1921] Uichanco — Reproduction in the Apliididw 97 



there is also a third form, the generation of 'sexupara/' that is, 

 parthenogenetic individuals ^\'hich give rise to amphigonous forms. 

 These are, as a rule, alate (Patch, 1920). In all specimens of 

 parthenogenetic aphids which I have dissected neither spermatheca 

 nor eolleterial glands were found. The absence of these structures 

 was apparently first reported by von Siebold (1839) and subse- 

 quently confirmed by other authors. In contrast with the amphi- 

 gonous forms, the walls of the ovary are uniformly thin and mem- 

 braneous, except at the vagina, where they are somewhat thicker. 



Parthenogenetic aphids are viviparous, the entire incubation pe- 

 riod being passed within the abdomen of the mother. This is a 

 very unique characteristic, in view of the fact that a similar case 

 does not occur in closely related families. The Phylloxeridte, which 

 is the only other family with the Aphididae in the superfamily 

 Aphidoidea, also have parthenogenetic generations, but they are 

 always oviparous. In certain other families of the order Homop- 

 tera, like the Aleyrodida^, parthenogenesis and viviparity are not 

 correlated. 



In parthenogenetic aphids, the development of the eggs proceeds 

 in the ovaries long before deposition, eggs in the blastoderm stage 

 having been observed within the abdomen of parthenogenetic em- 

 bryos. There is no formation of a vitelline membrane and chorion. 



Modifying Factors in the Production of Amphigonous 

 Individuals. 



As has been suggested elsewhere in the present paper, aphids 

 seem to have the tendency indefinitely to reproduce parthenogenet- 

 ically under favorable environmental conditions. As experimental 

 evidence favoring this view may be cited E wing's (1916) work on 

 Aphis avence Fabricius, in which he found that he could maintain 

 continuous and uninterrupted parthenogenetic reproduction through 

 as many as eighty-seven generations in the material he experi- 

 mented with on the Pacific Coast of the United States. In that 

 region, oviparous forms in any aphid species had not been kno^\Ti, 

 except in very few cases. His experiments, unfortunately, had to 

 be brought to a close through the dying of all the individuals in 

 his eighty-seventh generation from excessive heat ; otherwise, he 

 would probably have been able to observe parthogenetic reproduc- 



