THE PHYLA ARTHROPODA AND ONYCHOPHORA 



type of cycle, involving a gradual metamorphosis, is spoken of as paurometabolous 

 (Fig. 15.22). 



Somewhat different from the paurometabolous forms are those insects 

 whose eggs are laid in water and develop into aquatic nymphs, or naiads. 

 These immature forms are strikingly diflferent from the adult and usually 

 possess accessory structures, such as tracheal gills, related to their aquatic 

 habitat. During its aquatic life the naiad increases' in size by repeated molts; 

 at maturity it crawls out of the water and in a final molt becomes the winged 

 adult. This cycle includes a partial metamorphosis and is termed hemimetabolous 

 (Fig. 15.23). 



The most highly evolved insects, belonging to what may be considered the 

 most successful modern orders, have the most complex life cycles. The young 

 emerge from the eggs as larvae, which are completely different from the adult 

 in structure, in habitat relations, and very often in food habits and feeding 

 mechanisms. The larva, primarily a feeding stage, stores up reserves of 



Adult 



Fig. 15.23. Life cycle of a hemimetabolous insect. 



N^^d Embryo 



stages 



energy in its fat body and other tissues in the form of fats and glycogen. After 

 passing through a definite number of stages, each ending with a molt and an 

 increase in size, the larva transforms into a resting stage, the pupa. At the 

 conclusion of the molt marking the transition from larva to pupa, the general 

 outlines of the adult body form are laid down and are often visible externally. 

 The pupal stage may be brief, or it may last for several months. During this 

 period the insect does not feed, and nothing enters or leaves the body except 

 water vapor and respiratory gases. The pupa lives at the expense of reserves 

 laid down during the larval stages, and after a longer or shorter period of 

 quiescence, the organs of the adult begin to form. These structures are syn- 

 thesized from stored reserves and from materials that become available as the 

 special organs and tissues of the larva disintegrate. Eventually, the pupal 

 skin splits, and the adult which has been formed within it emerges. This 

 adult often has very small but perfect wings, which must immediately be in- 

 flated to full size and allowed to harden before they can be used. One of the 

 first acts of the adult is the ejection from the anus of a mass of nitrogenous 



455 



