Insects: life cycle

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Insects have parents. It seems odd to think of them that way, but a few species even have mothers who nurture them after they’re “born.”

An insect’s spark of life begins with a male and a female who meet, court and copulate. Mate-seeking behavior occurs at certain periods of the day or night, interspersed with non-mate-seeking behavior, like resting and feeding.

Mating can occur between insects on land, in the water or in the air, depending on the species. Most insects either mate “dog-style” or end-to-end. But there are many, and bizarre, variations.

Bedbugs and damsel bugs, for example, take copulation to a violent extreme — the male pierces the female’s body with needle-like genitalia and ejaculates directly into her body cavity, a process called (not surprisingly) “traumatic insemination.”

The table gets turned rather tragically on Praying Mantis males, because sometimes directly after mating, a hungry female will chomp off her suitor’s head and gobble him right up. Bedbugs aren’t the only insects with unusual penises: Some insects have a brush on the tip or one that’s football-shaped. The earwig has two penises and can choose which one he wants to use. Springtails inseminate secondhand. The males string packets of sperm in a circle around the female and she collects one into her genital opening, thereby fertilizing herself. The world of insect sex is a strange one, indeed.

Entomologists refer to the reproductive organs of insects collectively as genitalia. The female organs include two ovaries and a bursa copulatrix, the insect equivalent of a vagina. The male organs include two testes and an aedeagus (penis). Males and females both have glands and sex-appropriate structures for egg storage or sperm storage and the like. The male also has “claspers,” which he uses to hold on to the female during copulation.

Assassin Bug egg case.

Assassin Bug egg case. (WW)

Locating partners and courting between the sexes may involve one or more of several methods, including sight, sound, stroking, gifts, flight displays, or, as the fireflies do, flashing, and pheromones. (A pheromone is a chemical produced naturally in the body of insects and other animals. It induces a certain behavioral response in others of the same species. There are many different kinds of pheromones. Some, for instance, trigger alarm or mark territorial boundaries. (Humans produce pheromones, too.)

Most females deposit eggs in their chosen environment through an ovipositor, a tube-like organ that projects from the end of their abdomen. There are some species who give birth to live young and a few others who reproduce asexually.

Insects go through one of two different life cycles: Complete metamorphosis or incomplete metamorphosis.

Complete metamorphosis
The vast majority of insects go through “complete metamorphosis” (technically holometabolism), a four-stage progression: Egg to larva to pupa to adult. Here’s how it happens:

Egg stage
An insect hatches out of an egg laid by the mother on a “host” plant or other favorable environment. The environment she chose for the eggs usually offers an immediate food source for the young insect, called a larva (plural: larvae; LAR-vee). When first laid, the egg may be no larger than the period at the end of this sentence. Some females lay hundreds of eggs during their adult life, nature’s way of ensuring that at least some offspring will survive. Some species lay their eggs singly and others lay their eggs in cases containing dozens of larvae. Eggs may be laid in or on plants, in or on the soil, in yard debris, in water, and just about anywhere else that offers access to food for the larvae.

Larva stage
At this stage, an insect is still in his immature form and looks very different from his parents. Technically called “larva,” we know some insect larva by other names — butterfly and moth larvae are often called caterpillars and fly larvae are maggots. A larva doesn’t mate or reproduce. His sole job is to eat and grow, to take in nutrients for the dramatic stages to come. As his size increases we may notice him, but what’s more likely to catch our attention are the results of his voracious appetite, particularly if he’s an herbivore: Holes in leaves and tree trunks, skeletonized leaves, leaves with edges gone missing, flowers literally nipped in the bud, and fruits and veggies with bites taken out.

Firefly larva

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Firefly larva. (WW)

Not all larvae are herbivores. Many are ferocious predators. Ladybird beetle larvae are an example. They look like tiny dragons and they’re merciless hunting machines, eating just about anything small enough for them to overpower. Most larvae who go through complete metamorphosis change dramatically in their appearance as adults.

Larvae come in all sizes, with some growing very large. One example is Tobacco Hornworms (shown at the top of this page) who grow fat and up to 4 inches long. Miniscule at first, they eventually become easy to spot as they grow larger from munching on tomato plants. Tobacco Hornworms are sphinx moth that are so large as adults, they’re often mistaken for hummingbirds fluttering among flowers.

Most insect larvae are worm-like in shape. Fly larvae are those white, wormy, squirmy things called maggots. Beetle larvae are usually whitish and commonly called grubs, or brownish and called wireworms. Some larvae are parasites and there are others, like mosquito larvae, who live in water. Most larvae have legs, but some do not.

A larva’s skin can stretch only so far as he grows, so he sheds it several times, a process called molting. Since nothing goes unnamed, there’s a name for the stage between molts: instar. When a larva hatches he is a “first instar.” After he molts for the first time he’s a second instar, and after the second molting he’s third instar, and so on. There may be anywhere between six and eleven molts, depending on the species.

Pupa
After feeding for generally about two weeks the larva enters the pupa (pYOU-puh) stage, and is said to be “pupating.” This is the time when he constructs a chrysalis (cocoon) around himself and stays within it completely closed up. He doesn’t eat. He appears to be motionless, but within the chrysalis (KRIS-uh-lus) a great transformation is taking place. Embryonic cells within him begin to develop into adult features — jointed legs, mouthparts, eyes, wings, sex organs, antennae and all the rest. Depending on the species, a larva may remain in this stage from a few days to months to years.

The form and color of the chrysalis varies depending on the species of insect. Some species spin a silk chrysalis. There are others who make a chrysalis of their own sticky saliva mixed with mud, sand, dead leaves or wood. Or a chrysalis may be a cell in a nest full of cells constructed by the parent. A chrysalis may be buried or hanging from or within foliage. It may be loose or tight, soft or hard, or have many layers.

It’s a dramatic moment when an insect splits through his chrysalis: A full-sized adult emerges, ready to feed, mate and complete the last stage of his life. All his growing time is finished. When we see a tiny fly, he isn’t a young one still growing, he’s a full-grown member of a tiny fly species. Although he will probably eat (there are some species who don’t feed at all during their adult life), it will be solely to take in sufficient nutrients for mating and, for females, egg production. The adult stage is usually only a few weeks, but ranges up to a year or more.

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Composite image of Monarch adult emerging from chrysalis. Go here to watch a video of a Monarch’s “birth.” (Cathy Keifer)

Incomplete metamorphosis
About 12 percent of insects go through incomplete metamorphosis (hemimetabolism). This form of development has three stages: egg, nymph and adult.

Hemimetabolous insects don’t pupate. Called a nymph or naiad (NAY-ad), as in the case of aquatic species, like dragonflies, the hatchlings look a little bit like their parents from the moment they hatch. They lack wings at this stage, however. Most eat the same foods as their parents. As they eat and grow they molt several times to accommodate larger bodies. Most nymphs molt four to eight times. With their last molt they reach their adult size. At this time, they usually change color and body shape, to some extent. Nymphs don’t mate. It is only as adults that they mate and reproduce.

*Top photo: The Tobacco Hornworm caterpillar (Manduca sexta) grows to be very large, but he’s harmless, as this youngster demonstrates. (© Nathalie Fisher)

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