Review Sheet -- Test 4 (Week 15) Biology 1224 B Entomology; James Adams
Voltinism and Seasonality (Chapter 4, pages 84 - 86)
Voltinism -- number of generations in a year (univoltine, bivoltine, multivoltine); univoltine insects
include those that are strongly tied to seasons (warm/cold or wet/dry), particularly in harsh environments
(deserts). Some insects that are particularly large may also be univoltine (why?). Some insects may also
have different numbers of generations in different parts of their range (for instance, several moth species
may be univoltine in Canada, and multivoltine or continuously brooded in Florida).
Some insects have generation times longer than one year (insects at high altitude/latitude or other
harsh environments, periodical cicadas, etc.), and some may have continuous/irregular emergence of
adults throughout the year, as suggested above. As we discussed during the presentation of the different
orders, different life stages (larva, pupa, adult) may, of course, have different growth times themselves,
with some insects living for a total of several years (sometimes even longer than 10 and occasionally 20
years). Examples of long-lived insects? You may recall that most of the orthopteroid orders have
adults that live for multiple years. What others?
Seasonality -- Adults (and larvae) are active during different times of the year for different insects.
The term allochronic means "different times", and may apply to different activity times during a single
day as well as during the year. This may be important as a reproductive isolating mechanism for
species with similar mating behaviors/pheromones, etc. Most insects will exhibit some period of reduced
activity during harsher times of the year (dry or cold periods). A prolonged period of inactivity with low-
ered metabolic rate, typically regulated by hormones, is called diapause. Diapause may be obligate
(internally driven -- it will happen at a particular point regardless of what is going on externally) or facul-
tative (induced by external cues). Obligate diapause typically takes place in univoltine species, or species
that take more than one year to develop. Facultative diapause is typical in bivoltine or multivoltine insects
in seasonal environments (what are likely cues for diapause?). Pupal diapause is likely the most common,
since pupae are typically heavily sclerotized, inconspicuous, and non-motile. However, egg (determined
by the female egg-layer) and adult (reproductive) diapause are seen in some insects, and larval diapause
may be just about as common as pupal diapause. For species that take two years to develop (in the arctic,
for instance) the species may diapause in the larval stage the first winter, and the pupal stage the next.
Many species may require a certain set of circumstances while IN diapause, such as a period of chilling
during the winter, before they can actually break diapause.
For those with obligatory diapause, the timing of breaking diapause may also be genetically deter-
mined; however, even for these species, where entering diapause IS genetic, breaking of diapause may
be driven by external cues. For those with facultative diapause, both initiating AND breaking diapause
will be done using external cues (what are likely cues for breaking diapause?).
In some species that are bi-/multivoltine, the different generations in the year may exhibit different,
sometimes radically different forms, a phenomenon called seasonal polymorphism. Certain migratory
locusts are good examples, and we'll discuss other examples in class.
Key concepts for diapause: Photoperiod, with critical day length; temperature; rainfall
Thermoregulation (Chapter 5, pages 126 - 130)
Although generally considered ectothermic (cold blooded), various insects are active at a wide
range of temperatures. Additionally, insects living in temperate, subarctic or arctic habitats must have
at least one life stage that is freeze tolerant. Certain scorpionflies, collembolans (one of the few groups
of hexapods that occur around Antarctica), flies, winter stoneflies, and rockhoppers, are active as adults
at low temperatures and even on snow. On the other extreme, there are a number of adult insects that
are capable of activity at temperatures over 110E F (around 45EC). Most insects will cease activity at
either very low or high temperatures. At low temps, metabolic activity slows so that movement may be
difficult, though there are several things that insects can do to raise their body temperature over that of
the ambient (environmental) temps (see below). At high temps, insects typically seek shade; insects
have a limited ability to cool themselves (can= t afford to lose water), though insects with access to water
will open spiracles at high temps to increase convective (wind evaporation) cooling. In colonies of
honeybees, the workers pick up extra water and bring it to the hive. At higher temps, workers will
vigorously fan the wings to keep the hive cool. Still, most insects cannot cool themselves significantly.
specifically, this is where an organism gets most of its heat from the environment.
Incident sunlight will, of course, heat up the body of insects. Many insects living in cool
environments are darkly colored to increase heat absorption. Insects can increase their ability to
pick up heat by sitting on surfaces heated by the sun and by basking -- dorsal or lateral.
Endothermy -- specifically, this is the ability to metabolically generate internal heat.
Muscular thermogenesis, utilizing the wing muscles to generate heat, is common in
many insect groups. Muscle must be warm to generate the power and wingbeat frequency to get
an insect airborne, but the muscles can be "shivered" to get the muscles warmed up. Once warm,
the insects take flight, and the muscles continue to generate heat while in use. For species of
butterflies and grasshoppers that have been studied, thoracic temps during flight can be 5 - 10E C
(9 - 18E F) above ambient, and well insulated flying bees and moths can have thoracic
temperatures 20 - 30E C (36 - 54E F) over the surroundings. Insulation is accomplished by
hairs/scales (and some internal fat), which explains why the hairiest part of the body is often the
thorax. Thoracic temperatures can be precisely regulated, mainly by altering the heart
rate/hemolymph flow through the aorta, and controlling flow back to the abdomen from the
thorax by a large air sac. When warming up, flow is restricted back to the abdomen by inflating
the air sac, thus allowing heat to accumulate in the thorax. When the insects are well-heated, flow
to the abdomen increases, cooling the hemolymph, which is then pumped strongly by the heart
into the thorax, aiding in cooling the flight muscles.
Insect Cold Hardiness
Whether diapausing or not, insects in non-tropical regions of the world will require some
cold hardiness, though insects in soil and water environments are automatically buffered to some
extent from the extremes (both cold and hot) of the environment (though not the permafrost of
the arctic soil).
Some freeze-susceptible insects have minimal cold-hardiness, and will die if exposed to
freezing temps. Those that can survive freezing during the winter are called freeze-tolerant
insects. As indicated above, however, it will typically be just one life stage that is freeze tolerant,
unless development takes more than one year. Many insects can withstand having some extra-
cellular fluids actually freeze, and a few can withstand freezing of cellular fluids as well (though this
is not typical). Most insects that are freezing-tolerant have body fluids that can be supercooled,
without actual ice formation, down to as low as -20E C. This is typically due to cryopro-
tectant substances dissolved (like certain sugars, lipids, etc.) in the body fluids. Additionally,
many freeze tolerant and winter active insects have glycerol in the body fluids -- a natural anti-
freeze. Additionally, many insects will void the gut completely (including bacteria) to eliminate
any potential ice forming in the gut. Read the Arctic Ground Beetle story on pages 128 and 129.