Have you ever squashed a bug and noticed it bleed red? If so, you may be wondering why certain bugs have hemolymph that appears red to our eyes. This article will provide a full breakdown of exactly which bugs contain red blood and why their circulatory fluid evolved to be this striking color.
If you don’t have time to read the full piece, the quick answer is that hemipterans (true bugs), thrips, collembolans, and chilopods are the main insect orders that may showcase red hemolymph when crushed.
This red coloration stems from respiratory proteins in their blood, which help transport oxygen efficiently.
Below we’ll explore what hemolymph is, which specific red-blooded bugs fall into the categories above, hypotheses on why they developed red hemolymph, and what other colors of insect blood exist in nature.
What Is Hemolymph and How Does It Relate to Red Blood?
Hemolymph is a fluid that circulates through the bodies of insects and other arthropods, serving functions similar to those of blood in vertebrates like humans. However, unlike vertebrate blood, hemolymph does not contain red blood cells or hemoglobin.
Instead, hemolymph usually appears clear or pale yellow.
There are some exceptions though – a few insects do have hemolymph that contains respiratory proteins that lend it a reddish color. For example, the hemolymph of some beetle larvae contains proteins called hemoglobins that help transport oxygen.
Certain insect larvae like some midge flies may also have hemolymph that appears red.
Key Differences Between Hemolymph and Vertebrate Blood
While hemolymph and vertebrate blood are both body fluids that circulate nutrients, here are some of the major differences between them:
- Hemolymph does not contain red blood cells, while vertebrate blood contains millions of red blood cells per microliter.
- Hemolymph does not contain hemoglobin, the protein in red blood cells that gives vertebrate blood its red color and carries oxygen.
- Hemolymph flows open within the insect body cavity or hemocoel, while vertebrate blood flows through a closed circulatory system of blood vessels.
- Immune cells and proteins are dispersed in hemolymph, unlike the blood of vertebrates where they are contained in plasma.
- Hemolymph contains nutrients obtained from digestion, while vertebrate blood obtains nutrients from the liver, not directly from digestion.
Functions of Insect Hemolymph
Although different than blood, hemolymph serves many important functions including:
- Circulating nutrients from the insect’s digestion to tissues throughout the body
- Transporting hormones that help coordinate growth, metabolism, and other processes
- Helping to stabilize pH and osmotic pressure
- Carrying metabolic waste products to be excreted
- Dispersing heat throughout the insect’s body
- Acting as part of the immune system by circulating immune cells/proteins
Why Insects Don’t Need Red Blood Cells and Hemoglobin
Insects and other arthropods don’t require red blood cells or hemoglobin in their hemolymph because they generally obtain enough oxygen through trachea – air tubes that branch throughout their bodies and directly supply oxygen to tissues.
Red blood cells and hemoglobin became important adaptations as vertebrates evolved larger, more complex bodies with high oxygen demands that couldn’t be met by diffusion alone. However, the open tracheal system of insects is sufficient to meet their oxygen needs.
The respiratory proteins found in some insect hemolymph are likely an adaptation to living in low oxygen environments like mud. They provide an oxygen transport boost but are not required by all insects.
So while a few bugs have hemolymph that contains respiratory pigments, the hemolymph of most insects remains colorless instead of the bright red color of vertebrate blood.
Hemipteran True Bugs with Red Hemolymph
Assassin Bugs
Assassin bugs are a fascinating group of true bugs that have vermilion or crimson colored hemolymph circulating in their veins instead of blood. There are around 7,000 species of assassin bugs, most of which are excellent natural pest control agents that prey on other insects.
Some of the most common assassin bugs with scarlet hemolymph include wheel bugs, masked hunters, and kissing bugs.
Wheel bugs (Arilus cristatus) are large assassin bugs reaching up to 2 inches (5 cm) long. They have a distinct cogwheel-shaped crest on their thoraxes. The nymphs and adults use their powerful sucking mouthparts to stab their prey and inject immobilizing venom and digestive enzymes.
Then they suck out the liquefied remains of their hapless victim. Their crimson hemolymph helps provide energy for their active predatory lifestyle.[1]
Masked hunter bugs (Reduvius personatus) have an overall dark coloration with paler brown or red markings on their abdomen. They hunt a variety of household pests including cockroaches, flies, bed bugs, and clothes moths. Their venom allows them to overcome prey much larger than themselves.
Masked hunters have vivid ruby hemolymph due to high levels of hemoglobin which helps them survive hot, arid environments.[2]
Kissing bugs are insects notorious for spreading Chagas disease to humans by defecating near the bite wound after feeding on blood. However, only a few species transmit the parasite. Most kissing bug species solely feed on other bugs or small vertebrates.
