594 PHOTOPERIODISM IN INVERTEBRATES 



PHOTOPERIODIC CONTROL OF FORM DETERMINATION 



A number of multivoltine insects exhibit such striking seasonal 

 changes in form that considerable confusion has arisen over their 

 taxonomy. It is now known that these changes are often governed by 

 photoperiod, A classical example is provided by the nymphalid butter- 

 fly Araschnia. The differentiation of the characteristic wing pattern is 

 connected with the occurrence of a photoperiodically induced diapause 

 in the preceding pupal stage. Such diapausing pupae always yield the 

 spring (levana) form of the butterfly. The nondiapause pupae of the 

 first annual generation (resulting from the operation of long photo- 

 periods on the larvae) always give the summer {prorsa) form under 

 natural conditions of temperature (Danilyevsky, 1948; Miiller, 1954). 



This connection between diapause and form determination can also 

 be observed in several dimorphic Homoptera. In the pear sucker 

 Psylla pyri the individuals composing the overwintering generation are 

 larger and darker and have longer wings than the summer generation 

 insects. Moreover, the winter generation always exhibits reproductive 

 diapause. These alternative paths of development are controlled by 

 the photoperiod experienced by the early nymphal instars. But it 

 seems that the progeny of the winter and summer forms are not 

 strictly equivalent in their response to photoperiod. For under short- 

 day conditions the progeny of winter forms include a fair proportion 

 of the summer phenotype, whereas these are entirely wanting in the 

 offspring of summer forms (Bonnemaison and Missonnier, 1955). 



A further instance of photodetermination is met with in the jassid 

 Euscelis where the spring (incisus) and summer (plebejus) genera- 

 tions differ conspicuously in size, pigmentation, and penis shape 

 (Miiller, 1954). In the leafhopper Delphacodes the development of 

 long- or short-winged forms is controlled principally by the density 

 of larvae on the food plant; but photoperiod again exerts some effect, 

 for nymphs induced to enter diapause by exposure to short days yield 

 a relatively high proportion of brachypterous adults in comparison 

 with nymphs that have not experienced dormancy (Kisimoto, 1956). 



These problems have been most extensively studied in aphids. 

 Although it has been claimed that day length is responsible for the 

 .determination of winged and wingless parthenogenetic forms (vir- 



