CBP staining along the larval digestive tract.

(A-C) Schematics of the larval digestive tract (A), proventriculus (B) and sagittal section of half proventriculus (C, corresponding to the boxed region in B). The proventriculus forms at the anterior region of the digestive tract and is composed of the esophagus in the inner wall, foregut (ectodermal) cells (orange region in B,C) in the middle wall, and midgut (endodermal) cells (blue region in B,C) in the outer wall. The esophagus and foregut cells are lined by cuticle (green in C). The PM (yellow in C) is deposited in the proventricular lumen and moves posteriorly towards the midgut region (magenta in A). Different cell types, i.e PR, APR among others, have been identified in the proventriculus region [ 34 ]. (D-Q) L3 digestive tracts stained for chitin using CBP (magenta) and Dlg or α-Spec as cellular markers (green). Images show single sections (E-G, J, L-N,Q), projections of 2–3 sections (H,I,O,P) or a projection of several sections (D,K). In the wild type, chitin is deposited in the esophagus (purple arrows), anterior part of the proventriculus (white arrows), enriched in the proventricular lumen (yellow arrow) and in the PR cells (blue arrows) (D-G). In L3 Df(Chs2) escapers the enrichment in the PR region or cells is not detected (K-N). In the midgut region, a continuous layer along the midgut cells is observed in control (H-I’) but not in L3 escapers (O-P’) (yellow arrows and magnification of boxed regions in grey). Note the presence of unspecific CBP staining in the lumen of the gut in control and escapers (orange arrows). No differences are detected in the hindgut (ectodermal) region between control and L3 escapers in chitin deposition (J,Q). tr, trachea. Scale bars D,E,H,J,K,L,O,Q 50 μm; F,G,I,M,N,P 20 μm.