4

N. Chari and P. Srinivas

into an exoskeleton of chitin. The chitinous exoskeleton also serves as a protec-

tive cover, helps in attachment to muscles and acts as a water-tight barrier and

a sensory surface by developing sensory hairs (sensillae).

(2)

During the early stages of evolution, wings were useful for short-distance

passive flight, parachuting and gliding followed by flapping flight including

rotation of the wings. Flapping flight helps in the production of aerodynamic

forces such as lift, drag and thrust. Figure eight flapping contributes to lift and

thrust forces.

(3)

Some non-flying insects known as Apterygota and some others are fully flying

and known as Pterygota. Apterygota are primitive insects and the pterygota

are more advanced.

Respiration

(1)

The flying insects have adapted for aerial life and breathe by spiracles and the

tracheal system as in some other arthropoda such as scorpions, spiders and

centipedes.

(2)

The insect respiratory system includes paired segmental openings known as

spiracles and branched tracheal systems. Separate opening and closing mech-

anisms are present for the spiracles in the thorax and abdomen having separate

structures.

(3)

There are nine pairs of spiracles of which two pairs are found in the thorax and

seven pairs in the abdomen which differ structurally.

(4)

External air flows into the body through spiracles (stomata) and the branched

tracheal system supplies oxygen to different organs and cells. The tracheal

system has contributed to the evolution of insect flight in terms of O2 supply

and high oxidative metabolism.

(5)

Spiracles help not only in respiration, but also prevent evaporation of moisture,

entry of dust and small foreign particles into the respiratory system. Spiracle-

like structures are also present in some other arthropoda such as spiders,

centipedes and millipedes.

(6)

Air sacs are dilations of the trachea and are abundant in flying insects such as

grasshoppers, flies and bees.

(7)

Tracheal gills are present in some aquatic insect larvae such as larvae of

Odonata. Culex larvae have paired lateral branches leading to enter the respira-

tory siphons. Aquatic insects also breathe by ‘Plastron respiration’. Dytiscidae

and Bellastomidae breathe by plastron respiration.

Flight Apparatus

(1)

Membranous wings have elastic hinge-like joints containing axillary sclerites

located at the wing base. There is also resilin (elastomere) at the fulcrum and