PHOTOPERIODISM IN INSECTS AND MITES 595 



ginoparae) (Shull, 1928), most authors have concluded that this 

 factor controls the production of parthenogenetic and sexual indi- 

 viduals. Relevant species include the black bean aphis Aphis fabae 

 (Davidson, 1929; de Fluiter, 1950), the cabbage aphis Brevicoryne 

 brassicae, the peach aphis Myzus persicae (Bonnemaison, 1951), and 

 the pea aphis Acythosiphon pisum (Kenten, 1955). 



The sequence of events that leads to the appearance of sexual forms 

 (this is, the winged males and the wingless egg-laying females or 

 oviparae) differs according to species. In a species with host alterna- 

 tion such as A. fabae the first result of a shortened day length upon 

 the summer generations of virginoparae is the production of winged 

 offspring (gynoparae), which migrate from the summer host (broad 

 bean) to the winter host (the spindle tree). These gynoparae differ 

 slightly from the alate virginoparae in their sensory equipment and 

 also show differences in behavior (Kennedy and Booth, 1954). Their 

 progeny become the oviparae which in turn lay diapause eggs on the 

 winter host. It may be seen therefore that the connection with diapause 

 is in this case entirely indirect. 



Preliminary studies on the aphid Megoura viciae have revealed a 

 different pattern of development (Lees, unpublished work). For 

 example, there are no gynoparae in this "one host" species, the 

 absence of this form being clearly correlated with the absence of host 

 alternation. The oviparae (and males) arise directly from a vir- 

 ginoparous generation. When this aphid is reared in long days in un- 

 crowded conditions and at moderate temperatures, a succession of 

 virginoparous generations is produced. On the other hand, when young 

 parent virginoparae are put on short days they give birth to oviparae 

 (and males) exclusively. 



Experiment shows that the oviparae are determined long before 

 birth; indeed determination is actively proceeding when the parent 

 aphid is passing through the early nymphal instars. This is scarcely 

 surprising in view of the fact that embryogenesis is known to be 

 highly precocious in parthenogenetic aphids. In Megoura, as in other 

 species, each ovariole in the first instar aphid already contains two 

 embryos, and by the final moult the number has risen to seven. It is 

 therefore of interest to inquire how the fate of the embryos within the 

 mother is decided by photoperiod. There are two possibilities: either 



