Review Sheet -- Test 1 (Week 5) Biology 1224 -- Entomology; James Adams
Origin of Wings (Chapter 16, pages 323-326)
There is little question that wings are one, perhaps THE, important factor in the success of
insects: the wings aid in escape from predators, finding new resources, escaping or finding shelter
from the elements, finding (unrelated) mates, find oviposition places, etc. So, perhaps the most
important evolutionary question in the history of insects is: "Where did wings come from?"
The earliest fossil insects that have been found have evidence of wings, though fossils of
wingless hexapods from nearly as long ago have also been found. In today=s world, the few existing
wingless hexapods give little clue as to what structures might have become wings in the evolutionary
history of the insects.
First things first. Terrestrial hexapods have a protective, tough, waterproofed exoskeleton
necessary for their existence in a dry environment. Muscles for locomotion are in the thorax
(where the legs are). These could be considered preadaptations for flight -- items that evolved
before wings and yet necessary for wings to evolve later. The hardened cuticle provided the
materials for constructing a wing, and the musculature concentrated in the thorax provided the
musculature for the wings which, as you know, are also attached to the thorax.
While early insects were evolving and utilizing plant materials, plants were evolving, too,
and getting bigger. Insects today are still largely herbivorous, and large plants with lots of
resources would have attracted insects upward, along with their predators. Wings for moving
from one plant to another and escaping predators would be a definite selective advantage.
But how do insects get from no wings to wings? Two reasonable theories:
Paranotal Lobes -- projections, even small, off the sides of the thoracic
nota have been shown
to slow descent in model insects. Larger lobes would add gliding capabilities, and ability
to maneuver back to the trunk of your home plant (remember even wingless Archaeogna-
thans and ants can do this some; even small extensions improve this ability). Prothoracic
paranotal lobes are seen in some fossil insects, including some species that seem to have
the lobes articulated and veined. At some point, articulation with the thorax could have
allowed powered flight.
Lateral Gills of Aquatic Nymphs -- Some ephemeropteran (mayfly) nymphs have thoracic gills
that look a bit like wings, as do some fossil nymphal insects. The mayfly gills may be
"flapped" to help move the water for better oxygen exchange, and even help the insect
move through water. Adults of the fossil nymphs have "wings" that are too small for flight,
but still held outstretched -- may have allowed gliding in nymphs that went terrestrial. This
theory is further supported by the idea that these structures that may have become wings
would already have been articulated, as gills are today.
Flight and Flight Mechanisms (Chapter 5, pages 122 - 126; Chapter 21 for odonates)
Two different types of flight muscles exists in insects: direct and indirect.
Most flying insects use the large indirect muscles for upstroke and downstroke. These include
the longitudinal muscles (horizontal inside thoracic segments) and dorsoventral muscles
(vertical inside thoracic segments) -- neither set is directly attached to wing bases. The
wing base sits over the pleural wing process, which acts like a fulcrum.
When longitudinal contract, bulges thorax up which causes downstroke. When dorsoventral
contract, directly depresses top of thorax, which cause upstroke. Smaller direct muscles
pull leading edge downward on down stroke, and upward on backstroke to provide
appropriate lift and forward thrust. The two pairs of wings typically function together as
a unit and have some mechanism for keeping the hindwings linked to the forewings.
This is the basic mechanism for many insects, including most orthopteroidea, Lepidoptera
and some neuropteroidea. Some exceptions include the odonates, roaches and mantids,
where direct muscles attached to the wing base supply the entire downstroke. The direct
mechanism is particularly well developed in odonates, which are some of the absolute best
fliers, and which can beat wings independently and hover.
A second mechanism exists for orders with high wingbeat frequency -- Diptera, Coleoptera,
Hymenoptera, and some Hemipteroids. These insects require nerve impulses that can
generate several wingbeats per nerve impulse, resulting in typical wingbeat frequnecies of
100 to 300 per second in flies and bees, and up to 2000(!) in some small flies. This
mechanism involves typically one huge set of indirect muscles and interacting articulated
sclerites both at the wing base and the surrounding thorax, creating spring like activity
which provides some of the fast rebound activity for flapping wings this quickly. Again,
small direct muscles help modify orientation of the wing for hovering when necessary.
Some of the very smallest winged insects (thrips, for instance) have wings that are little more than
stiff, sclerotized rods with hairs or bristles off of the edges. Insects this small use the wings
to get airborne, then do little else with the wings unless they want to change direction. For
these insects, they more swim through the air than fly!
And speaking of small insects, even wingless insects that are tiny may end up at significant
altitude in the air. Some insects that have been picked up by winds and carried aloft include
fleas, ants, and springtails.