THE LARGE CABBAGE WHITE BUTTERFLY

The Large Cabbage White Butterfly is very common in early summer, and can be easily recognized by its large white wings with black markings. As is usual among insects, the body is divided into head, thorax and abdomen.

Head

The head bears a pair of large compound eyes and two long, many-jointed antenna; which end in small knobs. The antenna; are used as tactile organs, and it is probable that they are also used as organs of smell. The butterfly also possesses three small simple eyes, which are hidden among the hairs on the top of the head. It feeds on the sugary solution called nectar which is secreted by flowers, and its mouth-parts are structurally adapted to obtain this food. Of the mouth-parts, so well developed in the Cockroach, only the first and second maxilla; are present, the mandibles being vestigial or absent. Together the first maxilla; form a long tube called the proboscis, up which the nectar is sucked. When not in use the proboscis is coiled up spirally beneath the head, but it can be rapidly extended to reach the nectar hidden at the bottom of a flower. Each maxilla has a groove on its inner surface and forms one half of the tube, interlocking with the other half by means of small teeth running along the edge. The second maxilla; are much reduced, and are represented by a small plate lying beneath the proboscis. The plate bears a pair of small feelers or labial palps.

Thorax

The three segments of the thorax are not distinguishable owing to the thick covering of hairs. Each segment bears a pair of many-jointed legs which end in a small pad bounded by two claws. The two pairs of wings are thin chitinous outgrowths of the 2nd and 3rd thoracic segments, and have a characteristic network of thickened ribs or veins. The wings are covered by a layer of small scales, of various shapes, and which are generally stalked. The scales overlap each other like the tiles on a roof, and are easily brushed off. The scales not only cover the wings but are found over the whole body, and the name Lepidoptera, which is given to the order containing butterflies and moths, was suggested by these scales.

When at rest the wings of the Cabbage White are raised up over its back, so that the upper surfaces lie against each other and only the greenish-white lower surfaces are visible. In this position the butterfly harmonizes with a light green background so as to be difficult to see. The colour of the upper surface is creamish-white with black markings. The front wings of the female differ from those of the male in having more black markings.

Abdomen

The abdomen is long and covered by a dense layer of hairs, so that it is not possible to distinguish the ten segments from the outside. The anus opens at the posterior end of the abdomen.

The Life History of the Cabbage White Butterfly

The female Cabbage White lays its eggs in April or May, or, if it belongs to a later seasonal brood, in August or September. The eggs are yellow, conical in shape and have a surface marked with ridges. They are laid in batches of 20 to 100 on the underside of cabbage and nasturtium leaves, and after 8 to 10 days tiny caterpillars hatch from them. The caterpillar first eats the eggshell and then nibbles the leaf with a pair of strong toothed mandibles. Each caterpillar is attached to the slippery surface of the leaf by silk threads, which are formed by a little pointed process, called the spinneret, at the front of the head. After a few days the caterpillar stops feeding, its skin splits down the back near the head and it wriggles out. This moult or ecdysis, as it is called, is followed by a rapid increase in size. Four or five moults take place before the caterpillar is fully grown, when it is about il in. long. After the third moult the young caterpillars separate, and each feeds along the edge of a leaf with its jaws working on either side of the margin.

The Structure of the Caterpillar

The caterpillar is coloured bluish-green, with a narrow yellow streak passing down the middle of the back and a broader streak of the same colour running along each side. These streaks have black patches on them. Short white hairs spring from little wart-like protuberances which are scattered over the body. The body consists of a head and thirteen segments. The head has no large compound eyes like those of the butterfly, but has only six small black simple eyes on each side. The paired antenna; are tiny projections, scarcely visible among the hairs of the head. As in the Cockroach the mandibles lie behind a flap called the labrum, which hangs from the lower edge of the head. The mandibles work from side to side, and beneath them lie the first maxilla?. These are small jointed processes, behind which the second maxilla? are fused to form a plate from which the spinneret hangs downwards.

Each of the first three segments of the caterpillar bears a pair of jointed legs. A small claw terminates each leg. Behind the thoracic segments are the ten abdominal segments, certain of which bear fleshy projections called pro-legs or cushion feet. A pair of pro-legs is found on the 3rd, 4th, 5th and 6th abdominal segments, and the last segment also bears a pair called the claspers. A spiracle or breathing hole appears as a minute black dot on the side of the first thoracic segment and the first eight abdominal segments.

The Pupa

After about thirty days, when the caterpillar is fully grown, it descends from its leaf and proceeds to a place suitable for the development into the next or pupal stage. It climbs up some neighbouring tree or building to a dry spot, and spins a small pad of silk to which it holds by its claspers. It then supports itself in an upright position by silken threads which are passed round the fore part of the body and fixed on either side to the surface to which it is attached. Now the caterpillar sheds its skin and becomes a structure called a pupa, which is completely unlike the caterpillar stage. The pupa is shorter than the caterpillar and shows several features of the butterfly, for feelers, legs and wings can be seen flattened against the thoracic region, and the abdomen also is visible. The pupa is light green in colour, but in many other species of butterfly it is golden, and for this reason another name for the pupa is chrysalis. The pupa does not feed and is perfectly still except for occasional twitchings of the abdomen. Inside the pupa there is a considerable break-down and reorganization of structure, and it takes from two to three weeks for the butterfly to become fully developed. The cuticle of the pupa splits in the region of the thorax, and the mature butterfly or imago slowly works its way out. At first the wings are small and crumpled, but they soon expand and harden.

The Seasonal Cycle of the Cabbage White Butterfly

There are two broods of the Large White during the year, which occur in spring and summer. The winter is passed in the pupal form, and the butterflies which emerge from the hibernating pupa? form the first brood. From eggs laid by these insects the second brood is produced from July to September. The pupa? which develop from eggs laid at the end of the summer undergo the long resting stage which occurs during the winter.

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