THE PRODUCTION OF SILK

Caterpillars have the remarkable power of producing silk. Silk is a viscous fluid secreted by long glands, which are near the back of the caterpillar, and communicate with a little, tubelike organ near the jaws, called the spinneret, through which the silk is voided, instantly becoming, on contact with the air, a tough elastic fiber. Out of the silk thus secreted the caterpillars of butterflies spin threads, which they lay along the leaves and branches to guide themselves from place to place. From the silk many species weave little shelters, or tents, in which they are protected through the cold of winter. From the same delicate material they fashion the little knobs, buttons, and girdles by which the chrysalids are supported. The larvæ of butterflies do not spin cocoons: this is done only by the caterpillars of moths.

Fig. 15

A BUTTERFLY SET AND MOUNTED FOR DRYING

Caterpillars, as they develop, shed their skins a number of times. When the little caterpillar has “grown too big for its breeches” it anchors itself by a few threads to a fixed spot, the skin splits along the back, and, being securely tied in place, remains fast, while the caterpillar crawls out of it. The larvæ begins then to feed and grow again, but often treats the shed skin as it treated the shell of the egg, using it as a sort of “first course” before resuming the more substantial vegetable diet.

THE BUCKEYE

ORANGE-SKIRTED CALICO

GREAT SPANGLED FRITILLARY

After the caterpillar has molted four or five times it is transformed into a chrysalis, reverting to a stationary condition, fixed as immovably as it was fixed when it was only an egg. The process of transformation is wonderful, and well repays attention. Among butterflies there are three kinds of chrysalids,—those which are pendant from a knob of silk, those which are supported by girdles as well as by a silken knob, and those which are free and lie loose between leaves and rubbish, stitched together with a few strands of silk. The chrysalids of the “brush-footed butterflies” (Nymphalidæ) are always pendant; those of the other families are cinctured, or provided with girdles, except the “skippers” (Hesperiidæ), the chrysalids of which are free, and often are found on the ground, like the chrysalids of moths.

Figure 5 shows a caterpillar of the Monarch or Milkweed butterfly undergoing the change into a chrysalis. There comes a critical moment when the creature has wriggled itself nearly out of its skin, and when the only thing to keep it hanging in its place is a fold of this skin caught, as shown at c, between two segments or rings of the abdomen. Thus suspended, it feels about with the cremaster, as the spine at the end of its tail is called, which is full of minute curved hooklets at its end. As soon as the creature feels these hooklets securely gripping into the silk of the button above, it straightens out, and lets go its hold upon the old skin and assumes the form given in Figure 6, which gives the outline of the perfect chrysalis of this species,—a truly beautiful object, pale, pearly green in color, adorned with spots of burnished gold. Figure 7 shows the cinctured chrysalis of the Pipevine Swallowtail. After sufficient time has elapsed to permit of certain developments which take place in the chrysalis, the butterfly emerges. The thing, which has slept as if in a coffin, comes forth on airy wings to disport itself among the flowers. Little wonder that poets have seen in this transformation an emblem of the Resurrection!

Some of the butterflies of the United States belong to genera which are not confined to this country, but which occur also in the Eastern Hemisphere. Indeed, some few species are identically the same as are found in Europe and Asia. The Red Admiral (Vanessa atalanta) and the Mourning Cloak (Vanessa antiopa) are as familiar to English and German schoolboys as they are to boys in America. The Painted Lady (Pyrameis cardui), known also as the Thistle butterfly, is a cosmopolitan, and occurs all over the world, except perhaps in the hot jungles of the Kongo and the Amazons. The Mourning Cloak hibernates as a butterfly. In February, 1915, one of the guards in the Carnegie Museum found a specimen of this butterfly which had flown into the building. The day had been mild, and it had ventured forth from its hiding place under the eaves, or in a hollow tree. These butterflies may be found early in spring, as soon as the sap begins to flow, congregating in the sugar camps and sipping the drip of the maple trees. Comparatively few butterflies pass the winter in the winged form, but undergo its rigors as chrysalids or as larvæ.

Fig. 16

VIVARIUM

A breeding cage in which caterpillars may be reared until the butterflies are produced