Review Sheet -- Test 1 (Week 5) Biology 1224 -- Entomology; James Adams

Origin of Wings (Chapter 15, pages 308-312)

        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=d wings come from?"
        The earliest fossil insects that have been found have 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. 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.

Flight and Flight Mechanisms (Chapter 5, pages 117 - 119; Chapter 21 for odonates)
            Two different flight mechanisms exists in insects: direct and indirect.
    The Indirect Mechanism: Most flying insects use the indirect mechanism, which involves the
            longitudinal muscles (horizontal inside thoracic segments) and dorsoventral muscles
            (vertical inside thoracic segments) -- neither set is directly attached to wing bases, though
            there are smaller direct muscles attached to small sclerites at the base of the wing in
            order to alter the pitch of the wings.
        When longitudinal contract, bulges thorax up which causes downbeat. When dorsoventral
            contract, directly depresses top of thorax, which cause upbeat. 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.
    The Direct Mechanism: only the odonates (the dragonflies and damselflies) have this
            musculature. Considered to be some of the best fliers in the entire insect class, the
            odonates have even the big flight muscles are directly attached to wing base sclerites;
            one set for the downstroke, the other for the upstroke. These muscles involve some of
            those normally involve in walking (attached to the coxa) so that these insects cannot
            walk. The sets of muscles in the two winged thoracic segments can work independently
            so that the two pairs of wings can flap out of sync, and typically are somewhat out of
            phase during flight which may give some species incredible hovering capabilities.

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.