Fig. 4. Similarities and hypothetical relationships between surfaces in crustaceans and insects. (A) Expression of Vvl in thoracic appendages of Artemia franciscana. Four appendages of increasing maturity are shown, immunochemically stained with an anti-Vvl antibody: (a) onset of Vvl expression; (b) early uniform Vvl expression; (c) the beginning of Vvl upregulation in the distal epipod/gill; and (d) higher levels of Vvl in the distal epipod of near-mature appendages. (B) Higher magnification of mature epipod, showing high levels and nuclear localization of Vvl. (C-E) Expression of trh and vvl in thoracic appendages of Parhyale hawaiensis, visualized by in situ hybridization. (C) Expression of Parhyale trh in thoracic appendages, showing strong staining in epipods but no staining in the endopods/legs. (D) Expression of Parhyale vvl at an early stage of appendage development, showing specific expression in the primordia of epipods. Cells are visualized by nuclear staining with DAPI. (E) Expression of Parhyale vvl at a late stage of appendage development. (F) Engrailed staining in the distal epipods/gills of the crayfish Pacifastacus leniusculus. The anterior gill comprises exclusively non-en-expressing cells (labelled A), whereas the posterior gill comprises both non-en- and en-expressing cells (labelled AP). (G) Nubbin/Pdm staining in the same epipods/gills in crayfish. Nub is expressed only in the posterior gill (labelled AP). (H) Summary of hypothetical relationships between insect tracheae and crustacean gills. Colour code is cyan for appendages/legs (expressing Dll in all arthropods), red for AP gills and insect wings (straddling the AP boundary, and expressing nub and vvl/trh), and green for A gills and insect tracheae (comprising anterior cells only, and expressing vvl and trh, but not nub).