Arboreal nests often feature carton ______ constructed by termites. ______ is one gas that is poisonous to termites.

Lecturer: I’d like to talk to you today about termite nests. Now, termite
nests can be found in a variety of shapes and sizes depending on the age of
the mounds and the species that construct them. Four general types of nests
appear around the world: arboreal, subterranean, mound, and one-piece nests.
Arboreal nests are often extensions from nests below ground connected by
shelter tubes to a V-shape or a branch in a living tree. Termites try not to        Q31
wander unprotected in the open air but instead build tubes composed of carton.
Carton consists of earth and fecal material cemented together with salitionva.
This combination becomes very durable when allowed to dry and creates
perfect shelter tubes, sometimes covering the entire surface of a tree. These
shelter tubes are used to connect subterranean nests with arboreal nests and
also cover impenetrable surfaces to provide a dark and damp passage.
  Below ground nests are common where moisture remains at low levels     Q32
throughout the year for the reason that construction of a subterranean nest
minimizes evaporation and desiccation. In the case of subterranean termites,
gigantic systems of underground tunnels and galleries can reach hundreds
of feet to food sources, covering acres of land. A royal chamber is always       Q33
present in the centre towards the base of the nest where the reproductives are
kept moist and well fed. When a colony becomes too oversized for the queen
to keep up, secondary reproductives are created and satellite colonies emerge
nearby to supplement the primary colony.
  Although below ground nests are more or less protected from large
predators, there is a threat of invasion from tunnelling animals and anything
else that could enter the nest from underground. To protect a subterranean
nest from such attacks, an astounding feat of natural engineering arises as
a result of millions of years of defensive evolution. Using a combination
of earth, clay, sand, fecal material, and saliva the colony fabricates a thick,
rock hard, waterproof pavement surrounding the nest. This impermeable      Q34
barrier serves as a device to produce a self-sustaining environment as well
as for protection of the settlement. The watertight fortification also prevents
water loss and reduces diffusion of the queen’s scent; the scent is amplified
by confining it to the small area of the nest thus increasing its effectiveness.
A predicament develops, however, due to subterranean nests’ airtight
construction. Rising carbon dioxide levels are poisonous to termites and       Q35
several gases are produced that must be expelled from the nest. To clear this
obstacle, another inventive process takes place within the nest. Instead of
ventilating the nest with hot dry air from the exterior that would dehydrate     Q36
the colony, a system is set up to ventilate with cold moist air from below.
With differential air pressure and specialised air chambers, the nest is kept
well ventilated and water loss is all but eliminated.
  Termite mounds—also known as hills—are probably the most interesting
and straightforward indication of termite presence. Many species build
mounds, albeit for different reasons. Humans have contemplated termite
mounds for thousands of years attempting to fathom how insects so small,
blind, and mindless have such extraordinary and ingenious construction abilities.
A variety of mound shapes and sizes can be constructed from almost              Q37
any material found or excreted by termites. Mounds are above ground and,
depending on the species, can serve a plethora of uses including: a storage
room, temperature control device, emergency residence, refuge from flooding,
apparatus for ventilation, or as the entire nest itself.
  As with any structure in biology, form reflects function. A mound with         Q38
a large durable rounded peak, as is found in many tropical regions, functions
to repel water from the mound during heavy rainstorms. A tall slender
mound, similar to those often found in hot and temperate regions, acts to
regulate temperature throughout the day. Termites in different areas have
evolved to build mounds depending on local environmental conditions: daily
and seasonal variations in temperature, precipitation, light intensity, etc.
Magnetic mounds are at first puzzling; they take a thin wedge shape and are
always oriented in a north-south direction. Through a remarkable phenomenon
of evolution, the species that construct magnetic mounds have capitalised
on their position in reference to the Sun and Earth. The mounds always
point to the Earth’s north magnetic pole, though magnetism has nothing to do
with these termites. The reason for pointing their mounds in a north-south direction
is directly related, to the daily change in temperature associated with
the local environment. The days can be sweltering while the nights can be
frigid; how does the mound deal with the temperature extremes and stay at a        Q39
relatively constant temperature? It is the orientation of the mounds that allow
them to take advantage of the sun’s light energy. When the sun rises in the
morning, the broad eastern side of the mound absorbs heat to recover from
the cold night. In the middle of the day, only the top ridge of the mound is in        Q40
line with the sun’s rays; this prevents the mound from overheating in midday.
During sundown, the western side of the mound is heated to counter the upcoming
cold temperatures of night.