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Photo Source: Wikipedia in the world of locusts, females are like mysterious witches, breeding with "magic".

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"when Xinhe immature flying locusts arrive, the young seedlings eat up the withered stems." This is the description of locusts by Dai Shulun, a poet of the Tang Dynasty, in Tun Tian ci, which reflects the horror of locust plague. Locusts are notorious insects that can migrate thousands of kilometers in swarms, covering the sun like a dark cloud and devouring all plants along the way. It is estimated that a swarm of locusts of one square kilometer contains about 40 million locusts, which can eat 35000 people's food rations a day. The locust plague in East Africa in 2020 is a painful example, with locusts spreading all the way from Africa to India, affecting more than 20 countries and regions, and directly threatening the food supply of 11.9 million people along the way.

Locusts can cause such a serious agricultural disaster, which is closely related to their strong reproductive ability. Locusts lay eggs mainly in sandy soil. Each female locust can lay 60-80 eggs at a time and lay eggs at least three times in a lifetime. In about two weeks, these eggs can hatch nymphs. After molting five times, the nymph becomes an adult, and after another two to four months, it develops into a mature locust, and it can begin to reproduce. Therefore, as long as the environmental conditions are suitable, the number of locusts will increase exponentially.

In order to protect their offspring from being detected and preyed on by birds, animals and other insects, female locusts not only choose the right soil and humidity, but also lay their eggs 10 centimeters or more underground, which can also reduce the impact of wind and rain on their eggs. However, an adult female locust is no more than 4-7 centimeters long. how can a locust standing on the ground to lay eggs do such a difficult job?

Freely retractable female locusts use their bellies to make holes in the soil. Unlike male locusts, the belly of female locusts has a special structure called the ovipositor, which consists of two valves that look like shovels. One flap is located at the end of the abdomen and is inserted into the soil; the other is located at the end of the back and is mainly responsible for digging the soil. When laying eggs, the female locust will press the tip of the abdomen into the ground, and then skillfully control the movement of the spawner to periodically open, close and contract, etc., until the locust's belly extends to 10 cm or even 15 cm underground.

Female locusts spawning: t indicates the end of the abdomen, e indicates the locust eggs (source: original paper and Tel Aviv University), and then they will first determine whether the soil moisture and salinity at the end of the abdomen are appropriate, and if they are not suitable for spawning, or the spawning process is interrupted by other locusts, they will quickly retract their abdomen and stop the process in less than a minute or even dozens of seconds. If all goes well, they will carefully place the eggs in the egg pod, contract their abdomen while laying eggs, cover the eggs with secreted foam, and finally cover the hole where they lay eggs with soil. A complete spawning process lasts as long as an hour.

The abdomen of male (top) and female (lower) locusts (source: Australian Department of Agriculture, Fisheries and Forestry and original paper) the abdomen of locusts normally does not exceed 5 cm, but extends to 2-3 times the original length because of a special structure called internodal membrane (intersegmental membrane). This structure is very soft and connects every segment of the locust's abdomen, allowing the back of the locust's belly to be folded like a folding fan and spread out during spawning.

When unfolding these dorsal plates, locusts also need to use the abdominal muscles between the dorsal plates. This means that when the dorsal piece unfolds, the locust's abdominal muscles also stretch. The scientists observed under the microscope that when the dorsal piece was not unfolded, the muscle fibril of the locust's abdomen intertwined with each other, with many raised ripples on the side. When they lay eggs and stretch their abdomen, the originally entangled fibrils are elongated and arranged in parallel, the muscle surface becomes smooth, and even the nuclei of muscle cells are flattened.

Left: locusts' abdominal muscles (lm), dorsal plates (lt) and internodal membranes (ism) under resting (b), contraction (a), relaxation (c) and spawning elongation (d); right: abdominal muscles under locusts' elongation (ST) and contraction (CN) (source: original paper) humans and locusts both have muscles that can stretch and retract. why don't human skeletal muscles have such a strong ability to stretch?

Skeletal muscle is mainly composed of two kinds of protein fibers: thick filaments and fine filaments. They can slide against each other, causing the muscles to contract or stretch. If the thick filaments and fine filaments overlap more, the muscles contract; when the thick filaments and fine filaments overlap less, the muscles lengthen. In the human body, both thick and thin filaments in skeletal muscle are fixed to a structure called Z-disc, forming a structure called sarcomere. When thick filaments and fine filaments have little overlap, but continue to stretch, it will cause Z disk to break or tear, resulting in muscle damage.

Scientists speculate that when female locusts lay eggs, their abdominal muscles are extremely elongated and their Z-disk breaks, transforming into a different structure from muscle nodules but still capable of stretching and contracting-body Z (Z-body). Scientists have proposed two possible explanations for this phenomenon: one is the "tilt" model, which believes that the broken Z disk will tilt at an angle, thus expanding the length that the muscle can stretch; the other is the "sliding" model. It is believed that the broken Z disk will be misplaced and overlapped and further extended by sliding.

