108 WINGS CHAP. 



attended these efforts, and it is probable that no real homology 

 exists between the nervures of the different Orders of Insects. 

 We shall not therefore discuss the question here. AVe may, 

 however, mention that German savants have recently distin- 

 guished two forms of nervures which they consider essentially 

 distinct, viz. convex and concave. These, to some extent, alter- 

 nate with one another, but a fork given off by a convex one is 

 not considered to be a concave one. The terms convex and con- 

 cave are not happily chosen ; they do not refer to the shape of 

 the nervures, but appear to have been suggested by the fact that 

 the surface of the wing being somewhat undulating the convex 

 veins more usually run along the ridges, the concave veins along 

 the depressions. The convex are the more important of the two, 

 being the stronger, and more closely connected with the articula- 

 tion of the wing. 



The wings, broadly speaking, may be said to be three - 

 margined : the margin that is anterior when the wings are 

 extended is called the costa, and the edge that is then most 

 distant from the body is the outer margin, while the limit that 

 lies along the body when the wings are closed is the inner 

 margin. 



The only great Order of Insects provided with a single pair 

 of wings is the Diptera, and in these the metathorax possesses, 

 instead of wings, a pair of little capitate bodies called halteres or 

 poisers. In the abnormal Strepsiptera, where a large pair of 

 wings is placed on the metathorax, there are on the niesothorax 

 some small appendages that are considered to represent the 

 anterior wings. In the great Order Coleoptera, or beetles, the 

 anterior wings are replaced by a pair of horny sheaths that close 

 together over the back of the Insect, concealing the hind-wings, 

 so that the beetle looks like a wingless Insect : in other four- 

 winged Insects it is usually the front wings that are most 

 useful in flight, but the elytra, as these parts are called in 

 Coleoptera, take no active part in flight, and it has been 

 recently suggested by Hoff bauer x that they are not the homo- 

 logues of the front wings, but of the tegulae (see Fig. 56), of other 

 Insects. In the Orthoptera the front wings also differ in con- 

 sistence from the other pair over which they lie in repose, and 

 are called tegmina. There are many Insects in which the wings 



1 Zdtschr. wiss. Zool. liv. 1892, p. 579. 



