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Why do adult insects have 6 legs?

Why do adult insects have 6 legs?


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Apparently, there is an advantage to having 6 legs in the insect world. What is that advantage, if anything? Why would such an advantage exist for insects, but not for other, larger land animals?

What do those middle legs do that the front and hind legs cannot, if anything? Are they really essential, or do they just generate more muscle power?


Short answer
Six legs allow for locomotion, while maintaining a supportive tripod at all times.

Background
There are several million species of insects, all on 6 legs. This implies that any change in this number is promptly selected against. It is generally agreed that insects were derived from many-legged ancestors, e.g. centipedes.

One explanation is the tripod gait that results from having six appendages. This hypothesis, formulated more than 6 decades ago (Lanham, 1951), reasons that a reduction of the number of legs during evolution did not go further than 6, because locomotion of a small animal encased in a rigid exoskeleton is not effective with less than 3 pairs of legs. Insects generally walk by lifting the two outer legs on one side and the middle on the other side, sweeping them forward and placing them down together. Hence, insects support their rigid structures with a tripod at all times. Tripods are among the most stable configurations, and they never wobble (why on earth do tables have 4 legs?). Figure 1 shows an illustration of insect locomotion (Lanham, 1951).


Fig. 1. Insects' locomotion resembles a double tripod. Insects have a cyclic gait which consists of two phases, the stance phase and the swing phase. The stance phase is the power stroke, it pushes the body forwards in the direction of travel while the leg remains on the ground. Three legs are used is this phase by forming a tripod with the front leg and the hind leg on one side of the body and the middle leg on the other side. This formation is why this gait is known as the tripod gait. Source: Insect robotics.

Larger animals can afford to have less legs, because their vestibular systems have more time to maintain balance and adjust gait during locomotion. Because insects are so small, their strides are so quick that the nervous system may not be fast enough to deal with gait control. Instead, insects rely on the tripod gait to prevent any imbalance, rather than adjusting it. In addition, the exoskeleton effectively restricts small bodily movements to control balance. Larger animals such as mammals make small adjustments in their gait constantly to maintain balance. An insect has less opportunities to do so, because of their rigid exoskeleton (Lanham, 1951).

Reference
- Lanham, Science (1951); 113(2946): 663


Apparently, there is an advantage to having 6 legs in the insect world. What is that advantage, if anything? Why would such an advantage exist for insects, but not for other, larger land animals?

"Legs" is a tricky term, with insects. Colloquially, in the terrestrial world, they are things that an organism walks on. But the scientific definition of 'legs', for insects, relies on homology: many insects have six 'legs' because they all descend from a common ancestor, which had six legs. The anatomical structures derived from those legs, regardless of whether they are functionally used as paddles, or as claws, or as feeding appendages or elaborate mate-signalling devices, all get called 'legs'. Referring to all of these as 'legs' makes as much sense, from a functional perspective, as referring to our arms as 'legs'. We only do it to keep the homology clear when talking about evolutionary relationships between species.

The diversity of uses to which insects put their legs makes it clear that there is no single universal advantage of having six - it is just that completely losing (or gaining) appendages is a difficult evolutionary process, so it seldom happens over evolutionary time. Regardless, some groups of insects (members of the Coccidae and Diaspididae, for instance) have lost their legs completely - an apparent adaptation to their obligate parasitic lifestyle.

What do those middle legs do that the front and hind legs cannot, if anything? Are they really essential, or do they just generate more muscle power? If it's an advantage either way, why don't bigger life forms like mammals have 6 limbs?

In the insect groups which use six legs for locomotion, AliceD's answer provides an excellent description of how all six legs are used. However, there really is nothing special which can be achieved with six legs, which could not be achieved with some other number of legs: mantises handle their locomotion perfectly fine on just four of their six legs, and arachnids (not insects, I know - but with many of the same design challenges such as a rigid exoskeleton) manage equally fine on eight.


Why do adult insects have 6 legs?

Based on current literature, there is no advantage to an insect having six "legs". This is simply how the organisms evolved. Having six legs stayed because there were no selective pressure to alter the number of "legs".

Harvard Scholar Teaching Anatomy, Statistics, & Languages

Not all do! Some have four. Some have six but only use four regularly for locomotion.

