Salivary gland glue proteins are expressed and excreted by different cells at different times during the third larval instar. The appearance of fluorescence from expression of a Sgs3-GFP fusion protein can be used to stage larvae to fairly narrow windows of development.

The development and use of P{Sgs3-GFP} was described in Biyasheva et al. (2001) “Glue Secretion in the Drosophila Salivary Gland: A Model for Steroid-Regulated Exocytosis” Developmental Biology 231: 234-251.