36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



the larva presen-es the lan-al simpHcity of its thoracic segments. The 

 wing rudiments continue to grow in their pockets, and at the moult 

 to the pupa they become evaginated as well developed lobes resem- 

 bling the wing pads of hemimetabolous nymphs. Their final develop- 

 ment, together with that of the adult thoracic structure is then de- 

 veloped ^^-ithin the pupa, so that the emerging adult is fully able to 

 fly. In most cases the adult has to expand the wings and allow them 

 to dr\^ and harden, but in some aquatic groups (e.g., mayflies) the 

 adults emerge and fly immediately. 



It is clear that the evolution of the wings and of the thoracic 

 modifications that enabled the glider lobes of the early insects to be- 

 come organs of flight must have been a long and complex process. 

 The winged insects, however, owe almost all that they are today to 

 their wings. Note what simple creatures by comparison are the ap- 

 terj-gotes, which probably have changed httle since the time they first 

 became hexapods. The wings of higher insects freed the adults from 

 a ground existence, and many of them have taken advantage of their 

 freedom to adopt new kinds of food and new ways of feeding, for 

 which they have developed new types of mouthparts. The young in 

 such cases could not lead the lives of their parents, and have become 

 adapted to habitats and ways of living of their own. Thus it has 

 come about that the young of these insects have been specialized to 

 such an extent that they have lost all resemblance to their parents. 

 The adult development is then delayed to the end of the larA-al life 

 when the special larval tissues are destroyed in the pupa. This change 

 from larva to adult is commonly known as metamorphosis, but really 

 it is largely a replacement of the larva by the adult. 



Abdomen: The name "abdomen" for the third section of the 

 insect body does not clearly follow from its derivation ; however, 

 the insect abdomen does contain the principal viscera, and thus it may 

 be likened to the vertebrate abdomen. 



The primitive abdomen probably had 12 segments, this number 

 being present in some embryos and in adults of the Protura. The 

 terminal segment bearing the anus is probably a telson, since the 

 last embr}-onic appendages in the embryo, which are retained as the 

 cerci, pertain to the penultimate or eleventh segment. In most adult 

 insects, however, there are only 10 abdominal segments, and the 

 cerci are carried by the tenth segment and the anus is contained in 

 an apical lobe. 



The base of the abdomen may be broadly joined to the thorax or 

 narrowed to a petiole. In the Hymenoptera the first abdominal seg- 