The likely answer to your question is that six legs is not better or worse than four legs (depending on the type of locomotion) but instead that both are close enough to the optimal functional performance level that there's no selective pressure to have one or the other. In other words, having some legs helps dramatically with terrestrial locomotion, but there's no extra fitness cost associated with straying one or two legs away from that optimal number. It's much more likely that the first insects had six legs and so all their descendants have six legs, and that the first tetrapods had four limbs and so all their descendants have four limbs. Obviously, some lineages continued to diversify and adapt after this point in time, which is why you see variation today in both leg and limb number within these groups.

Evolution typically doesn't select for optimal but instead removes "not good enough" traits. Instead of asking "Why?", ask "Why not?"


Why can’t bugs be grub?

Billions of people around the world regularly eat bugs. Why do so many Westerners find the idea disgusting?

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November 19, 2018 at 6:45 am

One Friday morning in May, 11-year-old Sarah Nihan went to school and did something she had never done before. She pulled a dry-roasted cricket out of a bowl and carefully lifted it to her mouth. “At first I was a little iffy,” Sarah admits. “I made the mistake of looking it in the eyes.”

At the time, Sarah was a fifth grader at Ellis School in Fremont, N.H. Before her language-arts class held its bug buffet, the students had learned all about the benefits of eating insects. Packed with protein and vitamins, insects are quite nutritious. And raising them takes far less land and water than raising traditional livestock, such as cattle. So as a food source, insects are better for the planet.

Fifth and sixth graders in New Hampshire held a classroom Bug Buffet last spring. Anyone who couldn’t stomach these dry-roasted insects (or insect-containing snacks such as cricket chips or cricket pancakes) could opt to instead eat gummy worms. Robin Lee

The kids wrote essays on the environmental and health benefits of eating bugs, or entomophagy (En-tuh-MAH-fuh-jee). They read a book about a student who ate a stink bug as defense against a bully. They watched videos of Asian people relishing tarantula burgers. Yet Sarah still had to brace herself and count to three before popping that bacon-and-cheese-flavored cricket into her mouth. “I told myself that I’m not going to lose to a bug,” she says. But after chewing a few seconds, she cringed.

She’s not alone. To most North Americans and Europeans, the thought of eating insects triggers the same reaction: Ewwww.

This isn’t how people react to all foods they dislike. For example, people who dislike asparagus usually don’t say it’s disgusting. “They just say it tastes bad,” points out Paul Rozin. “But they’d say goat intestine is disgusting.” We seem to save our revulsion for certain animal products.

Rozin is a psychologist at the University of Pennsylvania in Philadelphia. He’s spent decades studying how some foods have become taboo. He and other researchers are trying to learn where this disgust comes from — and whether it can be unlearned.


Eating your first bug isn’t always easy, as participants in the Bug Buffet learned.
Curriculum with a Cause/Facebook

Geography matters

Insects aren’t gross to everyone. Indeed, some two billion people around the world savor them on a regular basis.

Most Westerners — people who live in North America and Western Europe — don’t eat insects. But the Western diet includes a number of foods that can seem just as gross when you stop to think about them. Cheeses are made with mold and bacteria. Escargot, a dish eaten in France and other countries, consists of cooked snails. Shrimp and lobsters look kind of like giant bugs. (In fact, they’re arthropods, the same group of animals that includes insects and spiders.)

So why do Westerners shun ants, grasshoppers and other creepy-crawlies? That question piqued the interest of Julie Lesnik. She’s an anthropologist at Wayne State University in Detroit, Mich. There, she studies how the human diet has evolved.

Even though many North Americans find the idea of eating bugs gross, much of the world enjoys snacking on insects. But bugs are far from the only animals many diners shun. Here, fried scorpions are sold as a street snack in Beijing, China. weiXx/istockphoto

Lesnik has always been a picky eater. She never intended to study edible insects, let alone eat them. But while doing research in South Africa, she found evidence that primate ancestors of early humans used bone tools to dig into termite mounds. That suggested ancient humans ate insects. So when and why did Westerners quit eating bugs?

Some researchers think hunting for insects became less popular as ancient people found easier food sources in farming. If the land could support crops and cattle, why go after tiny scattered bugs with fewer calories?

