PARTHENOGENESIS HETEROGAMY. 



543 



ovum a single (Forficula), or a number of yolk-forming cells (nutri- 

 tive cells), so that we can distinguish in the egg-tube alternate yolk 

 and germ compartments (rig. 453, a and b). In rare cases (Aphides} 

 there is at the end of each egg-tube a common larger chamber of yolk 

 cells, which are connected with the egg-chambers by means of " yolk- 

 cords " (tig. 453 c). 



Parthenogenesis and Heterogamy. In certain insects, partheno- 

 genesis, i.e., spontaneous development of unfertilized ova, has been 

 shown to obtain ; this occurs in the 

 Psychidce (Psyche), TineiJo- (Solenobia), 

 Coccidce (Lecanium, Aspidiotus) and 

 Chermes ; also in numerous Hymenop- 

 tera, especially in Bees, Wasps, Cynipidce, 

 and Tenthredinidce (Nematus). In the 

 Hymenoptera which live together in 

 the so-called animal communities, male 

 forms only are produced from the unfer- 

 tilized ova, (arrentokia). Chermes affords 

 an example of Heterogamy, in that two 

 different oviparous generations follow 

 one another ; a slender and winged 

 generation, and an apterous 



summer 



generation which is found in autumn 

 and spring and lives through the winter : 

 the males are, in most cases, not yet 

 known. The closely-allied Aphides (plant- 

 lice), which were formerly supposed to 

 present the phenomenon of an alterna- 

 tion of generations, behave in a similar 

 manner. In them the summer genera- 

 tions are very numerous, and are suc- 

 ceeded by a sexually-developed autumn " 



. . Fie;. 452. Micropyles (Mk) of insect 



generation, which includes winged males eggs (after R* Leuekart). , 

 a.-, well as the oviparous and often ap- 

 terous females (tig. 97. <i, />). In the 

 spring, viviparous Aphides are developed 

 from the fertilized eggs. These are mostly winged (tig. 99), and in 

 their organisation closely resemble true females. Their reproduc- 

 tive organs are, however, differently constructed, and are without 

 the receptaculum seminis. Since they never copulate, they have 

 often been regarded as asexual forms provided with germ tubes. 



upper part of the egg-shell of 

 Aiifliomyiti; b, egg of Droxojiliili 

 r, Ihirix c, stalke.l egg of 

 tegtaceug. 



