CHAPTEE XIX 



TEACHEATA 



Class III. INSECTA 



Chaeacteeistics. — Tracheata whose body is divided into three dis- 

 tinct regions : head, thorax, and abdomen. The head carries 

 the antennae and three pairs of mouth appevdages. Tlie 

 thorax is composed of three segments, each with a pair of legs, 

 and usually the posterior two segments hear each a pair of 

 wings. The abdomen is 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, (ii.) the charac- 

 ters of the wings, (iii.) 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, 



21 