Others explain the puzzle by looking at climate. Tropical countries get plenty of sun. That produces thicker vegetation, bigger insects and more kinds of them. People have better odds of finding an insect they like when they have lots to choose from year-round, Lesnik says. But farther north, where the seasons change, insects aren’t available during winter months.

Both ideas make sense. The earliest Europeans lived 18,000 to 22,000 years ago. That was during a period called the Last Glacial Maximum. Ice covered much of North America and northern Europe. To survive, people had to hunt deer and other large game. There wouldn’t have been many big, juicy bugs around. Could it be that insect-eating habits depend on where people live?

To test her idea, Lesnik gathered data on various factors that might affect whether cultures eat insects.

One such factor was agriculture. It’s likely that ancient hunter-gatherers chowed on insects. But people who raise animals and grow crops probably came to view insects as pests. That could make bugs less appealing.

Yet when Lesnik looked at a current map of insect-eating countries, she saw that agriculture was common in many of them. She also gathered data on the share of land in each country that’s good for farming. If agriculture were a key factor in insect eating, she’d expect people in farmable regions to eat fewer bugs. But that wasn’t the case.

She considered other explanations. For example, maybe the people who eat insects live in countries that are poor. Or maybe they don’t have enough farmed food to go around. If those theories were right, Lesnik would expect more insect eaters to be found in countries with crowded conditions or in low-income nations. Experts describe that last group as having a low gross domestic product, or GDP. (GDP is a way to measure the health and wealth of a nation’s economy.) However, Lesnik found no link between insect-eating and either GDP or population density. So insects aren’t just a fallback food for desperate people. Lesnik published her analysis last year in the American Journal of Human Biology.

Researchers have found that latitude — how far north or south of the equator you are — is the biggest predictor of who eats insects. People in warmer regions eat more of them. Yde Jongema

As it turns out, “Where you are in the world is the number one predictor of who’s going to be eating insects,” Lesnik says. Latitude is how far north or south you are from the equator. And in eight out of every 10 people, latitude alone predicts the likelihood that they’ll eat insects. Warmer parts of the world, Lesnik says, just have more bug-eating.

On the practical side, geography explains why early Westerners didn’t eat insects. But it doesn’t explain the emotional part — the disgust. And that disgust has not only persisted in Western culture but also crossed borders.

Yuck factor

Lesnik thinks Westerners’ “yuck” reaction to bugs came with travel.

As early Europeans began traveling farther, they met other cultures. In 1493, a member of Christopher Columbus’ expedition to the Caribbean wrote about what he saw: “They eat all the snakes, the lizards, and spiders, and worms, that they find upon the ground so that, to my fancy, their bestiality is greater than that of any beast upon the face of the earth.” In other words, he was comparing the people in the New World to animals.

Writings like this show that Europeans “considered the people they encountered beastlike because they ate insects,” Lesnik says. As Westerners colonized other cultures, they needed to make themselves feel superior to those cultures, she says. She suspects that this need strengthened Western disgust toward eating insects.

Disgust also can be learned by various messages shared within a culture, says Lesnik. We aren’t necessarily born thinking that insects are gross. “If a kid tries to put a bug in his mouth, many parents discourage that behavior and tell the kid it’s icky,” she observes.

Today, Europeans and North Americans aren’t the only ones who see eating insects as gross. The disgust is spreading to people in low-income nations who had been used to eating insects.

Arnold van Huis is a tropical entomologist at Wageningen University in the Netherlands. He noticed this shift in attitude while conducting a grasshopper study in the West African country of Niger. After adopting a Western lifestyle, “The people say, ‘We have a certain standard of living now, and we don’t eat insects anymore,’” van Huis reports. “They go for the hamburger instead of the nice grasshoppers.”

Changing minds

Can disgust be unlearned? For some people, education does the trick.

Six years ago, Robert Nathan Allen, a recent college graduate, was working as a bartender in Austin, Texas. At some point, his mom shared a video about edible insects. “She sent it as a joke — said it seemed like something wacky my dad and I would try,” Allen recalls. The video explained how bugs are good for us and good for the Earth, just as Sarah and her classmates had learned in school. “I thought this was just incredible,” Allen says.

People in the United States can buy buggy snacks including cricket-flour chips, cricket-protein bars and sour-cream-and-onion “Crick-ettes.” Robin Lee

He looked around for insect foods. He found the occasional bag of candied ants or chocolate-covered grasshoppers for sale. But there wasn’t much else available in the United States.

