x PHYLUM ARTHROPODA 239 



damp localities under bark, or dead timber, or stones. Four species occur 

 in South Africa, one in South America, one in the West Indies, one in 

 New Britain, one in New Zealand, and two in Australia. 



3. THE INSECTA 



The class Insecta comprises the Cockroaches, Grass- 

 hoppers, Dragon-flies, House-flies, Butterflies, Beetles and 

 Bees, with their many allies. Though the class is a very exten- 

 sive one, including as it does a larger number of species than 

 any of the other classes of the Arthropoda, it is nevertheless 

 characterised by a remarkable degree of uniformity, no such 

 extremes of modification occurring as are observable among 

 the Crustacea. The body of an Insect, like that of a Crusta- 

 cean, is segmented, and bears a series of pairs of jointed 

 appendages. The surface is covered with a chitinous cuticle, 

 forming an exoskeleton, which is sometimes comparatively 

 thin, sometimes thick and hard. Like the body of the 

 Crustacean, that of the Insect is divisible into certain regions. 

 In the Insecta these regions are quite constant in their 

 disposition, and are always three in number head in front, 

 thorax in the middle, and abdomen behind. The head is 

 found, when its development is traced, to be formed by the 

 union of the head-lobe of the embryo with some five seg- 

 ments, but in the adult no trace of segmentation is distin- 

 guishable in this region. The thorax always consists of 

 three segments, which are usually firmly united together. 

 The abdomen contains from seven to eleven segments. 



The appendages are also very constant in their arrange- 

 ment throughout the Insecta, though variously modified in 

 form in the different orders, in accordance with differences 

 in mode of life. The head (Fig. 131) bears a pair of 

 antenna, a pair of mandibles, and two pairs of maxillce. The 