However, in either case, the regular arrangement of thick and thin filaments has been destroyed after the abdominal muscles of female locusts are overstretched. Under the microscope, they no longer show light and dark patterns formed by the overlapping arrangement of thick and thin muscle filaments (right CN above), but become chaotic. This means that the structure of the mother's abdominal muscles may have changed permanently after laying eggs deep underground.

Human skeletal muscle structure and scientists' speculated abdominal muscle structure of locusts (source: antisensescienceblog and original paper) female locusts stretch their bodies as they lay eggs, which affects not only the abdominal muscles, but also their abdominal organs such as the intestines, the heart, and even nerves. There is a spinal cord on the back of vertebrates, and similarly, locusts have a corresponding structure called nerve cord, which has five ganglia in the abdomen. Generally speaking, for vertebrates such as humans, nerve elongation of more than 30% will cause irreversible damage. However, it is inconceivable that the locusts will stretch the nerve cord several times as much as before and still will not be damaged.

To compare whether locusts' nerves are naturally stretchable, the scientists compared the abdominal nervous system stretching abilities of male, presexual female (PM) and sexually mature female (SM) locusts. They dissected the nerve cords of the locusts and gradually stretched them with surgical forceps until the nerve cords were broken.

The abdominal ganglia of locusts (photo source: original paper) scientists found that the abdominal lengths of these locusts are basically the same, all about 3 centimeters, and there is no difference in the length of nerve cords among the three, and they look very similar. However, as soon as the stretching begins, the difference between the three becomes apparent immediately. The ventral nerve cord of male locusts can stretch up to 140.5% of the original length; most female locusts before sexual maturity can stretch to 164% of the original length, and some can stretch to 186%. But the ventral nerve cord of sexually mature female locusts can be stretched to 214% of the original length, or even 249%.

Later, the scientists abandoned manual stretching and used instruments to measure the elongation of locusts' ventral nerve cords. In the meantime, the ventral nerve cord of several sexually mature female locusts has even reached the maximum length that can be measured by the equipment, and is still unbroken. When the instrument slowly loosens these ventral nerve cords, they can return to their original length completely and are not as loose as rubber bands that exceed the elastic limit, and are still tight.

They also found that the stretchability of different ganglia on the ventral nerve cord was also different. For female locusts before sexual maturity, the stretchability of different interganglionic segments is relatively average, ranging from 145% to 186% of the original length. For sexually mature female locusts, the stretchability of different segments varies greatly, ranging from 172% to 257% of the original length. Among them, the most stretchable is the middle area of the ventral nerve cord, that is, the part between ganglia 2 and 4.

The abdominal nervous system of sexually mature male and female locusts (photo source: original paper) in fact, locusts are not the first species that humans have found that their nerves can reversibly stretch more than twice as long. For example, baleen whales need to swallow large amounts of sea water quickly when eating, and the volume of the mouth and abdominal cavity expands rapidly. Accordingly, their nerves elongate together. The baleen whale's strategy is to fold the nerve fiber bundles like an accordion and wrap them with the same height of folded adventitia and walls. In this way, the length of the tongue doubles, while the length of the nerve itself does not change. Of course, baleen whales also have some protective structures to prevent nerve overstretching.

However, scientists did not find such a structure in the nerve cords of female locusts. Compared with before sexual maturity, sexually mature female locusts have no special structure except that their nerves look thicker, and there are no special morphological changes before and after stretching. Scientists can only speculate that there may be some signal that changes the biomechanical properties of the nerve tissue during the sexual maturation of female locusts. However, what this signal is and how it works remains to be further studied.

The neural structure of the baleen whale tongue (source: original paper) the human body is sometimes injured by overstretching, but there are many wonderful creatures in the animal kingdom that can extend their bodies to eye-popping levels. without damaging any tissue. The physical structure of these creatures has provided a lot of inspiration for people to develop biomaterials and synthetic organs. By the time scientists fully discover the mystery of the super stretching of female locusts, maybe we will be able to develop prosthetics that can grow three times as long and run fast anytime and anywhere.

References:

Https://www.fao.org/ag/locusts/en/info/info/faq/index.html

Https://www.agriculture.gov.au/biosecurity-trade/pests-diseases-weeds/locusts/about/about_locusts

Https://royalsocietypublishing.org/doi/10.1098/rsif.2021.0955

Https://linkinghub.elsevier.com/retrieve/pii/0022191083900720

Https://antisensescienceblog.wordpress.com/2014/04/25/stretching-whats-the-big-whoop-debunking-one-of-the-oldest-exercise-myths/

Https://linkinghub.elsevier.com/retrieve/pii/S258900422201567X

Https://linkinghub.elsevier.com/retrieve/pii/S0960982215002845

This article comes from the official account of Wechat: global Science (ID:huanqiukexue)

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