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Hometown Field Guidesby Garden with Lilly

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Plate 2

The caterpillar, inside

Cut away along one side, like a museum model. Everything in here is busy turning milkweed into caterpillar.

Draft, awaiting expert review
Watercolor cutaway of a monarch caterpillar opened along one side: a large leafy-green stomach filling the middle, a narrower food pipe in front of it and a hindgut behind it holding round droppings, a long pale heart tube along the back, a beaded nerve cord along the belly, one looping white silk gland, creamy fat tissue, silvery branching air tubes rising from the breathing holes, and one small pale wing bud near the front.
  1. Food pipeChewed leaf bits slide down a short food pipe from the mouth into the stomach.
  2. StomachThe stomach is the biggest thing inside. It fills most of the body and digests leaves almost nonstop.
  3. HindgutThe last part of the tube takes back water and packs the leftovers into little poop pellets called frass.
  4. Silk gland (one of a pair)A long tube that makes liquid silk. Its twin is on the other side. The silk becomes thread as the caterpillar pulls it out of its spinneret.
  5. HeartThe heart is a long tube along the back. It squeezes blood forward toward the head, and the blood seeps back.
  6. Belly nerve cordA nerve cord runs along the belly like a string of beads. Ours runs along our back instead.
  7. Fat bodyCreamy packing tissue stores food energy. It's the lunchbox for the chrysalis, which can't eat.
  8. Air tubesSilvery air tubes branch from the breathing holes to every part of the body, bringing air straight in.
  9. Wing budsTiny flat pockets of cells hide inside the caterpillar. They are the start of butterfly wings.

Also inside, not pictured

  • Insect blood isn't kept in veins, and it doesn't carry air. It sloshes around the organs while the air tubes do the breathing.
  • Thin "cleaning tubes" pull waste out of the blood and pass it into the gut.
  • A small brain sits in the head, above the food pipe.
  • Other tiny groups of starter cells wait to build the butterfly's eyes, antennae and legs.

Sources: Oberhauser & Kuda (1997); USDA Forest Service, ch. 3; NC State University, General Entomology; Monarch Joint Venture.

Draft, awaiting expert review. Drawn with AI assistance (ChatGPT) from a written anatomy spec and checked against it. A lepidopterist or horticulturist reviews every plate that identifies a living thing before the guide ships.

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