<P> At the posterior angle of the wing base in some Diptera there is a pair of membranous lobes (squamae, or calypteres) known as the alula . The alula is well developed in the house fly . The outer squama (c) arises from the wing base behind the third axillary sclerite (3Ax) and evidently represents the jugal lobe of other insects (A, D); the larger inner squama (d) arises from the posterior scutellar margin of the tergum of the wing - bearing segment and forms a protective, hood - like canopy over the halter . In the flexed wing, the outer squama of the alula is turned upside down above the inner squama, the latter not being affected by the movement of the wing . In many Diptera, a deep incision of the anal area of the wing membrane behind the single vannal vein sets off a proximal alar lobe distal to the outer squama of the alula . </P> <P> The various movements of the wings, especially in insects that flex their wings horizontally over their backs when at rest, demand a more complicated articular structure at the wing base than a mere hinge of the wing with the body . Each wing is attached to the body by a membranous basal area, but the articular membrane contains a number of small articular sclerites, collectively known as the pteralia . The pteralia include an anterior humeral plate at the base of the costal vein, a group of axillaries (Ax) associated with the subcostal, radial, and vannal veins, and two less definite median plates (m, m') at the base of the mediocubital area . The axillaries are specifically developed only in the wing - flexing insects, where they constitute the flexor mechanism of the wing operated by the flexor muscle arising on the pleuron . Characteristic of the wing base is also a small lobe on the anterior margin of the articular area proximal to the humeral plate, which, in the forewing of some insects, is developed into a large, flat, scale - like flap, the tegula, overlapping the base of the wing . Posteriorly, the articular membrane often forms an ample lobe between the wing and the body, and its margin is generally thickened and corrugated, giving the appearance of a ligament, the so - called axillary cord, continuous mesally with the posterior marginal scutellar fold of the tergal plate bearing the wing . </P> <P> The articular sclerites, or pteralia, of the wing base of the wing - flexing insects and their relations to the body and the wing veins, shown diagrammatically, are as follows: </P> <Ul> <Li> Humeral plates </Li> <Li> First Axillary </Li> <Li> Second Axillary </Li> <Li> Third Axillary </Li> <Li> Fourth Axillary </Li> <Li> Median plates (m, m') </Li> </Ul>

The lateral surface of the insect thorax and abdomen is called the