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WILDLIFE OF NORTHERN ENGLAND

Teaching children about nature insects 2--the Cabbage White Butterflry

The large cabbage white butterfly is one of the most common of our English butterflies. It can be a serious pest of the cabbage family. Crops such as cabbage and Crucifer species along with garden favourites such as nasturtium can all have their foliage decimated by the caterpillars.

There are two broods and sometimes even three in the course of the year. The adult butterfly is on the wing, first in May and June and again during July and August, and if conditions remain favourable they may be encountered as late as September. Therefore they may be observed for the greater part of the summer.

Cabbage white rests with its wings folded so that only the under side of the wings is visible.

Photograph courtesy of Kropsoq { Creative Commons Attribution 3.0 generic license}

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Description of the cabbage white

THE HEAD---At the sides of this rounded head are relatively large black compound eyes. From the front of the head above the eyes there protrudes a pair of antennae which is club-shaped. This shape is the feature that distinguishes butterflies from the moths. However, the exact shape of the 'club' differs in different species.

Rolled up beneath the head is the long and delicate proboscis, which is capable of being protruded and utilised in the form of a tube for sucking nectar from flowers. this proboscis is composed of two half tubes, placed hollow to hollow,which are separable for undergoing cleansing, but capable of close opposition when desired for extracting nectar and other solubles.

THE THORAX--- Of the the three segments that form the thorax the middle one is the largest. The whole of the thorax is covered with black hairs. the three pairs of legs are located at one pair per segment. they are all walking or clinging legs, with two claws at their tips.

In some butterflies, as for an example, Purple Emperor, Peacock, Tortoise shell, and other Vanessids, All Fritillaries, White admiral, meadow browns and their allies have the first pair of legs reduced in length, have no claws and apparently employed as sense organs. The tibiae of the first and second pair have articulated spurs on the middle and at the ends, from the third pair the middle pairs are absent, in British butterflies only the skipper butterflies have them present in the third pair.

Skpper butterflies have the have the spurs on the middle of the tibiae.

Photograph by Dal

Wings.

The two pairs of wings are attached respectively to the second and third segments of the thorax. When fully spread their expanse from tip to tip is about two and a half inches,{6.4cm}. The front wings are rather longer and narrow and less round at their angles than the hind-wings and the former overlaps the latter to some extent, so that in flight the two wings of one side are pressed together as they strike the air, and beat as though united into one.

In both sexes the general colour of the upper-side of the wing is white, the colour  due to the thousands of overlapping scales.In the male the tip of the front wing has a very narrow black border which widens out as it reaches the front angle and passes more than half way along the outer edge, gradually fading out before reaching the hind angle. In the female the expanded portion of the black colouring is rather more extensive, and there are in addition two large black spots on the surface of the wing, one above the other, and separated by about as much white as their own width, situated rather more than halfway from the shoulder to the outer edge. Just beneath the lower of these spots the female also sorts a club-shaped black streak whose point is directed towards the shoulder.

On the hind wing of both sexes there is a black spot on the front edge in a line with the two on the fore-wing of the female. This spot is larger in the female and has the appearance of being two spots coalesced. the lower side of the wings are the same in both sexes, the male as well as the female having the two black spots upon the fore wing, however, they lack the dark markings with the exception of a small portion of the hind wings. The general colour of the underside is greenish yellow this is particularly true of fresh young butterflies that have just emerged.

When at rest the insect raises its wings, without any folding or creasing, over its back in the manner that the left meets the right and only the under side is visible. This affords them a degree of camouflage when they are sitting on the foliage of plants. The wings are strengthened by veins or nervures of which the hind parts are the strongest. The precise arrangement of the nervureor wing venation is of great importance in the classification of Lepidoptera the large family to which this species belongs. 

Life History of the cabbage white butterfly.

