Review Sheet -- Test 4 (Week 15) Biology 1224 B Entomology; James Adams
Thermoregulation (Chapter 5, pages 119 - 120)
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 rock-
hoppers, 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 45E
C). 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 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.
Voltinism and Seasonality (Chapter 4, pages 80 - 83)
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
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?
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 lowered 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 facultative (stimulated 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 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.
For those that exhibit obligatory diapause, the timing of breaking diapause may also be
genetically determined; however, even for these species, where entering diapause IS genetic, breaking
of diapause may be driven by external cues. For those with facultative diapause, just as there are
external cues to initiate diapause, there will also be external cues to break diapause (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, temperature, rainfall
Insect Cold Hardiness --
(Chapter 5, pages 120 - 122)
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 freezing-susceptible insects have minimal cold-hardiness, and will die if exposed to
freezing temps. Those that can survive freezing during the winter are called freezing-tolerant
insects. As indicated above, however, it will typically be just one life stage that is freeze tolerant.
Many insects can withstand having some extracellular 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. This is
due to substances dissolved (like salts, sugars, etc.) in the body fluids. Additionally, many freeze
tolerant and winter active insects have glycerol/ethylene glycol in the body fluids -- a natural
antifreeze. Unlike what the book indicates on page 122, there are many insects that have been
found to produce these compounds to act as a natural antifreeze during winter.