: head, thorax, and abdomen. The head carries 
the antennae and three pairs of mouth appendages. The 
thorax is conyposed of three segments, each with a pair of legs, 
and usually the posterior two segments bear each a pair of 
wings. The abdomen ws devoid of limbs, and consists of a 
varying number of segments; ten may be made out in some 
species, but the number is often less. 
The class Insecta includes an enormous number of species, 
probably far more than the whole of the rest of the animal 
kingdom put together. The single order of beetles—Coleoptera 
—contains more than 120,000 described species, and there is 
reason to believe that the flies—Diptera—are as numerous 
or even more so. At the present date, the total number of 
named species of insects must be very nearly a quarter of a 
million. 
The principles on which this enormous amount of material 
has been classified and brought into order rest upon (i.) the 
structure and arrangement of the mouth parts, (i1.) the charac- 
ters of the wings, (ili.) the relation of the first thoracic segment 
—the prothorax—to the rest of the thorax, and (iv.) the 
degree of metamorphosis. 
The arrangement of the mouth organs is intimately con- 
nected with the food of the insect; by the modification or 
suppression of some of the three pairs of oral appendages or 
parts of them, a very great diversity of structure is produced, 
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