Gavin Howe
Insects are considered small animals, but what laws limit their growth? Insects and arthropods are covered by an exoskeleton, which is periodically replaced through molting. This exoskeleton is thought to limit their growth due to its weight. Additionally, the insect respiratory system, which delivers oxygen through tracheal tubes, may also play a role in limiting their size. As insects get bigger, this type of oxygen transport becomes less effective, and they require more oxygen than can be supplied. Other factors, such as anatomy, genetics, and vulnerability to predators during molting, also influence the maximum size of insects.
| Characteristics | Values |
|---|---|
| Insect size limit | Oxygen delivery |
| Exoskeleton weight | |
| Molting | |
| Respiratory system | |
| Circulatory system |
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What You'll Learn
Oxygen levels
The relationship between oxygen levels and insect size is complex. Insects have a tracheal respiratory system, which consists of tracheae that transport oxygen throughout their bodies and remove carbon dioxide. As insects grow, their tracheal tubes lengthen to reach central tissues and increase in width or number to meet the oxygen demands of their larger bodies. However, the tracheal system can only compensate to a certain extent, and the opening where the leg and body meet presents a critical limitation. When tracheal size is constrained, oxygen supply becomes limited, thereby restricting growth.
Research has shown that hypoxia, or low oxygen levels, can significantly reduce the body size of various insect species, including fruit flies, mealworm beetles, cockroaches, and moths. Conversely, hyperoxia, or high oxygen levels, can lead to slightly larger insects, as seen in experiments with cockroaches and beetles. For example, male cockroaches reared under hyperoxic conditions were found to be larger than those raised in normoxic or anoxic environments.
Additionally, the effects of oxygen levels on insect size may vary depending on specific behaviours or life stages. For instance, the critical PO2 values, which represent the oxygen levels required to meet metabolic needs, rise dramatically late within each instar as metabolic rates increase with tissue growth. However, during the early larval stage, when insects are still small, the tracheae are dense, and oxygen supply is typically sufficient.
Furthermore, the advantages of larger body size in certain insect groups, such as predators that overpower their prey, must also be considered. Larger insects can take down larger prey, and this ability to overpower a wider range of animals may have contributed to the size of certain groups, in addition to higher oxygen levels.
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Insect exoskeletons
Insects, like other arthropods, have exoskeletons. An exoskeleton is a skeleton that is on the exterior of an animal, supporting the body's shape and protecting the internal organs. In contrast, an endoskeleton, like that of a human, is internal and enclosed underneath other soft tissues. Exoskeletons are made of hardened integument, which in the case of arthropods is called a cuticle. The cuticle is composed of fibrous chains of alpha-chitin within a matrix of silk-like and globular proteins, with the rubbery protein resilin being the most well-known. The relative abundance of these two main components varies, with softer parts of the exoskeleton having a higher proportion of chitin. The cuticle is soft when first secreted, but it soon hardens through a process called sclerotization, which involves forms of tanning that make the material hydrophobic. The exoskeleton also contains microfibers of chitin surrounded by a matrix of protein that varies in composition from insect to insect and even within a single insect. During sclerotization, individual protein molecules are linked together by quinone compounds, creating rigid "plates" of exoskeleton called sclerites.
The exoskeleton affords insects a larger surface area for muscle attachment relative to body volume compared to endoskeletons. This allows insects to have many more muscles than vertebrates, giving them incredible strength and optimal leverage for the movement of appendages. Invaginations (inward folds) of the exoskeleton add to its strength and provide an even greater surface area for muscle attachment. Ridge-like invaginations are called apodemes, while finger-like invaginations are called apophyses. In addition, certain epidermal cells in insects are specialized as exocrine glands, producing compounds such as pheromones and repellants that are released on the surface of the exoskeleton through microscopic ducts.
However, the exoskeleton also limits the growth of insects. As insects grow, they must periodically replace their exoskeleton through a process called moulting or ecdysis. A new exoskeleton is produced beneath the old one, and the insect will typically stay in a protected place during this vulnerable time. Once the new skeleton is at least partially set, the insect will expand it by plumping itself up. If an insect fails to shed its exoskeleton when it outgrows it, it may die or be prevented from reaching maturity and reproducing. Additionally, the weight of the exoskeleton relative to the insect's body size increases as the insect gets larger, which can limit how big they can grow. This is because the exoskeleton needs to cover the entire animal, making it heavy even in its lighter forms without calcium. If an insect were as big and heavy as a human, its muscles wouldn't be strong enough to support its weight.
The limitations on insect size are not solely due to their exoskeletons, but rather a combination of factors that affect the organism as a whole. For example, insects' method of breathing may also prevent them from growing larger. In the late Paleozoic era, when insects were larger than they are today, atmospheric oxygen levels were higher, leading to the theory that oxygen delivery may be a factor in keeping insects small.
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Insect breathing
Several factors limit the size that insects can grow to. One theory is that insect exoskeletons are not strong enough to support larger bodies. As insects grow, their exoskeletons would need to become thicker, but this is not always possible. However, larger arthropods on land do not have thicker exoskeletons than smaller ones, which contradicts this theory.
