Another salient feature is the difference that generally exists between the upper and lower surfaces of the wings. As a rule, the upper surface is the seat of the brightest colour. Most butterflies, perhaps all, close their wings when at rest, and the upper wing is generally dropped behind the under wing, so that only the tip is visible. The under surface is very frequently so mottled and coloured as to resemble the insect's natural surroundings, and so afford protection. It does not follow that this protective colouring need be dull, and only when we know the habit of the insect can we pronounce upon the value of such colouring. The pretty Orange-tip has its under wings veined with green, and is most conspicuous in a cabinet, but when at rest upon some umbelliferous plant, with its orange tip hidden, these markings so resemble the environment as to render the insect very inconspicuous. The brilliant Argynnis Lathonia, with its underside adorned with plates of metallic silver, is in the cabinet a most vivid and strongly-marked species; but we have watched this insect alight among brown leaves, or on brown stones, outside Florence, where it is very common, and find that these very marks are a sure protection, for the insect at rest is most difficult to see, even when it is marked down to its resting-place.

But some butterflies have parts of the under surface as gaily decorated as the upper; and this not for protection. This may be seen to some extent in our own species, for instance in the orange-tip of the Orange-tip, and the red bar in the upper wing of the Red Admiral (V. atalanta). If we watch these insects, the conviction that these are true ornaments is soon forced upon us. The insect alights, perhaps alarmed, closes its wings, and becomes practically invisible. With returning confidence it will gradually open its wings and slowly vibrate them, then close them again, and lift the upper wing to disclose the colour. This it will do many times running, and the effect of the sudden appearance and disappearance of the bright hues is as beautiful as it is convincing. None can doubt the love of display exhibited in such actions.

The delicacy of their organization renders butterflies peculiarly susceptible to any change, and hence they exhibit strong tendencies to variation, which make them most valuable studies. Not only do the individuals vary, but the sexes are often differently coloured. Where two broods occur in a season they are sometimes quite differently decorated, and finally a species inhabiting widely different localities may have local peculiarities.

We can thus study varieties of decoration in many ways, and we shall treat of them as follows:—

1. Simple Variation, in which the different individuals of a species vary in the same locality.

2. Local Variation, in which the species has marked peculiarities in different localities.

3. Sexual Dimorphism, in which the sexes vary.

4. Seasonal Dimorphism, in which the successive broods differ.

Fig. 3. Diagram of Butterfly's Wing.

A. Upper Wing. B. Lower Wing. a. Costal Margin. b. Hind Margin. c. Inner " d. Anal Angle. e. Costa. f. Costal nervure. g. Sub-costal do. g1-4. Branches of do. h. Median nervure. i. Sub-median do. j. Discoidal Cell. k. Discoidal Veins.
A.Upper Wing.
B.Lower Wing.
a.Costal Margin.
b.Hind Margin.
c.Inner "
d.Anal Angle.
e.Costa.
f.Costal nervure.
g.Sub-costal do.
g1-4.Branches of do.
h.Median nervure.
i.Sub-median do.
j.Discoidal Cell.
k.Discoidal Veins.

In order fully to understand the bearing of the following remarks it is necessary to know something of the anatomy and nomenclature of butterflies. [Fig. 3] is an ideal butterfly. The wing margins 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.