Gas exchange in insects 3.1.1
Insects
were one of the first organisms to adapt form aquatic to terrestrial life
need to avoid losing water espec at gas exchange surfaces - evolved a tough supporting exoskeleton made of chitin & covered by cuticle
cuticle
impermeable to water & almost impermeable to O2 & CO2
Tracheal system
open circulatory system
there are no blood vessel - tissues & cells are bathed directly in blood
with an open circulation insects can't easily direct the flow of blood to tissues that need it most
therefore they use a tracheal system - air filled tubes which can bring supply air directly to respiring tissues as O2 can diffuse quicker through air than through blood
air enters the tracheal system via a spiracle (pore) in each segment & then is transported into the body through a series of tubes called tracheae
tracheae divide into tracheoles - ends of tracheoles are open & filled with tracheal fluid
gas exchange occurs between air in the tracheole & the tracheal fluid -O2 dissolves into the tracheal fluid & then can diffuse into surrounding cells
some exchange can also occur across the thin walls of the tracheoles
tracheoles - stiffened & held open with bands if chitin to prevent collapse
Adaptation for increased O2 absorption
at rest the tracheoles are partially filled with tissue fluid but when active the fluid is reabsorbed & air can penetrate further down the tracheoles, increased SA of tracheole exposed to air
O2 can then diffuse through the walls at the end of the tracheoles into the tissue fluid
as respiration occurs the products accumulate in the cell, decreasing the water potential & thus forcing the fluid in the tracheoles to enter the tissue - exit of fluid creates a lower pressure in tubes & draws more O2 to the tissues where its needed
CO2 produced is detected by the chemoreceptor which make the muscles near the spiracles contract - pushes air out
Adaptations for efficient gas exchange
how insect prevent dehydration
waxy cuticle
is waterproof like in leaves to prevent evaporation of water
small SA:V ratio
minimises area over which water can be lost
keep spiracles closed most the time
short diffusion distance as all cells are only short distance form a tracheole
ventilation
movements of muscles in insects creates mass movement of air in & out of the tracheae - maintains steep diffusion gradient
prevents evaporation of water
insects are limited to small sizes because they rely mostly on diffusion & therefore need to keep diffusion distances short
ventilation
smaller insects can rely on diffusion alone as they have a large enough SA:V ratio, but larger insects can ventilate their tracheal systems by movements of their body can be done by;
tracheal systems have expandable walls which can be squeezed by the action of the flight muscles - repetitive expansion & contraction ventilates the tracheal system
movements of the wings alters the volume of the thorax, as thorax volume decreases air pressure in tracheal system increases & air is pushed out. when thorax volume increases air pressure in tracheal system decreases drawing air in
locusts
further developed ventilation system - open & close spiracle valves in particular order allowing the to take in air through their thoracic spiracles & expel air through abdominal spiracles
allows unidirectional flow of air through the tracheal system