As animals become bigger and more complex, the surface area to
volume ratio decreases, and it becomes impossible for the organism to get
enough oxygen for metabolic reactions through its surface. Instead, they need
to have specialised respiratory surfaces. Many aquatic insects have
‘respiratory siphons’ which allow the insect to breathe air from above the
surface of the water while their bodies remain submerged. The insect
breathes through ‘spiracles’, which are pores in the skin, which can be
opened and closed by valves. These lead into tracheoles, which end in the
tissues. Only the posterior spiracles are used, which are located at the end of
the siphon. Insects such as drone flies and mosquito larvae use this
technique.
One problem with respiratory siphons is that they limit the organism to
the upper levels of the water. Some insects, such as diving beetles, have
managed to overcome this by carrying air stores along with them when they
dive. By keeping air stores in contact with the spiracles, they are able to draw
upon these air stores to meet with their oxygen requirements.
The benefit of using air stores it that the bubble itself can act like a
physical gill, and draw oxygen from the surrounding water. This occurs
because after the beetle dives, a disequilibrium in the bubble occurs. The
oxygen is removed from the bubble, causing a decrease in the partial
pressure of the oxygen, and an increase in the partial pressure of nitrogen.
Oxygen then diffuses from the water into the bubble, and nitrogen diffuses
from the bubble into the water. This technique provides enough oxygen for
small, inactive insects to remain underwater for months, however, bigger, and
more active insects need to surface every few minutes to replenish their
oxygen supplies. Water temperature affects the amount of oxygen that can be
dissolved in water. As the temperature increases, the amount of water that
can be dissolved decreases. The amount of oxygen required by the beetle,
however, increases, thus the
physical gill is of little use.
A solution for some invertebrates is to develop a true physiological gill
to extract oxygen from the water. Gills are appendages for gas exchange, with
tracheae.
Oxygen diffuses from the water into the tracheae, and moves through the
tracheal system in to the tissues. Organisms that use gills for respiration
include mayfly nymphs, damsel fly larvae, and stone fly nymphs.
Finally, some organisms are able to obtain oxygen whilst underwater
by extracting it from aquatic plants by thrusting their spiracles into the
aerenchyma of plants.
