INSECTS. 



235 



CLASS III. INSECTA. 



In the Insects there is a distinct head bearing four 

 pairs of appendages antennae, mandibles, maxillae, and 

 labium which would indicate the existence of at least 

 four segments in this region. The respiratory organs are 

 trachea, a system of branching tubes ramifying the whole 

 body and opening to the exterior by spiracles in the sides 

 of some of the somites. Air is drawn into these tracheae 

 by an enlargement of some or all of the 

 somites, and forced out again by their con- 

 traction. The reproductive organs differ from 

 those of other arthropods in having their 

 ducts open near the tip of the abdomen. 

 The Insecta are divided into Chilopoda 

 and Hexapoda. 



SUBCLASS I. CHILOPODA (Centipedes). 



In the Chilopods, which include the 

 centipedes and similar forms, the head is 

 succeeded by a long series of body-seg- 

 ments, each with a pair of locomotor ap- 

 pendages (legs), and with no distinction 

 between thorax and abdomen. Most of 

 the group are carnivorous, and the larger 

 forms, at least, are provided with poison- 

 glands which open in the first pair of the 

 trunk appendages. The chilopods of north- 

 ern latitudes are small, insect-feeding forms, 

 but in the tropics occur the centipedes, the 

 larger species of which are said to be ex- 

 tremely venomous. 



Usually the Chilopods are associated with another group, 

 the DIPLOPODA (thousand-footed worms), as a class or 



Fl c'h i 6 i 4 o JTo'd 



