58 INTKODTJCTfON TO CLASSIFICATION. 



The head consists of at least five, and probably of six, coa- 

 lescent and modified somites, and some of the anterior segments 

 of the body are, in many genera, coalescent, and have their 

 appendages specially modified to subserve prehension. The 

 respiratory organs are tracheae, which open by stigmata upon 

 the surface of the body, and the walls of which are strengthened 

 by chitin, so disposed as readily to pull out into a spirally coiled 

 filament. 



XXIII. THE INSECTA. 



In this enormous assemblage of animals the respiratory 

 organs are like those of the Myriapoda, with a nervous and a 

 circulatory system disposed essentially as in this and the two 

 preceding classes. But the total number of somites of the body 

 never exceeds twenty. Of these five certainly, and six pro- 

 bably, constitute the head, which possesses a pair of antennae, a 

 pair of mandibles, and two pairs of maxillse ; the hinder pair of 

 which are coalescent, and form the organ called the " labium." 



Three, or perhaps, in some cases, more, somites unite and 

 become specially modified to form the thorax, to which the 

 three pairs of locomotive limbs, characteristic of perfect insects,* 

 are attached. 



Two additional pairs of locomotive organs the wings are 

 developed, in most insects, from the tergal walls of the second 

 and third thoracic somites. No locomotive limbs are ever 

 developed from the abdomen of the adult insect, but the ventral 

 portions of the abdominal somites, from the eighth backwards, 

 are often metamorphosed into apparatuses ancillary to the 

 generative function (Figs. 30 and 31). 



* The female Stylops is stated to possess no thoracic limbs. 