He started calling insect researchers on the phone. “I’ve got a bar in Austin and I want to serve bugs,” he would say. “What should I do?” Some people hung up. Others laughed him off the phone. But finally a professor confided that he cooks up a batch of bugs and brings them to school each year on the last day of class. “Everyone eats them. We all have a blast,” he said. “But please don’t tell anybody,” he implored of Allen, “because I don’t want the administration to make me stop.”

That phone call didn’t result in any new insect foods or recipes for Allen’s bar. But it did something bigger: It spurred Allen to action. “This professor was worried he’d be barred from serving a food that’s eaten by billions simply because it was stigmatized in our Western food culture,” Allen says. That made him realize there was “the need to educate the public and address the cultural taboo.”

Allen got in touch with researchers and business people who shared his goals. He discovered other campuses that host public insect-tasting events. Each year, some 30,000 people attend Purdue University’s Bug Bowl. And in February, Montana State University held its 30th annual Bug Buffet. This weeklong event features cook-off competitions, lectures and plenty of insect treats to sample.

In 2013, Allen founded an Austin nonprofit called Little Herds. The organization teaches the public about the benefits of edible insects, sometimes known as “mini livestock.”

Early on, the group set up tasting booths at local farmers’ markets. They gave talks at schools. They advertised at museums. Right away they realized their prime audience: children. Most parents wouldn’t dare reach for a roasted cricket before first sampling a nicer-looking food, such as a cookie made with cricket flour. But “little kids would just walk up and start chowing down on the crickets,” Allen says.

Fear of missing out — on bugs

Kids may be a somewhat easy sell when it comes to insects. That’s why some researchers have focused on adults. They’ve tried to figure out what traits make people likely to try insects. For a 2015 study, Rozin at the University of Pennsylvania and his colleagues gave 399 people from the United States or India an online survey about food. Participants saw pictures of cookies. Or breads baked with mealworm flour. And tacos or crepes containing whole grasshoppers. Then they asked the participants how willing they would be to sample those foods.

These researchers also asked the participants about their religion and politics. And they asked if participants agreed with statements such as: “eating bugs is disgusting,” “bugs are nutritious,” or “eating bugs puts you at risk for disease.” People also reported how willing they were to try new foods, how sensitive they were to disgust and how much they like risk and spontaneity.

Disgust was the most common reason people refuse to eat bugs, this study found. People who were most likely to try eating insects were those who weren’t easily grossed out, who didn’t mind unfamiliar foods and who liked new experiences (and telling others about them). Rozin’s team reported its findings, last year, in the Journal of Insects as Food and Feed.

Another team of researchers surveyed 368 meat eaters in Flanders, Belgium. According to their analysis, Westerners are most willing to replace meat with bugs if they’re young, male, open to new foods, environmentally conscious and already trying to eat less meat. Those 2014 findings were published in Food Quality and Preference.

Potential insect eaters may share another key trait: fear of missing out, often known as FOMO. In 2015, behavioral economists carried out a study to learn what types of messages might nudge people to try insects. At a shopping mall in England, the team lured shoppers to a table of dry-roasted crickets by posting three different signs. One described the health benefits of eating insects. A second tried to make eating insects seem normal. It showed a photo of family members at a restaurant and enjoying crickets. The third sign tapped into FOMO by showing a near-empty plate of roasted bugs with the plea, “Don’t miss your chance to try.”

The sign about health benefits did OK at attracting shoppers to the bug-foods table. The sign with the family photo did better. But the FOMO poster worked best.

It’s a marketer’s version of peer pressure. That tactic also seemed effective in the New Hampshire classroom. At the bug buffet, Sarah’s classmate Ruby Drake initially steered clear of insect foods. “I was just going to have the gummy worms,” she says. But after a friend begged her to try “one of the real bugs,” Drake picked up a roasted cricket.

Classroom questions

The taste test ended quickly. “It crumbled the minute I touched it, and that grossed me out,” Drake says. “I spit it out.”

But that crunchy critter wasn’t a deal breaker. Drake also tried cricket-flour chips. “Those were pretty good,” she says. “I would put them in my lunch box.”

