XI PHYLUM ARTHROPODA 619 



capable of being grouped in the same orders as the living repre- 

 sentatives of the class, and are looked upon as constituting at 

 least two orders, the members of which are all extinct. While the 

 Progoneata, and, more especially, the Symphyla, show marked 

 resemblances to the Insecta — more particularly to some of the 

 members of the order Aptera, the Oj^isthogoneata have features 

 connecting them through the Onychophora with the Annulata. 



CLASS IV.-INSECTA. 



The class of Insects (comprising the Cockroaches, Grass-hoppers, 

 Dragon-flies, House-flies, Butterflies, Beetles and Bees, with their 

 many allies) though it is a very extensive one — including as it does 

 a larger number of species than any of the other classes of the 

 Arthropoda — is yet characterised by a remarkable degree of uni- 

 formity, no such extremes of modification occurring as are 

 observable within the class Crustacea. 



Characteristic of all the members of the class is the presence of 

 three clearly-defined regions — the head, thorax, and abdomen. 

 There are present on the head, antenna, mandibles, and two pairs 

 of maxillae, the jaws being variously modified in the different 

 orders. All Insects have three pairs of thoracic legs, and most 

 have either one or two pairs of wings likewise borne on the thorax ; 

 the abdomen is not provided with paired appendages. 



The organs of respiration are tracheae similar to those of the 

 Myriapoda. 



The various systems of internal organs attain a very high 

 grade of structure in all the higher groups of Insects. In most 

 the development is complicated by the occurrence of a strongly- 

 marked metamorphosis. Insects are terrestrial or aerial, only a 

 few groups living on the surface of fresh or salt water ; but many 

 are aquatic throughout their larval condition. 



Many groups of Insects are remarkable for the high grade of 

 their intelligence as compared with the members of other classes 

 of the animal kingdom. This manifests itself mainly in a number 

 of instincts, often of a remarkable character, having to do with 

 the protection and rearing of the young; and in some cases leading to 

 the formation of communities consisting of individuals of various 

 different kinds (workers, soldiers, sexual individuals) for mutual 

 support and protection. 



1. Example of the Class— The Cockroach {Periplaneta 

 orientcdis or P. americana). 



The Cockroach, familiarly known by the misleading title of 

 " Black Beetle," is a common pest of kitchens, bakeries, and store- 

 rooms. It is nocturnal in its habits, rarely coming out of its 



