Mosquitoes are a common pest that can be found in many parts of the world. They are known for their itchy and irritating bites, but mosquitoes also play an important role in ecosystems and can transmit dangerous diseases like malaria, yellow fever, dengue fever, Zika virus, and more. Understanding the anatomy and physiology of mosquitoes can help us better understand how to control them and prevent the spread of the diseases they carry.
Mosquito Circulatory System
Like all insects, mosquitoes have open circulatory systems, meaning the insect’s blood (hemolymph) flows freely throughout the body and is not restricted to blood vessels. The hemolymph transports nutrients, wastes, and hormones between tissues and organs.
At the center of a mosquito’s circulation is its heart, which is a long, narrow tube located along the top of the abdomen, just under the exoskeleton. Mosquitoes, like all insects, have an open circulatory system rather than a closed circulatory system (like humans have).
Open vs Closed Circulatory Systems
In an open circulatory system:
- Hemolymph (insect blood) flows freely within the body cavity and surrounds the internal tissues and organs.
- Hearts pump hemolymph into the main body cavity where it circulates around tissues.
- Exchange of nutrients, gases, and wastes occurs directly between cells and the hemolymph.
In a closed circulatory system:
- Blood is confined to blood vessels and capillaries.
- It circulates in a closed loop system and does not directly interact with cells.
- Exchange occurs across capillary walls.
The open system allows for rapid circulation and helps facilitate gas exchange across cell membranes. However, it means insects don’t have as finely regulated internal environments as those seen in more complex animals.
Mosquito Heart Structure
A mosquito has one main heart that runs the length of its body on the dorsal (top) side. The heart is a long narrow tube with spiral valves along its length and openings called ostia on the sides.
The mosquito heart is divided into multiple chambers separated by the valves. As the heart contracts, hemolymph is pumped through the heart past the valves, which prevent backflow. This pushes the hemolymph from the abdomen area into the thorax and head.
Key facts about the mosquito circulatory system:
- There is 1 main heart that pumps blood (hemolymph).
- The heart is a long tube with valves to prevent backflow.
- Hemolymph fills body cavity and directly surrounds organs.
- No capillaries or veins to transport blood.
- The heart has layered muscles which contract and expand to pump blood.
In addition to the main heart, mosquitoes have accessory pulsatile organs that help circulate hemolymph in the head, wings and legs. They act as auxiliary hearts, supporting circulation.
Heartbeat
A mosquito’s heart rate depends on temperature and species but generally beats at a similar rate to a human heart – around 120 beats per minute. When a female mosquito takes a blood meal, the size of the abdomen expands. To accommodate this, the heart rate slows down to around 100 beats per minute.
The constriction of muscles surrounding the heart along with valves inside it allow blood to flow in one direction from the abdomen forward to the head and thorax. From there it will eventually make its way back to the abdomen in a continuous loop.
Hemolymph Composition
The hemolymph (blood) of mosquitoes:
- Is colorless or pale yellow.
- Is composed of plasma and hemocytes.
- Transports nutrients such as sugars, fats and proteins.
- Carries waste products.
- Contains hemocytes involved in immune responses.
Unlike vertebrate blood, the hemolymph does not transport oxygen. Instead, oxygen diffuses directly across mosquito cell membranes. Mosquito hemolymph also does not contain red blood cells like vertebrates.
Hemocytes
Hemocytes are insect blood cells involved in:
- Coagulation
- Defense against pathogens
- Wound healing
- Degrading foreign material
They are produced in the lymph glands and circulate within the mosquito’s hemolymph. Different types of hemocytes have been identified in mosquitoes including granulocytes, oenocytoids and prohemocytes.
Other Mosquito Circulatory Features
In addition to the heart and hemolymph, mosquitoes have other key circulatory system features including:
- Pericardial cells – surround heart and help regulate hemolymph composition.
- Ostia – openings along heart that allow input of hemolymph.
- Dorsal vessel – main tube-like heart that pumps hemolymph.
- Accessory pulsatile organs – support blood circulation in wings, antennae, legs.
Feeding Impacts on the Mosquito Circulatory System
Blood feeding by female mosquitoes provides protein for egg production but also significantly impacts circulation. Key effects include:
- Increased blood (hemolymph) volume.
- Lower heart rates after engorging.
- Changes in hemolymph composition and concentrations.
- Greater hemocyte numbers.
The large influx of blood causes the abdomen to expand to accommodate it. Circulatory adaptations allow mosquitoes to handle this increase in volume.
Blood feeding also exposes mosquitoes to potential pathogens present in the blood that can be transmitted during subsequent feedings.
Digestion
Mosquitoes have evolved adaptations to digest blood meals including:
- A peritrophic matrix that surrounds the blood meal.
- Digestive enzymes.
- Accelerated metabolism.
Nutrients from digested blood are circulated by the hemolymph and are used for activities like flight and egg production.
Comparison to Other Organisms
The mosquito circulatory system has some similarities and differences compared to other organisms:
Similarities
- Single dorsal (top) heart for circulation.
- Valves within heart prevent backflow.
- Heart pumps blood (hemolymph) forward.
- Open system where blood directly surrounds organs.
Differences from vertebrates like humans
- No veins or arteries.
- Heart is a tube vs chambers.
- No red blood cells or capillaries.
- No oxygen carrying pigments like hemoglobin.
Differences from other arthropods like spiders
- Only one main heart instead of multiple.
- Ostia openings down sides of heart.
- More regular hemolymph circulation.
While intricate compared to some invertebrates, the mosquito circulatory system is simple relative to mammals. However, it is finely adapted to facilitate blood feeding and egg production.
Conclusion
To summarize the key points:
- Mosquitoes have open circulatory systems with the blood (hemolymph) filling the body cavity.
- There is a single long tubular heart that pumps hemolymph forward when it contracts.
- Valves within the heart prevent backward flow.
- Hemolymph transports nutrients, wastes and cells but does not carry oxygen.
- Blood feeding expands the abdomen and increases hemolymph volume.
- The simple open circulatory system is adapted for the mosquito’s lifestyle.
Understanding the circulatory system provides insight into mosquito biology and behaviors like feeding and egg production. Given the impacts of mosquitoes on human health, discoveries related to their physiology can aid in developing new control methods and disease prevention strategies.