CHAPTER XXIV 



THE INSECTA PEEIPLANETA OEIENTALIS AND 

 AMERICANA 



THE Crustacea, of which Apus and Astacus have formed our 

 examples, are essentially aquatic arthropoda breathing by gills. 

 The centipedes, spiders, scorpions, and insects, are terrestrial 

 arthropoda, breathing air by means of tubular or sacculated 

 involutions of the external skin. Of these terrestrial forms it 

 will only be possible in this place to deal with a single example 

 of the very large and highly differentiated class of Insecta, and 

 we cannot select a better example than the common cockroach, 

 both because of its convenient size, and because it represents 

 one of the oldest and most generalised orders of the class. 



Ubiquitous as it is in these days, the house cockroach is not 

 a native of England, but has been introduced into this country 

 in comparatively recent years. It came over in ships trading 

 with the East, and was first mentioned as infesting London 

 houses in 1634. It was long before these household pests spread 

 from London into the country ; thus Gilbert White writes of 

 " an unusual insect " which had made its appearance in Selborne 

 in 1790, the intruder proving to be the cockroach. But as 

 the communications between seaport and inland towns im- 

 proved, and imported goods were carried into the country in 

 bulk, the cockroach spread more and more rapidly, and is now 

 almost ubiquitous. 



The common species, Periplaneta orientalis is a native of 

 tropical Asia ; a somewhat larger species, P. americana, is a 

 native of tropical America, and is as common as the Oriental 

 form in some English towns. In the former species the wings 

 and wing covers of the female are rudimentary, and in the male 

 do not reach to the end of the abdomen. In P. americana 

 both sexes are winged, and the wings and wing covers are 

 longer than the body. The allied genus Blatta is a native of 

 Europe, and comprises several species of smaller size and less 

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