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Insect Growth


By Ranger Steve Mueller

Kids change as they grow. Some change more than others. 

Monarch butterflies are worm-like creatures when they’re kids but as adults they have bright orange wings with black veins that serve as rigid support between a thin, scale-covered membrane that allows them lift for flight. Its growth process is referred to as complete metamorphosis with an egg, larva, chrysalis, and adult form. Milkweed bugs that feed on the same milkweed plant as monarchs, appear much like adults with the exception that adults develop wings that lay flat on their backs. The youth are called nymphs and have gradual metamorphosis from egg to nymph to adult instead of egg, larvae, pupa or chrysalis, to adult. 

When an aphid comes out of the egg it looks like a tiny version of the adult like the milkweed bug does. The aphid has a sucking mouth part, six legs, oval body, and two tiny spike-like projections rising from its rear end. Its eyes and antennae are like those of the adult’s. It sucks juices from a plant and grows, it will shed its outer skeleton that protects fragile inner body parts as it grows. A new soft skeleton forms inside the more rigid outer one. The rigid one splits allowing the insect to escape the old and the new soft one will expand with air before it hardens allowing room for internal growth. The shedding of exoskeletons will continue as the aphid grows to adulthood when it can reproduce. Gradual metamorphosis results with kids looking similar to adults.

Wooly aphids grow in a similar manner but secrete a waxy white covering over their bodies that hides their appearance. They live in masses like the more typical aphids you might see on plants in the garden or even on plants in your home. Many people have learned to use mild soap water to remove aphids from leaves if they are abundant enough to cause leaf wilting. Soap is a better alternative than insecticides. 

The woolly aphids are a chosen food for harvester butterfly caterpillars. The small butterfly is about the size of a nickel when wings are folded over its back and it lays eggs on alder shrubs where the wooly aphids live. When the egg hatches, the caterpillar crawls among aphids covering its body with the waxy secretion made by aphids to hide among its juicy prey. It has three tiny pairs of legs at its front end and fleshy prolegs farther back on its body that are not true legs. They are fleshy projects that help it move its worm-like body among the aphids it eats. Fluid is pumped in and out of the bulbous prolegs to make them function like legs. 

The harvester has a leathery exoskeleton that will split allowing the insect to crawl out when it gets too tight. The new one will expand from internal air pressure before hardening. When ready to transform to the winged reproductive adult, its growth process changes dramatically from that of an aphid or milkweed bug. It will form a chrysalis under its final caterpillar leathery skeleton and wiggle out of the youthful protective covering.

There it transforms to the winged adult. When the inner body transforms from a caterpillar, it will emerge from the chrysalis with tiny soft wings and a large liquid filled body. Fluid from the body is pumped into hollow veins of the wings causing them to expand like flat plates. The fluid hardens in the veins providing wing support needed for the butterfly to fly through its neighborhood to mate and lay eggs for new generations to continue.

The aphid and milkweed bug have gradual metamorphosis where the youth look much like the adult. The growth of insects that transform with a dramatically different appearance like bees, butterflies, and beetles is complete metamorphosis. A third type of metamorphosis is called incomplete and is seen in insects like the dragonflies where young are dramatically different from the adults, are called naiads, and lack a pupal stage.

Dragonfly naiads live in the water with a body form significantly different from flying adults. The kids breathing apparatus and mouth parts are uniquely adapted to different habitat nature niches than the air flying adults. The naiads are active in streams during winter. Incomplete and gradual metamorphosis are sometimes lumped together with only having egg, nymph/naiad, adult stages. They are different enough to be considered separate forms of metamorphosis. Insects with gradual metamorphosis live in a similar manner to adults as opposed to the greatly different life style of those like dragonfly kids and adults with incomplete metamorphosis. Insect kids grow with gradual, incomplete, or complete metamorphosis. 

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.

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