As for the dry-roasted crickets, Nihan says she would eat them again. However, she adds, “I’d probably brush my teeth afterward because the legs can get stuck in your teeth.”

Power Words

agriculture The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

annual Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.

arthropod Any of numerous invertebrate animals of the phylum Arthropoda, including the insects, crustaceans, arachnids and myriapods, that are characterized by an exoskeleton made of a hard material called chitin and a segmented body to which jointed appendages are attached in pairs.

bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

bug The slang term for an insect.

calorie The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.

cattle Also known as bovines (because they’re members of the subfamily known as Bovinae), these are breeds of livestock raised as a source of milk and meat. Although the adult females are known as cows and the males as bulls, many people refer to them all, generally, as cows.

climate The weather conditions that typically exist in one area, in general, or over a long period.

colleague Someone who works with another a co-worker or team member.

crop (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers.

culture (n. in social science) The sum total of typical behaviors and social practices of a related group of people (such as a tribe or nation). Their culture includes their beliefs, values and the symbols that they accept and/or use. Culture is passed on from generation to generation through learning. Scientists once thought culture to be exclusive to humans. Now they recognize some other animals show signs of culture as well, including dolphins and primates.

density The measure of how condensed some object is, found by dividing its mass by its volume.

diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

economy Term for the combined wealth and resources (people, jobs, land, forests and minerals, for instance) of a nation or region. It is often measured in terms of jobs and income or in terms of the production and use of goods (such as products) and services (for instance, nursing or internet access). Experts who study these issues are known as economists.

edible Something that can be eaten safely.

entomology The scientific study of insects. One who does this is an entomologist.

entomophagy A term for the human practice of eating insects.

equator An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed.

expedition A journey (usually relatively long or over a great distance) that a group of people take for some defined purpose, such as to map a region’s plant life or to study the local microclimate.

factor Something that plays a role in a particular condition or event a contributor.

FOMO Urban slang for fear of missing out.

geography The study of Earth’s features and how the living and nonliving parts of the planet affect one another. Scientists who work in this field are known as geographers.

gross domestic product Abbreviated GDP, this term refers to the monetary value (for instance, the dollar value) of all of the goods that are made and services that are performed in one year by everyone living within a nation.

host (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents.

hunter-gatherer A cultural group that feeds itself through hunting, fishing and gathering wild produce (such as nuts, seeds, fruits, leaves, roots and other edible plant parts). They can be somewhat nomadic and do not rely on agriculture for their foods.

insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

latitude The distance from the equator measured in degrees (up to 90).

livestock Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.

lizard A type of reptile that typically walks on four legs, has a scaly body and a long tapering tail. Unlike most reptiles, lizards also typically have movable eyelids. Examples of lizards include the tuatara, chameleons, Komodo dragon, and Gila monster.

mealworm A wormlike larval form of darkling beetles. These insects are found throughout the world. The ever-hungry wormlike stage of this insect helps break down — decompose, or recycle — nutrients back into an ecosystem. These larvae also are commonly used as a food for pets and some lab animals, including chickens and fish.

Niger A French-speaking nation in the center of the horn of Africa, it established its independence from France in 1960. It is one of the world’s poorest countries and largely rural. It also has the highest fertility rate of any country in 2018, which helps explain why half of its population was under the age of 15.

online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

peer (noun) Someone who is an equal, based on age, education, status, training or some other features. (verb) To look into something, searching for details.

politics (adj. political) The activities of people charged with governing towns, states, nations or other groups of people. It can involve deliberations over whether to create or change laws, the setting of policies for governed communities, and attempts to resolve conflicts between people or groups that want to change rules or taxes or the interpretation of laws. The people who take on these tasks as a job (profession) are known as politicians.

population (in biology) A group of individuals from the same species that lives in the same area.

primate The order of mammals that includes humans, apes, monkeys and related animals (such as tarsiers, the Daubentonia and other lemurs).

protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

psychologist A scientist or mental-health professional who studies the human mind, especially in relation to actions and behaviors.

spider A type of arthropod with four pairs of legs that usually spin threads of silk that they can use to create webs or other structures.

survey (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.

taboo A term for some activity that is considered wholly inappropriate and/or forbidden within a particular religious or social group. Many times the idea of this practice is so off limits that people won’t even discuss it in public.

tactic An action or plan of action to accomplish a particular feat.

tarantula A hairy spider, some of which grow large enough to catch small lizards, frogs and birds.

taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).

termite An ant-like insect that lives in colonies, building nests underground, in trees or in human structures (like houses and apartment buildings). Most feed on wood.

trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

vegetation Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.

vitamin Any of a group of chemicals that are essential for normal growth and nutrition and are required in small quantities in the diet because either they cannot be made by the body or the body cannot easily make them in sufficient amounts to support health.

