WING , WING VENATION AND MODIFICATIONS .

Among the invertebrate animals , only insects possess wings ,Wings are present in adult stage .

 Number of wings varies from two pairs to none . Certain primitive insects like silver fish and spring tail have no wings.

Ecto-parasites like head louse , poultry louse and flea are secondarily wingless . 

Wings are deciduous in ants and termites . There is only one pair of wings in the true flies .

 Normally two pairs of wings are present in insects and they are borne on pterothoracic segments viz . , mesothorax and metathorax . 

Wings are moved by thoracic flight muscles attached to their bases . 

Wings is flattened double layered expansion of body wall with a dorsal and ventral lamina having the same structure as the integument .

 Both dorsal and ventral laminae grow ,Meet and fuse except along certain lines . It is nourished by blood circulating through veins.

walls of these channels become thickened to form veins .

 The arrangement of veins on the wings is called venation which is extensively used in insect classification .

 The principal longitudinal veins arranged in order form the anterior margin are costa ( C ) , sub costa ( Sc ) , radius ( R ) , median ( M ) , cubitus ( Cu ) and anal veins ( A ) , Small veins often found inter connecting the longitudinal veins are called cross veins . 

In insects like dragon fly and damselfly , there is an opaque spot near the coastal margin of the wing called pterostigma .

 Margins and Angles

 The wing is triangular in shape and has therefore three sides and three angles . 

The anterior margin strengthened by the costa is called costal margin and the lateral margin is called apical margin and the posterior margin is called anal margin . 

The angle by which the wing is attached to the thorax is called humeral angle . 

The angle between the costal and apical margins is called apical angle . The angle between apical and anal margins is called anal angle . 

Wings region

 The anterior area of the wing supported by veins is usually called remigium . 

The flexible posterior area is termed vannus . The two regions are separated by vannal fold . The proximal part of vannus is called jugum .

 when well developed is separated by a jugal fold . The area containing wing articulation sclerites , pteralia is called axilla .

 Insects have evolved many variations of the wings , and an individual insect may possess more than one type of wing . 

Wing venation is commonly used as taxonomic character , especially at the family and species level . 

In most living insects ( the Neoptera ) , there are three axillary sclerites that articulate with various parts of the wing . I

In Neoptera , a muscle on the third axillary causes it to pivot about the posterior notal wing process and thereby to fold the wing over the back of the insect . 

( In some groups of Neoptera , such as butterflies , the ability to fold the wings over the back has been lost . ) Two orders of winged insects , the Ephemeroptera and Odonata , have not evolved this wing - flexing mechanism , and their axillary sclerites are arranged in a pattern different from that of the Neoptera : these two orders ( together with a number of extinct orders ) form the Paleoptera . 

Modifications

 1. Tegmina 

Tegmina ( singular tegmen ) are the leathery forewings of insects in the orders Orthoptera , Dictyoptera and Mantodea . The tegmina helps to protect delicate hind wings . They are not used for flight . E.g. grasshoppers , crickets and katydids ( order Orthoptera ) , cockroaches ( suborder Blattaria ) , Mantids . 

2. Elytra

 The wing is heavily sclerotised . Wing venation is lost . It is tough and protective in function . It protects hind wings and abdomen . It is not used during flight , but during flight they kept at an angle allowing free movement of hind wings . e.g. fore wings of beetles and weevils.

3. Hemelytra 

The basal half of the wing is thick and leathery and distal half is membranous.

  They are not involved in flight and are protective in function, e.g.  fore wing of heteropteran bugs.

  4. Halteres

 In true flies the hind wings are modified into small knobbed vibrating organs called haltere.  Each haltere is a slender rod clubbed at the free end (capitellum) and enlarged at the base (scabellum).  They act as balancing organs and provide the needed stability during flight.  e.g.  true flies, mosquito, male scale insect. 

 5. Fringed wings

 Fringed wings are usually reduced in size.  Wing margins are fringed with long setae.  These insects literally swim through the air.  e.g.  thrips.  

6. Scaly wings

 Wings of these insects are covered with small colored scales.  Scales are unicellular flattened outgrowth of body wall, inclined to the wing surface and overlap each other to form a complete covering.  Scales are responsible for color and are important in smoothing the air flow over wings and body.  E.g.  butterfly and moth.