Millipede legs grow in an unexpected way

In a groundbreaking discovery, researchers at the University of Tokyo have uncovered a new aspect of millipede growth that challenges long-held assumptions about these fascinating arthropods. Millipedes, known for their abundance of legs, have been trundling along the Earth for over 420 million years. 

As some of the first creatures to walk on land and breathe air, these invertebrates play a critical role in ecosystems by burrowing in the soil and decomposing plant matter. However, much is still unknown about their biology and development, particularly how they acquire their multitude of legs.

Traditionally, people believed that millipedes grew new segments without legs at the end of their bodies when they molted or shed their exoskeletons. In the subsequent molt, the previously added segments would re-emerge with fully formed legs. 

However, the University of Tokyo team has found that new segments actually contain minuscule leg bundles. They appear as transparent protrusions before molting and become fully formed legs afterwards.

Researchers are surprised and excited about the results

“In 1855, French entomologist Jean-Henri Fabre proposed the ‘law of anamorphosis.’ That law indicated a pattern in millipede growth, in which legless segments are first added to the body by molting and then legs are added onto that segment in the next molt. However, the physical changes that occur when segments and legs are added were not clear,” explained Professor Toru Miura. “Our study is significant in that it adds new morphological knowledge to the law of anamorphosis after 168 years.”

This finding has far-reaching implications not only for understanding millipede growth but also for the broader study of arthropods, which make up about 75 percent of all animal life on Earth. The fact that even a familiar creature like the millipede can yield new and vital insights into development and diversity is significant for researchers in the field.

“Since it is not yet known whether this phenomenon is widely seen in other millipedes, we will need to study other species for comparison. We would also like to continue our research into the molecular mechanisms involved in anamorphosis, as these are still largely unclear,” said Miura. “Our work has shown us that there are still many surprising discoveries to be made in nature, right under our feet.”

How the study was done

To make this discovery, the researchers used advanced microscopy techniques to study a common species of millipede called Niponia nodulosa. By employing a scanning electron microscope (SEM), the team was able to examine the microscopic surface details on the millipedes’ bodies and observe any changes during different stages of molting and growth. The researchers also used a confocal laser scanning microscope (CLSM) to analyze what was happening inside the millipedes.

“We expected that segments and legs would be added by molting, as happens in many other arthropods. However, we were surprised to find that drastic morphogenesis, the biological process of changing shape, actually started before molting,” said Miura.

A few days before molting, the scientists could already see two pairs of small, wrinkled legs beneath the cuticle of the previously thought legless segment. Observers noticed transparent protrusions on the outer surface of the millipede’s segment. As it approached the molt, a thin tissue membrane covered these and contained the new leg bundles.

“Arthropods generally undergo morphological changes through molting, but the phenomenon discovered in this study deviates from this. With these millipedes at least, transparent protrusions with legs appeared from the elastic cuticle (an external layer that provides muscular support) before they molted,” said Miura. “This result suggests that changing shape without molting via the cuticle may also be important in other arthropods.”

This breakthrough in understanding millipede growth underscores the importance of continued scientific inquiry into even the most familiar creatures. As the team at the University of Tokyo has demonstrated, new discoveries can reshape our understanding of the natural world and help to unlock the secrets of arthropod development, growth, and diversity.

More about millipedes

Millipedes are fascinating creatures with a long evolutionary history, and there is much to know about them. Here are some interesting facts and features about millipedes:

Classification

Millipedes belong to the class Diplopoda within the phylum Arthropoda. They are related to other arthropods such as insects, spiders, and crustaceans. Around 12,000 known species of millipedes exist, with many more possibly remaining undiscovered.

Anatomy

Millipedes have elongated, cylindrical, or flattened bodies, consisting of numerous body segments. Each segment typically has two pairs of legs, except for the first few segments near the head, which have one pair or no legs at all. The total number of legs varies among species, with some having as few as 30 and others over 400.

Size

Millipedes come in a variety of sizes, ranging from less than an inch to over 15 inches in length. The giant African millipede (Archispirostreptus gigas) is one of the largest species, capable of reaching lengths up to 15 inches.

Habitat

Millipedes inhabit diverse habitats worldwide, ranging from tropical rainforests to temperate woodlands and grasslands. They typically prefer moist environments, as they are prone to desiccation.

Diet

Millipedes are mainly detritivores, feeding on decaying plant material, fungi, and other organic matter. This makes them essential components of nutrient cycling and soil health in ecosystems.

Defense mechanisms

Millipedes possess several defense mechanisms to protect themselves from predators. Many species can curl up into a tight coil, exposing only their tough exoskeleton. Some millipedes also secrete toxic or foul-smelling chemicals from special glands called ozopores to deter predators.

Reproduction

Millipedes reproduce sexually, with males depositing sperm packets called spermatophores. The females then pick these up. Some species engage in elaborate mating rituals. After fertilization, females lay eggs in the soil or in specially constructed chambers. Millipede offspring, called nymphs, undergo several molts as they develop and grow, adding more segments and legs with each molt.

Lifespan

Millipedes have relatively long lifespans for arthropods, with some species living for several years. They continue to grow and molt throughout their lives, albeit at a slower rate as they age.

Importance in ecosystems

As mentioned earlier, millipedes play a crucial role in ecosystems by breaking down organic matter and returning nutrients to the soil. They also serve as a food source for various animals, such as birds, reptiles, and small mammals.

These are just a few aspects of millipedes’ biology and ecology, showcasing their remarkable adaptations and the integral role they play in the natural world.

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