Insect Breathing Not Dependent
Animals need oxygen to live. The process is called respiration. Oxygen
must get to cells within the animal. Vertebrates, including cows, hogs,
dogs, salamanders, snakes and humans, use a circulatory system to do the
job. Air is taken into the lungs where the oxygen is transferred to the
blood. The blood is pumped around to cells in the animal by the heart.
The system works pretty well.
But it is not the only system used by animals to supply oxygen to cells.
In fact, the majority of animals don't have lungs and can't use such a
system. These animals are the invertebrates. Invertebrates don't have
backbones. They also differ from the vertebrates in other important ways,
including how they breathe.
Invertebrates, including the insects, get oxygen to their cells through
a series of air-filled tubes called tracheae. These tracheae run within
a few cell diameters of each cell in the insect body. At that point, the
oxygen diffuses from the tracheae to the cell.
That brings us to another difference in the breathing system between
humans and insects. We all recognize that our nose and mouth are the openings
that allow air to get to our lungs. Insects, in general, use openings
called spiracles to do that job. There are a few exceptions, though. Some
very small insects can get the oxygen that they need by diffusion through
the cuticle; however, most have spiracles.
Spiracles are openings on the sides of insects that resemble portholes
on the side of a ship. These openings are located on the thorax and abdomen
of the insect. The number of spiracles varies between insects. The maximum
number is 10 pairs; such a system is called holopneustic (meaning complete
breathing) by scientists.
Not all insects have a full complement of spiracles. Some insects, like
fly maggots, have only one or two pairs of spiracles functional. These
insects are specialized for living in liquids, such as water or fluids
of rotting material.
Humans use active ventilation to get air into an out of their lungs.
We contract and expand our chest cavity by contracting or relaxing the
muscles of our diaphragm. This forces the air out of our lungs or allows
air pressure to fill them up.
Insects generally do not utilize active ventilation. They just let the
air move in and out of the spiracles passively. They do have muscles that
can be used to close the spiracles when necessary to reduce airflow. When
insects need additional oxygen in their system, they increase airflow
by telescoping the abdomen. This is much like physically forcing air into
and out of human lungs by chest compression during CPR.
Because insects don't need to force air into lungs like mammals, they
don't have noses. They do have a proboscis, however. Some human noses
of rather large size have sometimes been humorously called proboscises.
Insects do have proboscises, but they aren't used for breathing. An insect
proboscis is just used for eating!