THE COCKROACH. 293 



coloured. They show parallel differences between the sexes, 

 and great wealth of species within a narrow range. One 

 expects to find that insects, like birds, have a high body 

 temperature. 



General Characters. — Like other Arthropods, Insects 

 have segmented bodies, jointed legs, chitinous armature, and a 

 ventral chain of ganglia linked to a dorsal brain. Compared 

 ivith Peripatus and Myriopods, adult insects show concentra- 

 tion of the body segments, decrease iti the mimber and increase 

 in the quality of the appendages, and wings withal. 



Insects are terrestrial and aerial, and rarely aquatic 

 animals ; usually winged as adults, breathing by means of 

 trachea, and often with a metamorphosis in the course of their 

 groivth. 



The body is divided into three distinct regiofis, — head, 

 thorax, and abdomen. The head bears three pairs of mouth 

 appendages (= legs), and a pair of pre-oral out-growths — 

 the antennce ; the thorax bears a pair of legs on each of its 

 three segments, and, typically, a pair of wings on each of 

 the posterior two ; the abdomen has no appendages, unless 

 rudimentary modifications of these be represented by stings, 

 ovipositors, &'c. 



First Type of Insects, Periplaneta (or Blattd). 

 The Cockroach. 



The cockroaches found in Britain are immigrants, either 

 from the East {f. orientalis), or from America {P. americana) ; 

 the two species closely resemble one another. They are 

 omnivorous in their diet, and active in their habits, but 

 they hide during the day and feed at night. They are 

 ancient insects, for related forms occurred in Silurian ages ; 

 they are average types, for they are neither very simple nor 

 very highly specialised. Their position is among the 

 Orthoptera, in the same order as locusts and grasshoppers. 

 The young are hatched as miniature adults, except that 

 wings are absent ; in other words, there is no metamorphosis 

 in development. 



The skin consists of an external chitinous cuticle and a 

 subjacent cellular layer — the epidermis or hypodermis — 

 from which the cuticle is formed. The newly hatched 

 cockroaches are white, the adults are dark brown. 