The eggs are encountered in May and again in late July and August. They are deposited in patches of between 50-100, or even more, on the under side of cabbage leaves and other crucifer plants as well as on the foliage of garden favourites such as nasturtium.

The eggs are flask-shaped and water proof and of a yellowish colour. The young caterpillars emerge within ten days or so, biting their way through the shells with strong mandibles and very soon begin to devour the foliage.

The eggs of the cabbage white butterfly  are clustered together on the under side of the foliage.

Photograph by Dal.

Caterpillar

The caterpillar feeds ravenously and grow rapidly moulting their skins four or five times as the exo-skeleton becomes to small for the growing internal organs. prior to each shedding of the skin, a small carpet of silk is spun over a portion of the leaf, and upon this the caterpillar rests, obtaining a firm foothold and anchorage for the husk shortly to be anchored. The full grown caterpillar is about two and a half inches long. 

The head is of a slaty brown colour with black spots, its under side is yellowish. It is relatively hard, a firm skeleton being required for the attachment of the muscles that move the mandibles upon so much use is made in the lifetime of the caterpillar.

The body colouring is yellowish green with black spots and blotches. The whole surface is covered with whitish hairs. The first three segments each have a pair of true, jointed legs ending in a curved claw, the joints are very short.  These three segments form the thorax.

The next two segments have no legs of any sort, the third, fourth, fifth and sixth segments of the abdomen each have a pair of 'false' legs or ' claspers' and there is a fifth pair of claspers of slightly different shape at the extreme hind end.  the claspers are non-jointed, fleshy outgrowths and are adorned with rows of little hooks on their soles for gaining a firm foothold. All Lepidoptera caterpillars, except for a very few legless forms, have six true legs upon the thorax, and they never have claspers upon the first two segments of the Abdominal segments.

During the larval life, immense stores of fat are accumulated  in the body, for this is the main feeding period in the insects life time. The adult butterfly consumes very little food. Indeed many adult species of Lepidoptera are incapable of taking in any food.

Cabbage white caterpillars---

Photograph by Dal

Pupation

When the caterpillar has eaten sufficient food and grown to its full size it will leave the food plant, and after a short journey over the ground , climbs up a wall or fence or tree trunk, and there spins once again a little silken platform similar to that constructed before each shedding of the skin.However, at this stage it also places a girdle of silk around the middle of its body and each end is attached to the vertical surface it has selected.

The caterpillar rests with its head uppermost, though it may place itself horizontally on occasion, the hind most claspers secure themselves by the hooks to the platform of silk. During this period of inaction the length of the caterpillar becomes much shorter. After some hours the skin cracks open. With a series of spasmodic jerks and wriggles the skin is worked off downwards towards the tail.At this stage and shape the structure is known as the chrysalis or pupa, and is very different in form to that of the caterpillar.

At first it is quite soft, and its skin is semi-transparent. Later the pupil hardens it only measures about an inch long. the hind end terminates in a sharp spike flanked by two smaller projections. On the dorsal surface, which is directed outwards away from the surface of attachment, is a series of similar pointed elevations of which the one from the middle segment of the thorax is the largest. Thus the whole pupa is very angular in aspect. The colour of the pupa is very much dependent on the colour of the surface to which it is attached.

As for an example, should they be attached to the surface of a treated, dark shed or fence, the pupa will also be dark coloured, while those secured to a white window frame will be very pale.

During the pupal stage extensive changes of the internal organs take place, and, during this cycle of pupation the caterpillar makes the necessary modifications into what will be the emerging flying adult butterfly. When the imago {adult butterfly} is ready to emerge during the spring [or summer} the colour of the pupae change a little. Eventually the skin near the back end breaks open and the butterfly crawls out, wet and limp, with small crumples wings that are full of fluid.

the wings soon lengthen out, hanging down with their edges parallel with the abdomen. After a few hours they dry. The liquid within disappearing and soon after this has taken place the butterfly flits away taking its first aerial experience and the cycle of life starts over again. 

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