2 
INTRODUCTION. 
surface of the body; in the latter the respiration is 
effected by means of extremely beautiful and delicate 
air-tubes, which convey the oxygenating element to all 
parts of the body. The Sub-Kingdom Arthropoda may 
be represented in a tabular form thus :— 
Arthropoda. 
Tracheata r Class 1. Insecta. 
(breathing by tracheae) 
(aerial) 
Divisions -< 
Class 2. Myriapoda. 
Class 3. Arachnida. 
Bran chi at a 
(breathing by bran¬ 
chiae) (aquatic) 
s 
^ Class 4. Crustacea, 
At present we are only concerned with Class I., the 
Insecta, which it will be necessary to define, in order to 
distinguish it from the two other air-breathing classes 
of the same sub-kingdom. Insects are air-breathing 
animals having three well-marked divisions of the body, 
the head, thorax, and abdomen, always distinct one 
from the other; the thorax, or middle portion, is 
composed of three segments, the prothorax, to which is 
attached the first pair of legs, the mesothorax with the 
second pair of legs and first pair of wings, and the 
metathorax with the third pair of legs and the second 
pair of wings, where these last organs ace present. The 
head always carries two antennre. Insects are generally 
furnished with wings, and undergo a series of trans¬ 
formations (: metamorphosis ) before arriving at their adult 
