A gene named Broad is found in caterpillars and, is essential for the change from larva to pupa to complete the metamorphosis. If a caterpillar is without this gene it is incapable of developing, and making the important change to a pupa. And, so fails in its quest to become a butterfly.
Reference, Scientific American, [for the information on the Broad Gene]. All photographs are the copyright of Nick Broomer
The 1st photo depicts a small White larva at 25mm in length, and 2-3mm wide. This is the normal size of a 4th instar larva that is ready to pupate.
|A final instar Small White larva at 25mm in length and, ready to pupate|
The 2nd photo depicts another 4th instar Small white larva that has grown to the enormous size of 35mm in length [10mm above the normal size for pupation for this species] and, 5mm wide. Far to big to pupate. If this larva had managed to complete its journey and become a butterfly, it probably would of been called a Large Small White. The reason for this abnormality is that this larva was missing that vital gene, the Broad gene and carried on eating until it died without developing any further and not being able to pupate.
|A final instar Small White larva at 35mm in length, should only grow to 25mm in length|
|Two Small White final instar larva, the one on the left is ready to pupate|
the one on the right has failed to pupate and carried on eating
and eventually dies
|Final instar White Admiral larva that failed to pupate|