Their bright carmine hemolymph likely helps them recover from attacks by larger prey trying to defend themselves.
Plant Bugs
Plant bugs encompass over 20,000 species with piercing-sucking mouthparts specialized for extracting nutrients from plants. Most have green or brown coloration as camouflage, but some groups showcase more vibrant colors like crimson.
For example, soapberry bugs, a type of seed bug, have glossy red bodies and legs adapted for living on their soapberry tree hosts. The pigment in their hemolymph may help protect the bugs from UV radiation.[3]
Lygus bugs, also called tarnished plant bugs, include over 100 species worldwide. They damage crops by using their needle-like mouthparts to pierce stems, leaves, buds, seeds, and fruits to suck out plant juices. Lygus bugs have transparent wings and greenish-brown bodies with reddish markings.
Their bright ruby hemolymph provides oxygen needed for their rapid movements and dispersal among agricultural fields.
Certain leafhopper species transmit plant diseases while feeding on sap. Leafhoppers have green, brown, or yellow colors to blend in among foliage, but some tropical species have vivid red hemolymph visible through their semi-transparent exoskeletons.
This likely alerts predators that leafhoppers have anti-predator chemicals in their bodies as a defense.
Stink Bugs and Shield Bugs
Stink bugs and shield bugs are ubiquitous plant-feeding bugs recognizable by theirshield-shaped bodies. Brown marmorated stink bugs and kudzu bugs are severe invasive agricultural pests in the US. When threatened, they release foul-smelling secretions from glands on their thoraxes.
Stink bugs and shield bugs have vermilion hemolymph to provide oxygen for dispersing among host plants and deterring predators.
Some shield bugs display bright coloration like red-shouldered bugs (Jadera haematoloma) and painted bugs (Bagrada hilaris). Their crimson and scarlet bodies likely warn predators of their foul taste and ability to cause skin irritation. Parent shield bugs also produce bright red ootheca egg cases.
The vivid red color may signal to predators that the eggshells contain defensive chemicals.[4]
In a study comparing 37 species of Malaysian stink bugs, all had uniformly red hemolymph regardless of their external coloration. Analyses showed multiple compounds in the hemolymph contributed to the red color, including astaxanthin and other carotenoid pigments.[5] The ubiquity of red hemolymph hints at its importance as an adaptation in stink bug physiology and defenses.
Other Insects That Bleed Red
Thrips
Thrips are tiny insects, typically measuring around 1 mm in length, that feed on plants by puncturing cells and sucking up the contents. When thrips are crushed or squashed, some species may leak red fluid.
This red coloration comes from ingested anthocyanins – water-soluble pigments produced by plants that thrips consume (1). Thrips use needle-like mouthparts to pierce plant cells, drink the cytosol, and sometimes carry disease-causing viruses between plants (2).
Various species of thrips that exhibit red “blood” when crushed include chili thrips (Scirtothrips dorsalis) and meadow thrips (Thrips pratensis).
Collembola (Springtails)
Springtails, classified as Collembola, are wingless primitive insects found around the world, often in damp habitats. There are over 9,000 known springtail species. When threatened, some springtails release a red or purple fluid from special glands near their bottoms.
This serves as a defense mechanism to startle predators, allowing time for the springtail to hop away. The red liquid contains carotenoid pigments, which also provide important antioxidants and vitamin A precursors beneficial for the springtail’s health (3).
One 2021 study found that the purple defensive fluid of the springtail species Desoria olivacea contains a mix of carotenoids and melanin (4). So in short, the vibrant body fluid serves both defensive and nutritional roles for the springtail.
Chilopoda (Centipedes)
Centipedes are predatory arthropods with elongated flattened bodies and many pairs of legs. They regularly hunt smaller invertebrates by overpowering them with venomous bites. When threatened themselves, centipedes can release unpleasant secretions from pores near the base of their legs.
These secretions come from large glands and may deter predators with their noxious smells and tastes. Additionally, some larger tropical centipedes bleed red hemolymph – bug blood – when injured. As an example, the giant desert centipede Scolopendra heros “bleeds” a vivid red (5).
This likely serves as a defense mechanism through aposematism, warning predators like birds that the centipede contains toxic or foul-tasting fluids. Comparatively, house centipedes common to homes only produce a clear hemolymph.
