Many epithelial tissues undergo extensive remodelling during morphogenesis. How 
their epithelial features, such as apicobasal polarity or adhesion, are 
maintained and remodelled and how adhesion and polarity proteins contribute to 
morphogenesis are two important questions in development. Here, we approach 
these issues by investigating the role of the apical determinant protein Crumbs 
(Crb) during the morphogenesis of the embryonic Drosophila tracheal system. Crb 
accumulates differentially throughout tracheal development and is required for 
different tracheal events. The earliest requirement for Crb is for tracheal 
invagination, which is preceded by an enhanced accumulation of Crb in the 
invagination domain. There, Crb, acting in parallel with the epidermal growth 
factor receptor (Egfr) pathway, is required for tracheal cell apical 
constriction and for organising an actomyosin complex, which we propose is 
mediated by Crb recruitment of moesin (Moe). The ability of a Crb isoform unable 
to rescue polarity in crb mutants to otherwise rescue their invagination 
phenotype, and the converse inability of a FERM-binding domain mutant Crb to 
rescue faulty invagination, support our hypothesis that it is the absence of 
Crb-dependent Moe enrichment, and not the polarity defect, that mainly underlies 
the crb invagination phenotype. This hypothesis is supported by the phenotype of 
lethal giant larvae (lgl); crb double mutants. These results unveil a link 
between Crb and the organisation of the actin cytoskeleton during morphogenesis.