Western (n. the West) An adjective describing nations in Western Europe and North America (from Mexico northward). These nations tend to be fairly industrialized and to share generally similar lifestyles levels of economic development (incomes) and attitudes toward work, education, social issues and government.

Citations

Journal: A. van Huis. Did early humans consume insects? Journal of Insects as Food and Feed. Posted online September 25, 2017. doi: 10.3920/JIFF2017.x006.

Journal: M.B. Ruby et al. Determinants of willingness to eat insects in the USA and India. Journal of Insects as Food and Feed. Posted online August 17, 2015. doi: 10.3920/JIFF2015.0029.

Journal: J. Lesnik. Not just a fallback food: global patterns of insect consumption related to geography, not agriculture. American Journal of Human Biology. Posted online February 1, 2017. doi: 10.1002/ajhb.22976.

Journal: W. Verbeke. Profiling consumers who are ready to adopt insects as a meat substitute in a Western society. Food Quality and Preference, Vol. 39, January 2015, p. 147-155. doi: 10.1016/j.foodqual.2014.07.008.

About Esther Landhuis

Esther Landhuis is a freelance journalist in the San Francisco Bay Area. She worked on her high school newspaper and spent a decade studying biology before discovering a career that combines writing and science.

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Contents

Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax, and posterior abdomen. [6] [7] The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), [8] followed by six segments, all closely fused together, with the following appendages:

Segment I. None Segment II. Antennae (sensory), absent in Protura Segment III. None Segment IV. Mandibles (crushing jaws) Segment V. Maxillae (chewing jaws) Segment VI. Labium (lower lip)

The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). [9] In true insects (class Insecta) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. Similar appendages are found on the heads of Myriapoda and Crustacea, although these have secondary antennae. [10]

The thorax is composed of three segments, each of which bears a single pair of legs. [11] As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on the abdominal segments of some immature aquatic insects. [12] In most insects the second and third thoracic segments also support wings. [13] It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves. [14]

The abdomen follow epimorphic development, where all segments are already present at the end of embryonic development in all the hexapod groups except for Protura, which has an anamorphic development where the hatched juveniles has an incomplete complement of segments, and goes through a post-embryonic segment addition with each molting before the final adult number of segments is reached. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are twelve, and in Collembola only six (sometimes reduced to only four). [15] [16] The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. [17] [18] [19]

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. [20] These were then considered subclasses of a subphylum called Uniramia or Atelocerata. [21] In the first decade of the 21st century, however, this was called into question, and it appears the hexapoda's closest relatives may be the crustaceans. [22] [23] [24] [25]

The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha, [26] or as several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (springtails) [27] or the Protura. There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere. [28] [ better source needed ]

Molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago - coinciding with the appearance of vascular plants on land. [29]


Pest Has Six Legs, and .


American cockroach (1 3/8-2 1/8", 34-53 mm)


bumblebee (1/4-1", 6-25 mm)


moth (1 1/2-1 7/8", 38-46 mm)


house fly (1/8-1/4", 4-7.5 mm)


termite, winged reproductive (3/8", 10 mm)


carpenter ant, winged reproductive (1/8-1/2", 3.5-13mm). Photo: Jon Yuschock, Bugwood.org

These horizontal lines (the abdominal segments) will show if the pest has no wings.


bed bug (3/16", 4-5 mm), photo by Gary Alpert, Harvard University, Bugwood.org


carpenter ant (1/8-1/2", 3.5-13mm)


silverfish (1/2-3/4", 12-19 mm), photo by Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org


cat flea, (1/8", 2.5 mm),(1/8", 2.5 mm), photo by Pest and Diseases Image Library, Bugwood.org


springtail (1/32-1/8", 1-3 mm)


termite worker (1/8-1/4", 5-7 mm)


book louse (1/32-1/4", 1-6 mm)

