Those small, silky, oval objects, of yellowish colour,
frequently found in groups on walls and palings, are the cocoons of these useful little flies, spun round about and over the remains of the dead caterpillar their victim. "These," as Mr. Westwood observes, "ignorant persons mistake for the eggs of the caterpillar, and destroy; thus foolishly killing their benefactors."
Happily these devastating caterpillars have plenty of enemies to prevent their continued multiplication, and to reduce their number speedily when it exceeds certain limits. Besides the ichneumons, mentioned above, the feathered tribes do much towards keeping them down. Mr. Haworth, in his "Lepidoptera Britannica," says, with reference to this: "Small birds destroy incredible numbers of them as food, and should be encouraged. I once observed a titmouse (Parus major) take five or six large ones to its nest in a very few minutes. In enclosed gardens sea-gulls, with their wings cut, are of infinite service. I had one eight years, which was at last killed by accident, that lived entirely all the while upon the insects, slugs, and worms which he found in the garden."
The pretty egg of this butterfly is figured on Plate II. fig. 1: it may be found commonly enough, with a little searching, on cabbage-leaves, either at the end of May or beginning of August.
The caterpillar, which, besides cabbages, consumes various other cruciferous plants,—also Tropæolums, or, as they are erroneously called, "Nasturtiums,"—is green,
shaded with yellow on each side, and covered with black points, on each of which is situated a hair.
By way of compensation for the damage it inflicts, it has been suggested that a durable green dye might be extracted from the caterpillars of cabbage butterflies, since it is extremely difficult to eradicate the stain made by a crushed caterpillar on linen. If this strange and novel dye should ever take its place among the vagaries of fashion, the shopkeepers could find a familiar French name, as the word chenille, applied to another commodity, means simply "caterpillar," so "chenille green" would be the phrase for the colour afforded by smashed caterpillars.
The chrysalis ([Plate I]. fig. 15) may be found almost anywhere, laid up under ledges of garden walls, doorway, or any convenient projection, not too far from the creature's food. Wanting an individual just now, to sit for his portrait, I had only to step out of my door, and within a hundred yards espied a candidate for the distinction, ready to hand, under the coping-stone of a gate-post.
A female specimen of the butterfly is figured on Plate IV. fig. 2. The male may be readily distinguished by the absence of the black spots and dashes on the upper side of the front wings.
The winged insect may be seen throughout the warm season from April to August.