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Psyche 
[Vol. 91 
light, and inhibited when either less than 8 or more than 14 hr of 
light occur. Type IV is its reciprocal, in which diapause is inhibited 
over a relatively narrow range and induced both above and below. 
Of the four types, Type I is by far the commonest in temperate- 
latitude insects and Type IV the rarest. Low-latitude insects have 
rarely been examined for photoperiodic responses, though the 
different patterns of seasonality in low latitudes might be expected 
to produce a rather different picture than that seen in temperate 
zones. 
Type III curves are known from a variety of insects. Among 
Lepidopterans, two important species that show them are the 
European Corn Borer, Ostrinia nubilalis (Hbn.) (Beck, 1962) and 
the Large White, Pieris brassicae L. (Danilevskii, 1961). The 
adaptive significance of the short-day threshold is unclear. Nor- 
mally the entire population would have been induced by day-length- 
temperature interaction to enter diapause before the inhibition 
threshold was reached. Thus, the photosensitive stages would never 
be exposed to such short photoperiods in nature, and the existence 
of a short-day threshold would seem nonadaptive. This paper 
reports a situation in which Type III curves were demonstrably 
adaptive for two multivoltine Lepidopterans in nature. 
The Systems 
Papilio zelicaon Lucas and Pieris rapae L. are very common, 
widespread multivoltine butterflies in lowland California. Both have 
been monitored phenologically for up to 13 yr along a transect 
parallel to Interstate Highway 80 from sea level at the Suisun 
Marsh, Solano County, to treeline in the Sierra Nevada. We are 
concerned here with the phenology of populations at low elevations 
between the San Francisco Bay area and Sacramento. The flight 
seasons for both species may reach ten months (Table 1), with four 
(P. zelicaon) to about six (P. rapae ) generations per year. Both 
species have facultative pupal diapause under conventional photo- 
period-temperature control (Sims, 1980; Shapiro, unpublished data); 
both have Type III diapause-induction curves. Diapause is ir- 
reversibly determined no later than the fourth instar. P. zelicaon is 
the stronger diapauser and is unrecorded at any of our sampling 
sites between 1 December and 18 February. In some years the 
period with no rapae flying may, however, be as short as four weeks. 