Wings covered by a hard shell and divided by a vertical line:

Arrow marks division between the elyatra: hard, shell-like modified wings found in beetles.


carpet beetle (1/16-1/8", 1.8-3.2 mm), adult, photo by Natasha Wright, Florida Department of Agriculture and Consumer Services, Bugwood.org


ladybug (1/32-3/8", 0.8-10 mm), photo by Russ Ottens, University of Georgia, Bugwood.org


beetle in flight, photo by Erich G. Vallery, USDA Forest Service - SRS-4552, Bugwood.org

Very short and stubby wings:


earwig (1/4-1", 5-25 mm), photo by Gary Alpert, Harvard University, Bugwood.org

Wings appear in an X pattern:


boxelder bug (1/2", 11-14 mm), photo by Link Elmore, Bugwood.org. Overlaid X in photo on right shows how the wings make an X pattern.


Ants and termites are social insects, and only the queens and males ever have wings. Soldier and Worker ants and termites (all sterile females), never have wings. You are likely to see only worker ants and termites, although at times large swarms of winged ants and termites will gather when they are looking for mates.

Queen carpenter ant (.5-.7", 13-17 mm)

Queen termite (3/8", 10 mm)

Pavement ant workers worker ants do not have wings.

Photo: Sarah Vanek, Bugwood.org

Soldier and worker termites have no wings.


Examples of Arthropods

When you think of a stereotypical arthropod body, you probably think of an ant. Ants have hard exoskeletons and jointed legs. They also have bodies which are clearly segmented into a head, thorax, and abdomen.

Ants show one type of social organization that has been developed by arthropods. Ants, bees, and termites are all what is called “eusocial” organisms – organisms living in extreme degree of cooperation, with “colonies” that almost operate like a single organism themselves.

Most arthropod species are not eusocial, but eusocial colony life is one of the fascinating roads that arthropod evolution has taken.

Spiders

Spiders are also arthropods, possessed of hard exoskeletons, segmented bodies, and jointed limbs.

Spiders typically eat smaller arthropods, such as gnats and flies – though they will eat any living thing they can catch, and some particularly huge spiders have been known to eat birds or rodents!

Spiders have evolved a variety of strategies for catching their prey – some spin sticky, nearly invisible webs that prey animals wander into and get stuck. Others are active hunters, including jumping spiders which can jump at extreme speeds using special mechanisms in their legs.

Some spiders combine these two strategies, such as “trap door” spiders, which set traps by creating hiding places for themselves – and then jumping out to grab unsuspecting prey animals that wander by!

Lobsters

With lobster being considered a luxury food today, it’s easy to forget that lobsters are in the same family as spiders and ants.

Crustaceans can grow bigger underwater than on land – and lobsters can grow to weigh nearly 50 pounds!

Lobsters’ body design has changed little in the last 100 million years, and their anatomy is spectacularly weird. The lobster’s kidneys are located in its head, its brain in its throat, and its teeth in its stomach. Its “ears” for picking up sound are located in its legs, and its tastebuds, like those of insects, are in its feet.

Butterflies

Butterflies are the most famous example of arthropod metamorphosis.

At some point in their lifecycle, all arthropods go through a drastic change from their larval stage to their adult form. But butterflies are the only ones whose adult forms are so beautiful that we pay attention to this change.

The common features of exoskeleton, jointed limbs, and segmented body can be seen in adult butterflies.


The Institute for Creation Research

After my recent university lecture on scientific creationism, a student referenced a Bible passage in Leviticus regarding insects to imply that the Bible contained errors and that Christianity and creation thinking are false.

The passage? "Yet these may ye eat of every flying creeping thing, that goeth upon all four, which have legs above their feet, to leap withal upon the earth (including) the locust . . . the beetle . . . and the grasshopper after his kind" (Leviticus 11:21,22). Tucked within a list of dietary regulations for the people of Israel, it refers to a number of animals whose exact identification is obscured by antiquity. But let's look closely.

First, we must recognize that modern day taxonomic categories, like species, genus, family, etc., are not the same as the Biblical "kind." Even the term "creeping thing" finds wide application meaning, in general, small animals which exist in great numbers. In this chapter it is used for insects (v. 21), various small mammals and reptiles (vv. 29,30), as well as animals which "move" in the ocean (v. 10).

