T7ie Colouration of Insects. 71 



are described as the Costal, which is the upper strong edge of 

 the wing, the Hind margin, forming the outside, and the Inner 

 margin, forming the base. The nervures consist of four principal 

 veins ; the Costal, a simple nervure under the costa, the Sub-costal, 

 which runs parallel to the costal and about halfway to the tip emits 

 branches, generally four in number ; the Median occupying the centre 

 of the wing and sending off branches, usually three in number, and 

 the Sub-median below which is always simple. There are thus two 

 simple nervures, one near the costal the other near the inner margin, 

 and between them are two others which emit branches. Between 

 these two latter is a wide plain space known as the discoidal cell. 

 Small veins called the discoidal pass from the hind margin towards the 

 cell, and little transverse nervures, known as sub- discoidal, often close 

 the cell. By these nervures the wing is mapped out into a series of 

 spaces of which one, the discoidal cell, is the most important. 



The nervures have two functions, they support and strengthen 

 the wing, and being hollow serve to convey nutritive fluid and after- 

 wards air to the wing. 



The wings are moved by powerful muscles attached to the base of 

 the wings close to the body and to the inside of the thorax, all the 

 muscles being necessarily internal. "There are two sets which 

 depress the wings ; firstly a double dorsal muscle, running longi- 

 tudinally upwards in the meso-thorax ; * and, secondly, the dorso- 

 ventral muscles of the meso and meta-thorax,f which are attached 

 to the articulations of the wings above, and to the inside of the 

 thorax beneath. Between these lie the muscles which raise the wings 

 and which run from the inner side of the back of the thorax to the 

 legs." % When we consider the immense extent of wing as compared 

 with the rest of the body, the small area of attachment, and the great 

 leverage that has to be worked in moving the wings, it is clear that 

 the area of articulation of the wing to the body is one in which the 

 most violent movement takes place. It is here that the waste and 

 repair of tissue must go on with greatest vigour, and we should, on 

 our theory, expect it to be the seat of strong emphasis. Accordingly 

 we commonly find it adorned with hairs, and in a vast number of 

 cases the general hue is darker than that of the rest of the wing, 

 and so far as we have been able to observe, never lighter than the body 



* The middle division of the thorax. 



t Hinder division of thorax. 



% Dallas in Cassell's Nat. Hist., vol. vi., p. 27. 



