The Insect’s Wings
Introduction
Insect wings are adult outgrowths of the insect exoskeleton that enable insects to fly. They are found on the second and third thorasic segments, and the two pairs are often referred to as the forewings and hindwings, respectively, though a few insects lack hindwings, even rudiments. Insect wings do not constitute appendages in technical parlance, as insect only have one pair of appendages per segment.
The wings are strengthened by a number of longitudinal veins, which often have cross-connections that form closed "cells" in the membrane. The patterns resulting from the fusion and cross-connection of the wing veins are often diagnostic for different evolutionary lineages and can be used for identification to the family or even genus level in many orders of insects.
Fully functional wings are present only in the adult stage, after the last moult. The one exception is the order Ephpemareoptera, in which the penultimate instar possesses well-developed and functional wings, which are shed at the final moult. Wings are only present in the subclass Pterygota, with members of the archaic Apterygota being wingless. Wings may also be lost in some pterygota clades, such as the fleas and lice.
Origin of insect’s wing
Regarding the origin of wings the following three theories have been postulated:
(1) The flying fish hypothesis of Oken (1831) considered the wings to be homologous with the nymphal abdominal gills of some aquatic ancestors and seen in some modern Ephemerids.
(2) The hypothesis of Muller (1873) postulating that wings arose from lateral tergal expansions or the paranotal lobes of the thorax.
(3) Wigglesworth (1963) has suggested that the wings arose in tiny, passively airborne insects and the presence of light cuticular thoracic expansions facilitate take off. The development of muscles helping their twisting, also would afford control while landing. The next step in efficiency would be the appearance of flapping muscles, increasing the take off and landing efficiency.
Structure of wing
A generalized pterygote wing is more or less flattened or triangular, with an anterior notal margin, a distal margin and a posterior anal margin. The anterior area of the wing supported by veins is usually called remigium while the posterior area comparatively flexible, is termed the vannus. The two regions are separated by the vannal fold. The proximal part of the vannus is called jugum. The area containing the wing articulation sclerite is termed as the axillary field. Sometimes at the posterior margin of the wing base there occurs additional membranous folds, the alula or calyptus.
Wing Venation
Insect wings have rigid veins which support the wing in flight. Venation is the term given to the arrangement of the veins on the wing. The wing venation vary in different insect groups, scientists tracked that all different insect wings are evolved from the same ancestor, i.e. wings had evolved only once in the insects history. The archedictyon is the name given to a hypothetical scheme of wing venation proposed for the very first winged insect. Since all winged insects are believed to have evolved from the common ancestor, the archediction. According to current dogma, the archedictyon contained 6-8 longitudinal veins. These veins and their branches are named according to a system devised by John Comstock and George Needham the Comstock-Needham System:
In the insect history, the fossil records show that the early insect wing had 8 pairs of main veins. Each pair diverged from wing base into anterior convex and posterior concave sector (e.g. MA and MP). The two sectors often fused into one veinal stem near the wing base. In evolution insect wing is in most case modified in reduction of veins.
The wing veins of archedictyon plan are listed below:
1. Precoasta (PC): This vein is fused with costa in all extant insects, mostly unrecognisable.
2. Costa (C): At the leading edge of the wing, strong and marginal, extends to the apex of the wing, it is unbranched.
3. Subcosta (Sc): The second longitudinal vein, mainly the subcosta posterior sector (ScP). Sc is reduced or fused with R in most Hemiptera.
4. Radius (R): The third vein, usually the strongest vein on the wing, with branches usually cover the largest area of wing apex. RP (radius posterior) is often referred to as radial sector (Rs) and the end branches as R1-5.
5. Media (M): The fourth longitudinal vein, MA and MP usually with 4 branches each. In some insect groups MA fused with R so only MP on the medial area. In this case the MP1-4 are often referred as M1-4.
6. Cubitus (Cu): Fifth longitudinal vein, CuA (Cubitus Anterior) may branch to 4 or fewer veins. CuP (Cubitus Posterior) is unbranched, lies near the claval fold and reach the wing posterior margin.
7. Anal veins (A): Veins behind the cubitus, AA (anal anterior) and AP (anal posterior) are usually separated by the anal fold. In Neoptera, AA is always fused with Cu or CuP. In the hind wings of most orthopteroid insects, there is a large anal area where anals branch several times to form a fan-like folded wing.
8. Jugal (J): Small veins in the jugal area, found only in Neoptera.
Cross-veins are transverse veins joining longitudinal veins to provide rigidity of the wings. Their names are based on the position relative to longitudinal veins. In Orthoptera and Odonata the cross veins are numerous but in other insects there is a definite number of cross veins, with specific positions and designations. The main cross veins are:
(i) Humeral cross vein (h)-lies between the costa and the subcosta.
(ii) Radial cross vein (r)-lies between the R1 and fisrt four of Rs.
(iii) Sectorial cross vein (s)-lies between two forks of Rs.
(iv) Median cross vein (m)-lies between the second and third media M2 and M3.
(v) Median-cubital cross vein (m-cu)-runs between the media (M) and cubitus (Cu).
(vi) Radial-median cross vein (r-m)- runs between the radius and media.
The black pterostigma is carried near the wing tip, between RA1+2 and RA3+4.