Another factor that may limit insect size is their respiratory system. Insects breathe in a completely different way from humans, and the way they breathe may prevent them from growing larger. In the late Paleozoic era, when insects were larger, the atmosphere had a higher oxygen content of about 32% compared to 21% today. This suggests that higher oxygen levels could enable insects to grow bigger.
Now, let's focus on insect breathing:
The key characteristics of Insect Breathing include:
- Quick, precise strikes: This style emphasizes speed and agility, utilizing multiple quick stabs and thrusts to create openings for fatal strikes.
- Poison: Shinobu's medical knowledge allows her to create deadly poisons and identify an opponent's weaknesses. By coating her sword with poison, she can ensure that a single hit is often enough to take down an enemy.
- Visualization: Users of Insect Breathing visualize themselves as insects, mainly butterflies, when unleashing their techniques.
- Zig-zag movements: This style incorporates zig-zag patterns to confuse opponents, making it difficult for them to predict the angle of attack.
- Collaboration: Insect Breathing can be combined with other breathing styles, such as Water Breathing, to create collaborative attacks that bombard the opponent with multi-directional strikes.
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Insect moulting
Insects, like other arthropods, are considered "small" animals. However, there are limits to how large they can grow, and one of the factors that restrict their growth is their exoskeleton. The exoskeleton, or cuticle, is periodically replaced through moulting, or ecdysis. Moulting is a complex and vulnerable process that requires a lot of energy.
The process of moulting begins with the separation of the cuticle from the underlying epidermal cells (apolysis) and ends with the shedding of the old cuticle (ecdysis). In preparation for ecdysis, the arthropod becomes inactive, and the old exoskeleton separates from the epidermis. A digesting fluid is secreted into the space between the old cuticle and the epidermis, but this fluid remains inactive until the upper part of the new cuticle has formed. The new exoskeleton is constructed inside the old one, and when it is ready, muscular contractions and the intake of air cause the insect's body to swell until the old exoskeleton splits open. The remnants of the old exoskeleton are called exuviae.
After moulting, an arthropod is described as teneral or callow, and it is "fresh", pale, and soft-bodied. Within a short period, the new cuticle hardens and darkens through a process called sclerotization or tanning, which gives the exoskeleton its final texture and appearance. During this short phase, the insect expands, as growth is otherwise constrained by the rigidity of the exoskeleton. Growth of the limbs and other parts covered by the exoskeleton is achieved by transferring body fluids from soft parts before the new skin hardens.
The number of moults varies between species and sexes, but spiders, for example, generally moult between five and nine times before reaching maturity. In some insect species, the number of moults is constant, typically ranging from three to fifteen, but in others, it may vary depending on temperature, food availability, or other environmental factors. Each stage of development between moults is called an instar or stadium, and each stage represents the end of one growth stage and the beginning of another.
Moulting is a vulnerable process for insects, as they cannot breathe while they shed their exoskeleton. Larvae stop breathing for 45 minutes to an hour during moulting, and their oxygen consumption spikes after moulting. This dramatic drop and spike in oxygen consumption have been likened to "having your lungs ripped out". However, it is not clear whether oxygen deprivation causes damage, and insects may have evolved to deal with this stressful part of their life cycle.
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Insect circulatory systems
Insects, like all other arthropods, have an open circulatory system, which differs from the closed circulatory system found in humans and other vertebrates. In a closed system, blood is contained within vessels such as arteries, veins, capillaries, or the heart itself. In an open system, the blood, or hemolymph, flows freely within body cavities, making direct contact with all internal tissues and organs.
Hemolymph is pumped forward from the hind end and sides of the body along the dorsal vessel, which extends from the hind end through the thorax to the head. The dorsal vessel is a continuous tube with two regions: the heart, restricted to the abdomen, and the aorta, which extends forward through the thorax to the head. The heart's contraction rate varies from species to species, typically ranging from 30 to 200 beats per minute.
Hemolymph passes through a series of valved chambers, each containing a pair of lateral openings called ostia, to the aorta and is discharged in the front of the head. Accessory pumps carry the hemolymph through the wings, antennae, and legs before it flows backward again to the abdomen. The body cavity is divided into three compartments, or blood sinuses, by two thin sheets of muscle and/or membrane called the dorsal and ventral diaphragms.
Hemolymph is not involved in respiration but is responsible for transporting nutrients, salts, hormones, and metabolic wastes throughout the insect's body. It contains free cells called hemocytes, most of which are phagocytes that help protect the insect by consuming microorganisms. Additionally, the circulatory system plays a crucial role in defense, sealing wounds through clotting, encapsulating and destroying internal parasites, and producing distasteful compounds that deter predators.
The size of insects is limited by various factors, including their exoskeletons, respiratory system, and circulatory system. The exoskeleton, or cuticle, can constrain growth due to its weight, especially in larger insects. The respiratory system, which involves gaseous diffusion and mechanical ventilation in active species, may also influence the maximum size of insects. The circulatory system's role in oxygen delivery could be a factor in keeping insects relatively small.
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