[Drosophila] banding in cultures

[Mark Thorson]

In my best, most productive cultures, the pupae
are more-or-less uniformly distributed around
the sidewall of the culture vessel.  In some
cultures, the pupae tend to be bunched up in
a band about halfway up the sidewall.

Does anyone know what causes this?  Some of
these cultures are good performers, so I can't
say for sure that banding is detrimental to
productivity.