Likewise the term "flying" applies both to flying insects and birds (vv. 13-19). Obviously, the context and description must take precedence in identification, and in this case, the "four legged insect" applies, in particular, to the grasshopper/locust kind.

In our modern classification system, all insects have at least six legs. They are members of the large and varied arthropod phyla, which includes also the eight-legged spiders, the multi-legged centipedes, as well as crabs&mdashanything with segmented legs. Some insects also have wings, but these don't count as legs.

Today, locusts are considered migratory grasshoppers. They all have two large hind legs, quite different in appearance, size, and function from the front four legs. Their front legs are used for "crawling, clinging, and climbing," while their back legs rest "above" their front legs and feet, and are used for "jumping." Furthermore, the Hebrew word translated "beetle" actually comes from the verb "to leap," implying a similar leaping insect, not our modern beetle. Thus, the Biblical description of grasshoppers turns out to be exactly anatomically correct. Far from being an embarrassment to Bible believers, this passage bears sterling testimony to the accuracy and inspiration of Scripture. As always, arguments which claim that the Bible is wrong are themselves wrong, and the Bible still stands!

It was interesting that when the student made the claim of the Bible's error, it followed my 90-minute lecture on the scientific evidence for creation. I had not directly mentioned the Bible. The question, as were several others, was a smoke screen&mdash—an effort to sidestep the issue. But even if the Bible has an error (which it doesn't), that still doesn't produce transitional fossils, or identify beneficial mutations, or account for the exquisite design in living things. Such diversionary tactics are really a futile attempt to avoid the personal implications of creation.

For if creation is true and the Bible is trustworthy, then the Creator has authority over our lives and lifestyles, and someday we must stand before our Maker and give account for our actions and choices. And that is the real issue. No smoke screen can cancel that appointment.

*Dr. John Morris is President of ICR.


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Difference Between Insects and Arachnids

Definition

Insects: Insects are small arthropods who possess six legs and one or two pairs of wings.

Arachnids: Arachnids are wingless arthropods, having a body with cephalothorax, abdomen, eight appendages, and no antennae.

Habitat

Insects: Insects are mostly terrestrial. Some insects can be aquatic and parasitic.

Arachnids: Arachnids are mainly terrestrial, and some are parasitic.

Appendages

Insects: Insects consist of three pairs of appendages.

Arachnids: Arachnids consist of four pairs of appendages.

Wings

Insects: Many insects have wings.

Arachnids: Arachnids do not have wings.

Mouthparts

Insects: Insects possess mandibles.

Arachnids: Arachnids possess chelicerae.

Division of the Body

Insects: The body of insects is divided into head, thorax, and abdomen.

Arachnids: The body of the arachnids is divided into cephalothorax and abdomen.

Antenna

Insects: Insects have one pair of antenna.

Arachnids: Arachnids do not have an antenna.

Insects: Insects have compound eyes.

Arachnids: Arachnids have one to six pairs of simple eyes.

Respiration

Insects: Respiration of insects occurs through the trachea.

Arachnids: Respiration of arachnids occurs through trachea and book lungs.

Blood Color

Insects: Insects have colorless blood.

Arachnids: Arachnids have blue color blood.

Metamorphosis

Insects: Insects undergo complete metamorphosis.

Arachnids: Arachnids undergo a series of molts.

Examples

Insects: Butterfly, beetle, bee, ant, fly, termite, grasshopper, true bugs, and louse are examples of insects.

Arachnids: Spider, acari, amblypygid, and scorpions are examples of arachnids.

Conclusion

Insects and arachnids are two types of arthropods with jointed appendages. The main difference between insects and arachnids is the anatomical structure of each type of animals. Insects have six legs and wings. Arachnids have eight legs and no wings. There are some other differences of the body of insects and arachnids.

Reference:

1.” What are insects?” Australian Museum, Available here.
2.“Arachnids.” Education in India, Available here.

Image Courtesy:

1. “Spider-characteristics” By Kaldari – Own work by uploader. Spider image from CDC Public Health Image Library (public domain).[1], Public Domain) via Commons Wikimedia
2. “Close Whopper New Insect Insect” (Public Domain) via MaxPixel

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things


Watch the video: Why do insects have six legs? (February 2023).