Entomology: May 10, 2006
Flight!
I. Benefits of Insect flight
A. More efficient in terms of energy spent/distance
B. Effective means of escape from predators
II. Insect flight records/abilities
A. Speed
1. Odonata (Austrophlebia costalis) recorded at 98 km/hr (published result)
2. Diptera (Hybomitra hinei) recorded at 145 km/hr in pursuit of a female
B. Wingbeat frequency
1. Over 1000 beats/min recorded in a Dipteran (Forcipomia sp.)
III. Anatomy
A. Basic wing structure
1. Outgrowths of tergal and pleural plates of thorax
a) Double layer
2. Supported by tracheal tubes and hemocoel branches
a) Hemolymph provides stiffness for wings
B. Wing location/attachment
1. Dorsal surface
a) Wings extend from tergal plates of thorax
2. Base of wing is flexible
a) Basal sclerites imbedded in base of wing which articulate with pleural wing processes.
C. Muscles
1. Direct muscles
a) Attach basal sclerites to wings
· Not generally the main flight muscles, except in some weaker fliers (Ex: Blattodea, Isoptera)
· Allow for changes in wing angle
· Responsible for wing flexion over abdomen at rest in neopterous insects.
2. Indirect muscles associated with thorax
a) Overall thorax structure
· Three types of plates (tergal, pleural and sternal) form a flexible "box" that is an integral part of flight mechanics.
b) Muscles
· longitudinal flight muscle
è extend lengthwise along the tergal plates
· dorso-ventral flight muscle
è attach tergum to pleural and sternal plates
3. Resonating muscle fibers
a) Contract repeatedly with single nerve stimulation
b) Appear to have intrinsic rhythmicity
· Allows for faster wingbeats than would be possible with non-resonating fibers
IV. Mechanics of flight
A. Key principle of flight generated by flapping motion
1. Object: To cause air to move swiftly backwards and downwards at a faster rate than air in front and above
a) The flying object will move in "equal and opposite" directions to these air forces
2. Forces which reduce effectiveness
a) friction/shear forces
b) turbulent flow
B. Basic flight mechanism
1. Downstroke: created by contraction of longitudinal muscles
a) tergal plate arches upward in its center, wings are pulled downward
2. Upstroke: created by contraction of dorso-ventral muscles
a) tergal plate pushed downward, wings pushed upward (cantilever effect)
3. Click mechanism: Spring effect of elastic skeletal structure
a) At top of upstroke, the wings are stable
b) As the contraction of the longitudinal muscles bows the tergal plate upward, the pleural plate is also bowed outward.
· This creates a stretch, storing elastic energy, a type of potential energy
è much like a rubber band
c) The “snap back” of the pleural plate, once the wings are just past horizontal, pushes the wings forcefully downward
d) A similar mechanism operates to push the wings upward forcefully during the upstroke
e) Net result: stable positions are fully down and fully up, while a horizontal position is unstable.
4. Wing position
a) Upstroke: Wings sweep both upward and backward
b) Downstroke:
c) Wings sweep both downward and forward
5. Twisting
a) Allows lift to be generated on both upstroke and downstroke
6. Turning
a) Unequal strokes with left vs. right side
b) Unequal angle of attack with left vs. right side
c) Use of hindlegs as a rudder to direct movement
C. Four-winged vs. two-winged flight
1. Potential problem of four-winged flight: forewings can create turbulence for hindwings, making hindwing movement inefficient
2. Weak four-winged fliers
a) Orthoptera, Neuroptera, Isoptera
3. Effective four-winged flight
a) Odonata
· Hindwings stroke prior to forewings
· Wings may "counterstroke" so they are at the complete opposite position and don't interfere.
4. Effective two-winged flight
a) Coleoptera: Elytra remain stationary during flight, and only hindwings used
b) Diptera: hindwings reduced to stabilizing halteres
5. Coupling of hindwings and forewings: Hemiptera, Trichoptera, Lepidoptera, Hymenoptera.
NOTE: Study questions for April 28 through today’s lecture will be provided to you on Friday.