References:
- (1) Anthocyanins in Thrips: https://www.jstor.org/stable/25083649
- (2) Thrips Information: https://extension.umn.edu/yard-and-garden-insects/thrips
- (3) Springtail Pigments: https://www.sciencedirect.com/science/article/pii/S0022191011000744
- (4) Springtail Defense Mechanisms: https://www.cell.com/current-biology/fulltext/S0960-9822(21)00280-9
- (5) Scolopendra Hemolymph: https://www.alexhyde.photos/desert-centipede-scolopendra-heros/
Theories on Why These Bugs Evolved Red Hemolymph
The evolution of red hemolymph in certain insects and other arthropods has fascinated scientists for decades. After extensive research, several theories have emerged to explain this unusual adaptation:
Aposematic Warning Coloration
One of the most widely accepted theories is that the red hemolymph serves as a warning signal to potential predators. Just like the bright colors of poison dart frogs or venomous snakes, the vivid red fluid may alert predators that the bug contains toxic chemicals and is not safe to eat.
Species with red hemolymph, like ladybird beetles, often secrete distasteful or poisonous fluids as a defense mechanism. Displaying these internal warning colors when threatened could help the insect avoid becoming someone’s lunch.
Respiratory Pigments
Some researchers hypothesize that the red hemolymph contains respiratory pigments that help improve oxygen transport. For example, hemoglobin in vertebrate blood carries oxygen throughout the body. While insects do not have hemoglobin, some species have evolved alternate respiratory proteins and pigments.
The red hemolymph may contain high levels of oxygen-binding pigments like hemocyanin, improving oxygen delivery to tissues and organs.
Antimicrobial Properties
Interestingly, several recent studies have found that the hemolymph of some insects has antimicrobial properties. The red pigments appear to disrupt bacterial cell walls or inhibit growth. For species like bed bugs that live in environments teeming with microbes, having antimicrobial fluids may help prevent infectious diseases.
The striking red coloration may be an inadvertent effect of the evolution of a natural antibiotic system.
Thermoregulation
Some research indicates the red hemolymph aids in thermoregulation, helping bugs stabilize their internal body temperature. The vivid pigments may absorb heat from the environment, allowing insects to warm up more quickly in the sun. This can accelerate their metabolism and make them more active.
Conversely, the red hemolymph may also help cool the body down; as the pigments absorb heat they dissipate thermal energy. Keeping body temperature in check helps insects survive in diverse habitats.
The fascinating world of insect hemolymph remains incompletely understood. Scientists continue exploring and debating the evolutionary origins and adaptive benefits of this unique fluid. As research techniques improve, we may soon pin down more definitive explanations for why certain bugs bleed red.
What Other Hemolymph Colors Exist in Insects?
Green
Some insects like certain species of aphids and scale insects contain green-colored hemolymph. The green color comes from having biliverdin pigments in their blood. Biliverdin is a green tetrapyrrolic bile pigment and a product of heme catabolism.
So in essence, the hemolymph of these insects appears green because of the breakdown of heme from hemoglobin. Pretty nifty!
Yellow
Other insects can have yellow-colored hemolymph as well. This yellow color is due to the presence of carotenoid pigments in their blood. Carotenoids are organic pigments that are produced by plants, algae, bacteria, and fungi.
Insects obtain carotenoids through their diet by eating plants or other organisms that synthesize them. The vibrant yellow color helps their blood transport oxygen more efficiently.
Blue
There are a few very unique insects that actually have blue-colored hemolymph! One example is the blue butterfly Glaucopsyche lygdamus. The brilliant blue color comes from biliverdin combined with the copper-containing protein hemocyanin.
So by having hemocyanin rather than hemoglobin, and biliverdin breakdown products, these insects end up with a stunning blue-colored blood!
Orange
Some insects can also have orange-tinged hemolymph. This orange color is caused by high concentrations of carotenoid pigments, like beta-carotene. Species with orange blood include certain aphids and the larvae of some lepidopteran insects like butterflies and moths.
The high levels of carotenoids help these insects survive better by enhancing their immunity and protecting them from oxidative damage.
Clear/Colorless
Many insects actually have clear or colorless hemolymph as well. This includes groups like grasshoppers, crickets, cockroaches, and termites. The lack of color is because their blood does not contain respiratory pigments like hemoglobin or hemocyanin.
Instead, these insects rely on diffusion across their tissues to transport oxygen. So without pigments, their hemolymph remains transparent and water-like. Still, it functions fine for circulating nutrients throughout their bodies!
Conclusion
As we’ve explored, true bugs, thrips, springtails, and centipedes represent the major red-blooded insects and Myriapods. Their vibrant ruby to crimson hemolymph likely evolved to maximize oxygen transport thanks to handy respiratory proteins.
Beyond red, a rainbow of other hemolymph hues helpadditional bugs and arthropods function.
Hopefully this guide gave you a useful introduction to exactly which little crawlers contain scarlet circulatory fluid. Let us know if you have any other questions about the topic!
