XT PHYLUM ARTHROPODA 603 



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, or immersed in, 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 

 orientalis 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 

 lurking-places in the daytime, and is almost omnivorous in its 

 diet. It is a good example of the Insecta, not only on account of its 

 large size, which renders it convenient for dissection, but also 

 because of its generalised structure, which makes it a fairly central 

 member of the class, devoid of any extreme modifications. 



Three regions are very distinctly recognisable in the body of 

 the Cockroach (Fig. 498). In front is the head, elongated vertir 

 cally, bearing the very long, slender feelers or antennce and the 

 large eyes, and contracted behind to form a narrow neck. In the 

 middle is the thorax, consisting of three segments, bearing the 

 three pairs of legs and the two pairs of wings. Behind is the 

 abdomen, consisting of ten segments covered over above by the 

 wings in the male. The entire surface is invested by a chitinous 

 cuticle, which is especially thickened on the head, on certain parts 

 of the thorax, and on the anterior pair of wings. 



The head consists of four parts the epicranium behind, com- 

 prising the region between and behind the eyes ; the dypeus, 



