[THE BUTTERFLIES OF THE BRITISH ISLES.]
[PART I.]
THE LIFE CYCLE OF A BUTTERFLY.
As is the case with all other Lepidoptera, butterflies pass through three very distinct stages before they attain the perfect form. These stages are:—1. The egg (ovum, plural ova). 2. The caterpillar (larva, larvæ). 3. The chrysalis (pupa, pupæ). The perfect insect is called the imago (plural imagines).
The Egg.
Butterfly eggs are of various forms, and whilst in some kinds the egg-shell (chorion) is elaborately ribbed or fluted, others are simply pitted or covered with a kind of network or reticulation; others, again, are almost or quite smooth. If the top of an egg, such as that of the Purple Emperor (Plate [28]), is examined under a good lens a depression will be noted, and in this will be seen a neat and starlike kind of ornamentation. In the middle of this "rosette" are, present in all eggs, minute apertures known as micropyles (little doors), and it is through these that the spermatozoa of the male finds entry to the interior of the egg and fertilization is effected. The changes that occur in the egg after it is laid are of a very complex nature, and readers who may desire information on this subject are referred to Sharp's "Insects," Part I., in the "Cambridge Natural History," where also will be found much interesting and instructive matter connected with the caterpillar and chrysalis, to which stages only brief reference can here be made.
The Caterpillar.
The second stage is that of the caterpillar, and in some species, such as the Red Admiral, this is of very short duration, a few weeks only, whilst in others, as for example the Small Blue, it usually lasts for many months. There is considerable diversity both in the shape and, where it is present, in the hairy or spiny clothing (armature) of caterpillars. All, however, are alike in one respect, that is the body is divided into thirteen more or less well-defined rings (segments), which together with the head make up fourteen divisions. In referring to these body-rings, the first three nearest the head, each of which is furnished with a pair of true legs (thoracic legs), are called the thoracic segments, as they correspond to the thorax of the perfect butterfly. The remaining ten rings are the abdominal segments; the last two are not always easily separable one from the other, and so for all practical purposes they may be considered only nine in number. These nine rings, then, correspond to the abdomen of the future butterfly. The third to sixth of this series have each a pair of false legs (prolegs), and there is also a pair on the last ring; the latter are the anal claspers.
The warts (tubercles) are the bases of hairs and spines, and are to be seen in most butterfly caterpillars, but they generally require a lens to bring them clearly into view. These warts are usually arranged in two rows on the back (dorsal series) and three rows on each side (lateral series).