EPOW - Ecology Picture of the Week

Lined (or Spotless) Grass-Yellow

Lined (or Spotless) Grass-Yellow (Eurema laeta), Family Pieridae
Mary River National Park, Northern Territory, Australia

Explanation:  It is the dry season, here in northern Australia; want to know how I know?

It's because of this butterfly.  No, not because of some seasonal migratory pattern ... but instead because of how it looks.  And in the photo, we are looking at the underside of its wings, as it perches.   

This is the "dry season form" of this butterfly species, Eurema laeta, known as the lined (or spotless) grass-yellow.  And, this species has two forms, here in Australia:  sana, the wet season form, with yellow underneath ... and lineata, the dry season form, which is brown underneath.  Our specimen is brown underneath, thus form lineata, thus it must be the dry season.

The species also occurs in Japan, southeast Asian, India, and Sri Lanka, but apparently only here in Australia does it show the two forms by wet and dry season.  Why?  Perhaps it is a local adaptation to the weather extremes found in northern tropical Australia?

And also, other species of this same butterfly family also show seasonally variable forms ... in Australia ... including variations in color pattern and size, with 4 of the 5 species of this family there having smaller adults during the warm, wet summer-autumn seasons, and larger adults during the cooler, dry winter-spring seasons.  

Except that 5th species shows a reverse pattern of size.  What a puzzler!  Studies suggest that the reason for the difference has to do with juvenile overcrowding and limitations of resources during the cooler dry season, limiting body growth and size.  But only for that 5th species.
  

Information:
     Jones, R.E.  1992.  Phenotypic variation in Australian Eurema species.  Australian Journal of Zoology 40(4):371-383.