  The mayfly nymph breathes under water through an oscillating array of
wing-shaped tracheal gills. As the nymph grows, the kinematics of these gills
change abruptly from rowing to flapping. The classical fluid dynamics approach
to consider the mayfly nymph as a pumping device fails in giving clear reasons
to this switch. In order to understand the whys and the hows of this switch
between the two distinct kinematics, we analyze the problem under a Lagrangian
viewpoint. We consider that a good Lagrangian transport that distributes and
spreads water and dissolved oxygen well between and around the gills is the
main goal of the gill motion. Using this Lagrangian approach we are able to
provide the reason behind the switch from rowing to flapping that the mayfly
nymph experiences as it grows. More precisely, recent and powerful tools from
this Lagrangian approach are applied to in-sillico mayfly nymph experiments,
where body shape, as well as, gill shapes, structures and kinematics are
matched to those from in-vivo. In this letter, we show both qualitatively and
quantitatively how the change of kinematics enables a better attraction,
stirring and confinement of water charged of dissolved oxygen inside the gills
area. From the computational velocity field we reveal attracting barriers to
transport, i.e. attracting Lagrangian coherent structures, that form the
transport skeleton between and around the gills. In addition, we quantify how
well the fluid particles and consequently dissolved oxgen is spread and stirred
inside the gills area.
