a, compound eye; b, palp; c, antenna; d, proboscis.
It may be well now to briefly consider some of the structural details of the perfect butterfly, so a beginning will be made with the head (Fig. [6]). When looking at the head of a butterfly, the first thing to attract the attention is the very large size of the compound eye (a), which seems to take up the largest share of the whole affair. Although so bulky and so complex in the matter of divisions, or facets, as they are termed (the facets are not shown in figure), the power of sight is not really very keen. A butterfly can see things in a general way readily enough, but it seems unable to clearly distinguish one object from another. When engaged in egg-laying, the female butterfly rarely fails to place her eggs on a leaf or spray of the plant that the future caterpillar will feed upon, and it has been suggested that in making this unerring selection the insect is guided more by the sense of smell than by that of sight.
The horns (c) (antennæ), or feelers, as they are sometimes called, which adorn the head, are now considered to be organs of smell. These are composed of a number of rings or segments, which vary in the different kinds of butterfly, as also does the shape of the terminal rings forming what is known as the club. In Fig. [7,] e (Purple Emperor) and f (Marbled White) represent the gradually thickened club; in g (Brimstone) and h (Dark-green Fritillary) the clubs are more or less abruptly formed. Our Skippers have well-developed clubs; these may be hooked at the tip as in i (Large Skipper), or blunt at the tip as in j (Chequered Skipper); at the base of the Skipper's antenna, that is at the point where it is inserted in the head, there is a tuft of rather long hairs.
Of the various mouth parts it will only be necessary to refer to the suction-tube, Fig. [6,] d (proboscis), often called the "tongue," which is perhaps the most important, at least to the butterfly itself, as this organ is, in a way, as useful to it in the perfect state as were the very differently constructed strong biting jaws (mandibles) of its caterpillar existence. These latter in the butterfly are only microscopically represented, and the suction-tube of the perfect insect is an extension of the maxillæ, which in the caterpillar are not conspicuous. When not engaged in probing the nectaries of flowers for the sweets they contain, the suction-tube is neatly coiled up between the palpi (Fig. [6,] b). Its great flexibility is due to the many rings of which it is composed. Although seemingly entire, it is really made up of two tubes, each being grooved on its inner side, and forming, when the edges are brought together, an additional central canal, through which the sweets from the flowers and other liquids are drawn up into a bulb-like receptacle in the head, whence it passes into the stomach. When it is remembered that the passage of sweet, and no doubt sticky, fluid through the central tube would most probably result in its walls becoming clogged, there is reason to suppose that the method of construction permits of the canal being cleansed from time to time.
Fig. 7.
Antennæ of Butterflies.
