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pairs of mouth appendages, variously modified ; the thorax, 

 which consists of three segments (often united) has three 

 pairs of limbs for locomotion and usually one or two pairs 

 of wings (on second and third segments) ; and the abdomen, 

 which as a rule is without appendages, generally has ten 

 segments, but two or more of the hinder ones are frequently 

 telescoped. 



There is a ramifying system of respiratory air-tubes 

 (tracheae). Fat- cells in the haemocoele form a diffuse 

 fatty body. The central nervous system consists of a 

 nerve ring with cerebral ganglia, and a double ventral 

 nerve cord with, typically, one pair of ganglia to a segment. 

 The excretory organs are tubular outgrowths of the hind- 

 gut (Malpighian tubules). The sexes are separate ; but 

 parthenogenesis occurs The life history generally exhibits 

 metamorphosis (larva, pupa, and imago), the larval forms 

 (maggots, grubs, or caterpillars) being markedly different 

 from the adult* or imagines. 



Insects are terrestrial, aerial, and aquatic ; their in- 

 stincts and their adaptive features are remarkable, they 

 present many of the most striking examples of Resemblance 

 and Mimicry. Some insects are social, living in communities 

 and showing specialisation of individuals, and some are 

 parasites; many are of great economic importance, e.g., 

 honey bees and other " beneficials," insects injurious to 

 agriculture and forestry, and those blood-sucking species 

 and germ-carriers which spread disease in Man and animals. 



Structure of a Typical Insect. 

 The Cockroach (Periplaneta or Blatta). 

 Describe the External Features of the Cockroach. 



The chitinous cuticle is leathery, except about the joints. 

 The head, vertically long, is connected by a slim neck to 

 the thorax. The abdomen is flattened. 



The head bears two large compound eyes, two long and 

 jointed antennae, a pair of mandibles without palps (be- 

 t \\cni which is the mouth), and two pairs of maxillae. The 

 largest of the three thoracic segments is the prothorax ; 

 its tergum (the pronotum) overlaps the neck. The meso- 



