A black swallowtail, newly emerged from its chrysalis. Courtesy photo, Glenn Cushman

Eclosure is happening all around us, but few people notice.

Even SpellCheck tries to make this entomological term into “enclosure.” In fact, it’s from the French word “éclore” meaning “to hatch” and is used for that magical moment when a butterfly emerges from its chrysalis.

A monarch chrysalis resembles a jade jewel with a spatter of gold. Courtesy photo, Steven R. Jones

After bursting out of a tiny egg, a caterpillar (“cat” to the cognoscenti) munches on its host plant like a ravenous teenager. Different species depend on different host plants though many are eclectic in their choice. Black swallowtail cats will dine on dill, fennel, parsley, and more. Other caterpillars will eat only one host plant. The classic example is the monarch which must have milkweed and only milkweed to survive.

When the caterpillar gets too big for its skin, it molts, shedding its exoskeleton — a process that happens up to five times for some species. At the final stage or “instar” the caterpillar pupates (forms a chrysalis.) Although the method of attaching themselves to a stem or twig varies, most caterpillars make a silken pad and hold themselves on with a silken thread.

Their outer skin hardens into a jewel-like chrysalis. Inside the chrysalis, the caterpillar’s body turns into a goo that gradually differentiates into cells that form a butterfly. In about two weeks eclosure happens, and the adult emerges.

Unfortunately, caterpillars are vulnerable to predation by wasps, especially the non-native European paper wasps, and other insects. These predators lay their eggs inside the caterpillar. When the eggs hatch, the growing wasp larvae dine on the still-living caterpillar, eating it from the inside out. Lepidopterist Jan Chu recommends bringing garden caterpillars indoors to raise in netted cages away from predators. If you do this, provide plenty of their preferred food and be sure there is enough space for the butterfly to move its wings on emergence.

This monarch butterfly eclosed a few years ago in October, perched on Steve’€s hand for an hour to warm up, and then took off for Michoacan. Courtesy photo, Stephen R. Jones

Several years ago, Ruth Carol saw a magpie devouring black swallowtail caterpillars and dashed out to rescue the remaining ones, carefully placing them on a large bouquet of dill. She was delighted with her dining room centerpiece until suppertime when she discovered that caterpillars are prodigious projectile poopers.

The process of raising and releasing a butterfly can be fraught with anxiety. Several hours may elapse between eclosure and flight, and the hovering human is sure the newly emerged butterfly will not make it. The butterfly creeps around, occasionally pumping its wings. Then it may become quiescent and appear dead. We hold our breath as it lets go a thin stream of waste and feebly moves it legs. Eventually the wing pumping becomes more frequent. And, suddenly, the butterfly takes to the air.

To find caterpillars, check their host plant and look for chewed-on leaves. We’ve been dismayed this year to see masses of milkweed with little damage as we normally find a variety of colorful beetles, milkweed bugs, and, occasionally, a monarch caterpillar.

Beautiful butterfly videos are available on youtube. Good sources for identification and to learn about different species are and . For photos and information on local species, see Butterflies of the Colorado Front Range by Janet Chu and Stephen Jones.

This monarch butterfly eclosed a few years ago in October, perched on Steve’s hand for an hour to warm up, and then took off for Michoacan. Courtesy photo, Stephen R. Jones

Other August Events

  • Dickcissels — like subdued versions of meadowlarks with less melodious voices — are usually uncommon but not this year. Look for them in moist meadows where you may also see bobolinks and snipe.
  • Black witches may cling to your screen doors or invade your homes. Actually, they are beautiful and uncommon moths that have become fairly numerous in the past few weeks.
  • The tundra turns autumnal by mid-month. Look for crimson big-rooted spring beauties in the talus where pikas are harvesting forbs.

Ruth Carol Cushman and Stephen Jones are authors of “Wild Boulder County” and “The North American Prairie.”

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