(b) That a bristle at the base of the hind-wing hooks on a catch in the fore-wing of the same side.
3. The vapourer moth.—About the end of June examine rose trees, fruit trees, willows, oaks, etc., for caterpillars of the vapourer moth. They may be recognised ([Fig. 228]) by the reddish warts and the tufts of hair which stand out from various parts of the body. Notice that the caterpillars fall into two groups according to size, being, when full-grown, about 2 in. and 1¼ in. long respectively. Keep the larvae in a breeding-cage until they pupate and change into moths; notice that the moths (the females) which come from the large caterpillars have no wings; while the male moths, which are derived from the small caterpillars, have well-developed wings.
A typical butterfly.—The great beauty of butterflies and moths, and the ease with which the stages of their wonderful life-history can be followed, have made these insects favourite objects of study among naturalists of all ages.
Among the commonest of butterflies are the well-known cabbage-whites, the caterpillars of which work so much havoc upon crops of cruciferous plants ([p. 95]). The eggs are laid in May and September upon the leaves, and soon hatch out into small larvae called caterpillars, which feed voraciously and grow rapidly. The larval skin is shed from time to time as it becomes too small. The caterpillar ([Fig. 225], A) is somewhat worm-like in appearance, but insect characters may easily be recognised in it. The head is small and shiny. It carries six eye-spots, but the large compound eyes of the adult are not yet visible. A pair of short feelers is present; and the mouth-parts are obviously comparable with those of the cockroach, although the labium ([p. 353]) has become converted into a spinneret, which gives out the silk threads used for the protection of the pupa. The mandibles, which are used in gnawing leaves, are stout and toothed; the first maxillae are rather small. Behind the head come the three segments of the thorax, each of which bears a pair of short jointed legs; wings have not, as yet, developed. The abdomen really consists of ten segments, although only nine can be seen without dissection. Segments 3 to 6 of the abdomen bear short, unjointed legs called pro-legs or cushion feet, and the last segment bears a pair of appendages called the anal feet. Spiracles are to be seen on the first eight abdominal segments. In the larva of the green-veined cabbage-white, each spiracle is reddish and surrounded by a yellow border.
Fig. 225.—Cabbage White Butterfly. A, larva; B, pupa; C, perfect insect.
(All × ⅔.) (B and C from photographs by Mr. A. Flatters)
When the caterpillar has attained its full size, it stops feeding and seeks out a sheltered place—often a chink in a wall. Silken threads are given off by the spinnerets until a little heap of silk is formed into which the hooked end of the abdomen is fixed. Then a girdle of silk is made, passed round the thicker fore-part of the body, and so attached to the wall that the animal is supported in an upright position. The larval skin now splits and is peeled off, and the pupa, or chrysalis, stage ([Fig. 225], B) is entered upon. All the external parts of the butterfly are complete at the time of pupation, but profound changes are still necessary in the internal organs; it is to allow these changes to take place in tranquillity that the resting, or pupal, stage is interposed between larval and adult life.
At last the new organs are ready for their work, the pupal skin cracks, and the perfect insect ([Fig. 225], C) emerges. The head now carries a pair of large compound eyes, and two slender feelers with knobbed ends. The jaws also have been remodelled in accordance with the completely different manner of life upon which the insect is now entering. The mandibles are now only doubtfully recognisable; the first pair of maxillae are elongated and grooved, and are closely applied to each other to form a long tube called the proboscis ([Fig. 226], Mx. 1), which, when not in use for sucking up the sweet juices of flowers, is kept coiled up beneath the head; the palps of the labium (Lab. Pa.) project like tusks on the sides of the head. The thorax is provided with two pairs of broad wings, which are covered with minute overlapping scales—forming a delicate “bloom” which is readily detached by rough handling. In many butterflies and moths the scales are gorgeously coloured and arranged in symmetrical patterns. The name Lepidoptera,[32] which is applied to this order of insects, was suggested by the scaly covering of the wings. When the butterfly is at rest the wings are either fully expanded horizontally, or are held vertically over the back, the upper surfaces of the fore wings being in contact. The first segment of the thorax, which bears the first of the three pairs of legs, is greatly reduced; the third segment also is but small.