PHYLUM ARTIIROPODA 267 



Class Insecta 



The insects, the most abundant of all animals. The characters 

 head is distinct, and there are typically three pairs of 

 legs, all attached to the thorax. In the majority of 

 species there are two pairs of wings ; but these may be 

 reduced to two, as in the flies (Diptera), or may be 

 altogether absent. When the wings are absent, they 

 may be primitively so (e.g., Collembola), or the animals 

 (such as the louse and bedbug) may be evidently derived 

 from winged ancestors. There is usually a distinct 

 metamorphosis, or change in form during growth. This 

 may be extreme (complete), as in the butterflies, which 

 hatch from eggs as caterpillars, pass through the dor- 

 mant chrysalis stage, to emerge as an adult (imago) 

 totally unlike either caterpillar or chrysalis. The wings 

 are never developed until the adult stage is reached, and 

 after reaching the adult or imago stage the animal grows 

 no more. If it ever seems to do so, it is only because 

 the body becomes distended with eggs. 



The classification of insects is in an unsettled condi- 

 tion, owing largely to differences of opinion as to the 

 number of orders and other divisions to be recognized. 

 Some authors recognized even five different classes, 

 four of which are based on the primitively wingless 

 forms often treated as an order Aptera. Our present 

 treatment represents a less extreme point of view, but 

 like all other classifications is subject to revision. We 

 begin with the groups which seem to be most primitive. 



Order Protura 



Minute wingless terrestrial insects, slender in form 

 and without antennae. Some are without a tracheal 

 system. There are six legs, the front pair held forward 



