53 SPECIFIC DISTINCTIONS. 



readily catches the eye, and undoubtedly it is a valu- 

 able character, always provided that other points are 

 not ignored, as unfortunately they too often are in 

 descriptive works. The body is rarely sculptured, 

 sometimes it is more or less pilose, but excellent 

 characters are to be obtained from the antennae, as to 

 their length, thickness, pilosity, &c. ; of the clypeus, 

 as to whether it is truncated or incised at the apex ; 

 by the form of the head and its sutures; by the 

 arrangement and position of the nervures in the 

 wings ; by the legs, as to the length of the spurs, of 

 the tarsal joints, and the form of the spurs. Most of 

 the specific characters, in fact, are slight morpho- 

 logical variations, which, so far as we can see, are of no 

 use to the species ; but in one organ we find a wonder- 

 ful amount of variety in structural detail. This is in 

 the saw, of which, indeed, it may be said that its form 

 affords us an almost infallible criterion of specific 

 distinctness. We can easily see why there should be 

 so much variety of form in the saw when we consider 

 not only how manifold are the modes of depositing the 

 ova, but also how different in texture, &c., are the 

 substances in which they are laid. And as the same 

 species follows always the same mode of oviposition we 

 can understand, also, how this organ scarcely or never 

 varies in structure ; for a variation, however slight in 

 the shape of the teeth, &c., might prevent the eggs 

 being sunk in the proper manner in the substance of 

 the leaf, and thus might lead to the death of the 

 embryo. Contrariwise, we find great variation in the 

 other specific distinctions in colour, sculpture, &c., 

 because they are, so far at least as we can see, of 

 secondary importance to the insects, and therefore 

 variation has had some play. 



