THE ANATOMY OF IX SECTS. 43 



rentia, like the oviducts unite and form an unpaired duct, the ductus 

 ejaculatorius. Accessory glands open into this duct. 



All insects are developed from eggs. But there are some ap- 

 parent exceptions. Thus many flies retain their eggs till after they 

 are hatched, if a proper place for laying them is not found earlier; 

 and in some flies (the Pupipara) the young attain a considerable 

 development before they are born. In the Plant-lice (Aphididae) 

 there is a remarkable alternation of reproduction by budding with 

 the sexual reproduction. 



