498 INVERTEBBATE MORPHOLOGY. 



from these various portions on each side unite to form a vas 

 deferens which may dilate into a vesicula seminalis and then, 

 uniting with its fellow of the opposite side, forms the ductus 

 ejaculatorius. Occasionally the vesicula is unpaired arising 

 from the point of union of the two vasa deferentia, and very 

 frequently accessory glands occur. The ductus ejaculatorius 

 opens usually on the ventral surface of the tenth abdaminal 

 segment, and projections of the body-wall in the vicinity of 

 the orifice form a groove or tube through which the sperma- 

 tozoa, usually united into spermatophores, are introduced 

 into the bursa copulatrix of the female. 



Parthenogenesis occurs as a normal process in certain 

 Insects, though always associated with true sexual reproduc- 



// 

 Fig. 2Z0.— Aphis mali. Winged and Wingless Forms (from Packard). 



tion. Examples of it are found in certain Coccidse {Aspidio- 

 tus) and in some of the Gall-wasps (Cynipidse), the fertilized 

 ova producing both males and females, while in the Bees, for 

 example, in which both fertilized and unfertilized ova are de- 

 posited, the latter give rise to drones or males alone, while 

 workers or queens, i.e. the females, develop from the fertilized 

 ova. Occasionally heterogony occurs, as in the Plant-Kce 

 (Aphidse). These forms under favorable conditions of tem- 

 perature and food produce viviparously usually wingless indi- 

 viduals, not, however, from true ova, but by a process which 

 may rather be compared to internal budding, as in the Kedise 

 of certain Trematoda. Generation after generation of such 

 individuals may be produced during the summer, but on the 



