316
E. B. EDNEY
Growth in insects entails periodic shedding of the integument and
the development of a new one by a highly complex series of events controlled by the underlying epidermal cells. Finally, the greater part of the
cuticle is in a real sense "living," since it is traversed by fine pore canals
which at times, at least, contain cytoplasmic extensions of the epidermal
cells, and since it forms part of the general metabolic pool, as Locke
(1961, 1964) has pointed out.
The existence of an impermeable lipid layer has been demonstrated or
inferred in spiders, solpugids, ticks, scorpions, and even in certain isopods.
Two other aspects of the general organization of insects concern us here:
the respiratory system, because respiratory membranes are necessarily sites
of water loss, and the "excretory" system insofar as this is significant in
osmotic and ionic regulation. The respiratory system consists of ramifying
tubules, the tracheae, which are morphologically invaginations of the integument, and like it, lined with a lipid layer. Tracheae terminate internally
in still finer tubules, the tracheoles, which are morphologically intracellular.
The main trachéal trunks open at the surface by segmentally arranged
spiracles.
The fact that the trachéal system is internal rather than superficial is
by itself of no advantage to an animal insofar as water conservation is
concerned, because a reduction in the rate of diffusion of water outward
occasioned by the long diffusion paths from the moist surfaces to the dry
outside air involves an even greater reduction in the rate of 0_> diffusion
inward. What does matter is that it is possible, with an internal system,
to reduce ventilation to the necessary minimum by closing the spiracles
(for further treatment, see Bursell, 1970). The extent to which loss of
water from the respiratory membranes is so controlled will be considered
below.
The excretory system consists of a number of blind Malpighian tubules
opening proximately directly into the hindgut. Waste nitrogen is excreted
mainly as uric acid which is nearly insoluble in water and therefore exerts
very little osmotic pressure; it can be eliminated as a semisolid mass.
Glandular areas in the rectal walls function in cooperation with the Malpighian tubules as osmotic and ionic regulators. These matters also will be
considered further in the context of particular cases.
III. Water Balance
A. WATER IN THE ENVIRONMENT
Humidity may be expressed either on a relative or an absolute scale,
and each form has its appropriate applications. The relationship between
E. B. EDNEY
Growth in insects entails periodic shedding of the integument and
the development of a new one by a highly complex series of events controlled by the underlying epidermal cells. Finally, the greater part of the
cuticle is in a real sense "living," since it is traversed by fine pore canals
which at times, at least, contain cytoplasmic extensions of the epidermal
cells, and since it forms part of the general metabolic pool, as Locke
(1961, 1964) has pointed out.
The existence of an impermeable lipid layer has been demonstrated or
inferred in spiders, solpugids, ticks, scorpions, and even in certain isopods.
Two other aspects of the general organization of insects concern us here:
the respiratory system, because respiratory membranes are necessarily sites
of water loss, and the "excretory" system insofar as this is significant in
osmotic and ionic regulation. The respiratory system consists of ramifying
tubules, the tracheae, which are morphologically invaginations of the integument, and like it, lined with a lipid layer. Tracheae terminate internally
in still finer tubules, the tracheoles, which are morphologically intracellular.
The main trachéal trunks open at the surface by segmentally arranged
spiracles.
The fact that the trachéal system is internal rather than superficial is
by itself of no advantage to an animal insofar as water conservation is
concerned, because a reduction in the rate of diffusion of water outward
occasioned by the long diffusion paths from the moist surfaces to the dry
outside air involves an even greater reduction in the rate of 0_> diffusion
inward. What does matter is that it is possible, with an internal system,
to reduce ventilation to the necessary minimum by closing the spiracles
(for further treatment, see Bursell, 1970). The extent to which loss of
water from the respiratory membranes is so controlled will be considered
below.
The excretory system consists of a number of blind Malpighian tubules
opening proximately directly into the hindgut. Waste nitrogen is excreted
mainly as uric acid which is nearly insoluble in water and therefore exerts
very little osmotic pressure; it can be eliminated as a semisolid mass.
Glandular areas in the rectal walls function in cooperation with the Malpighian tubules as osmotic and ionic regulators. These matters also will be
considered further in the context of particular cases.
III. Water Balance
A. WATER IN THE ENVIRONMENT
Humidity may be expressed either on a relative or an absolute scale,
and each form has its appropriate applications. The relationship between