A - Hypothetical wingless ancestor,
B - Paranotal,
C - Epicoxal,
D - Endite-exite
The origins of insect wings and the evolution of flight has been a subject of frequent debate, with a number of hypotheses currently being considered. Here we will explore the three most popular theories: the paranotal hypothesis, the epicoxal hypothesis and the endite-exite hypothesis.
The Paranotal Hypothesis
This theory suggests that insect wings developed from paranotal lobes, which evolved de novo from the thoracic terga (Gullan & Cranston, 2010). Originally non-articulated, innervation and musculature would have been secondarily-derived. At first, the lobes could have acted as parachutes to create a softer landing, and then grew larger and more complex. This model suggests an increasing effectiveness of the wings, commencing with parachuting, then gliding and finally active flight (Wootton, 1976).
Quartau (1985) presented a detailed overview of the literature on the paranotal theory and concluded that it would have been unlikely for flight by flapping wings to evolve from gliding, as it is already a successful flight technique.
The Epicoxal Hypothesis
This theory implies that wings derived from a tracheated gill or gill-cover (Woodworth, 1906). The tracheal gills have their own rudimentary muscles and are equipped with tiny winglets that continually move for respiration. This model suggests that the gills started off as exits for the respiratory system, then were modified for basic locomotion, and finally developed into wings.
This hypothesis, although theoretically sound, presents a number of problems. The main concern here is that if wings developed from fully aquatic insects, then invasion of land would have had to happen twice: once for wingless insects and once for insects with wings (Gullan and Cranston, 2010). Although this could have happened, is it unlikely.
The Endite - Exite Hypothesis
Kukalova-Peck (1978) first put forth the theory that insect wings developed from exite (outer) leg segments at the lateral body region. Trueman (1989) then extended this idea to include endite (inner) segments. He believed that the exite and endite parts fused during the evolution of the wing. This hypothesis is the most favourable as it is supported by the fossil and molecular record (Jockusch and Nagy, 1997).
The Paranotal Hypothesis
This theory suggests that insect wings developed from paranotal lobes, which evolved de novo from the thoracic terga (Gullan & Cranston, 2010). Originally non-articulated, innervation and musculature would have been secondarily-derived. At first, the lobes could have acted as parachutes to create a softer landing, and then grew larger and more complex. This model suggests an increasing effectiveness of the wings, commencing with parachuting, then gliding and finally active flight (Wootton, 1976).
Quartau (1985) presented a detailed overview of the literature on the paranotal theory and concluded that it would have been unlikely for flight by flapping wings to evolve from gliding, as it is already a successful flight technique.
The Epicoxal Hypothesis
This theory implies that wings derived from a tracheated gill or gill-cover (Woodworth, 1906). The tracheal gills have their own rudimentary muscles and are equipped with tiny winglets that continually move for respiration. This model suggests that the gills started off as exits for the respiratory system, then were modified for basic locomotion, and finally developed into wings.
This hypothesis, although theoretically sound, presents a number of problems. The main concern here is that if wings developed from fully aquatic insects, then invasion of land would have had to happen twice: once for wingless insects and once for insects with wings (Gullan and Cranston, 2010). Although this could have happened, is it unlikely.
The Endite - Exite Hypothesis
Kukalova-Peck (1978) first put forth the theory that insect wings developed from exite (outer) leg segments at the lateral body region. Trueman (1989) then extended this idea to include endite (inner) segments. He believed that the exite and endite parts fused during the evolution of the wing. This hypothesis is the most favourable as it is supported by the fossil and molecular record (Jockusch and Nagy, 1997).