WINGS 107 



for the actual continuity of the limb at the chief joint — the 

 knee — can be demonstrated in many Insects by splitting the 

 outer integument longitudinally and then pulling the pieces a 

 little apart ; while in other cases even this is not necessary, 

 the knee along its inner face being membranous to a consider- 

 able extent, and the membrane continuous from femur to tibia. 



Turning to the wings, we remark that there may be one or 

 two pairs of these appendages. When there is but one pair it is 

 nearly always mesothoracic, when there are two pairs one is invari- 

 ably mesothoracic, the other nietathoracic. The situation of the 

 wing is always at the edge of the notum, but the attachment 

 varies in other respects. It may be limited to a small spot, and 

 this is usually the case with the anterior wing ; or the attachment 

 may extend for a considerable distance along the edge of the 

 notum, a condition which frequently occurs, especially in the case 

 of the posterior wings. The actual connexion of the wings 

 with the thorax takes place by means of strong horny lines 

 in them which come into very close relation with the little 

 pieces in the thorax which we have already described, and which 

 were styled by Audouin articulatory epidemes. There is 

 extreme variety in the size, form, texture, and clothing of the 

 wings, but there is so much resemblance in general characters 

 amongst the members of each one of the Orders, that it is usually 

 possible for an expert, seeing only a wing, to say with certainty 

 what Order of Insects its possessor belonged to. We shall allude 

 to these characters in treating of the Orders of Insects. 



Each wing consists of two layers, an upper and a lower, and 

 between them there may be tracheae and other structures, 

 especially obvious when the wings are newly developed. It has 

 l^een shown by Hagen that the two layers can be separated 

 when the wings are recently formed, and it is then seen that 

 each layer is traversed by lines of harder matter, the nervures. 

 These ribs are frequently called wing -veins, or nerves, but as 

 they have no relation to the anatomical structures bearing those 

 names, it is better to make use of the term nervures. The 

 strength, number, form and inter - relations of these nervures 

 vary exceedingly ; they are thus most important aids in the 

 classification of Insects. Hence various efforts have been made 

 to establish a system of nomenclature that shall be uniform 

 throughout the different Orders, but at present success has not 



