Modules:

• Integument
• Respiration
• Circulation
• Metamorphosis
• Hormones
• Moulting
• Dormancy
• Circadian Rhythms
• Feeding & Nutrition
• Excretion & Water Balance
• Sensing touch, movement & sound
• Sensing taste & odours
• Olfactory Orientation
• Eyes & Vision
• Neural Integration
• Flight & locomotion
• Memory & Learning
• Reproduction
• Semiochemicals
• Bioluminescence
• Thermoregulation
• Insecticides mode of action

Circulation

Objectives

• Know the main structural components of the circulatory system
• Know the range of functions of the circulatory system

Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

Topic Outline

• circulatory function
• dorsal vessel and circulation; videos of heartbeat
• respiration, thermoregulation,haemolymph & haemocytes

Activities

• Minilecture: Circulation

 

Circulatory Function

Insects have an open circulatory system: the body cavity is filled with circulating blood or haemolymph. Thus the tissues are bathed in the circulating fluid. This is very different to the situation in vertebrates where the circulatory system is separate from the body cavity. The dorsal vessel is the main circulatory organ. It creates directional flow of haemolymph to the extremities, although accessory pulsatile organs aid this task.

The haemolymph has multiple functions:

• nutrient transport, for example, from gut to tissues, from fat body to tissues.
• metabolite store: reservoirs of water, proteins, excretory material
• distribute hormones: corpora cardiaca are situated adjacent to the dorsal vessel, and neurohemal organs release neurosecretions into haemolymph
• pigments: some insects have pigmented haemolymph

Diagrammatic representation of the blood circulation in an insect with a fully developed circulatory system. Arrows indicate the course of circulation. A. Longitudinal section. B. Transverse section of thorax. C. Transverse section of abdomen (modified from Chapman, 1982, based on Wigglesworth, 1972)

 

Mini-lecture: Circulation Presented by D. Merritt.

download video (.m4v)14MB alternative format (.mov)16MB link to pdf file

 

Dorsal vessel

Like most other tissues the dorsal vessel is segmentally arranged. The abdominal component of the dorsal vessel is generally referred to as the heart and the head/thoracic component as the aorta. The division between heart and aorta varies among the insect groups.
The dorsal vessel is composed of a single cell layer of striated myocardial cells, i.e.it takes the form of a muscular tube.

Diagrammatic crosssections of various insects showing the main sinuses of the haemocoel and the positions of heart, alimentary canal and nerve cord. (From Chapman, 1982.)

Ostial valves

Ostial valves are slit-like openings in the wall of the dorsal vessel, usually also segmentally distributed. They are divided into incurrent and excurrent ostia.

Excurrent ostia can be simple non-contractile openings allowing passive flow from the dorsal vessel, e.g. in the grasshopper. Excurrent ostia can open into segmental vessels (non-muscle, connective tissue tubes which guide the outflowing haemolymph into particular regions of the body).

Diagrammatic representation of the incurrent ostial valves as found in Bombyx seen in horizontal (left) and transverse (right) sections of the heart. Arrows indicate the direction of blood flow. (From Chapman, 1982.)

Dorsal and Ventral diaphragm

The dorsal diaphragm lies just under dorsal vessel. The ventral diaphragm is often situated immediately overlying the ventral nerve cord which is them said to lie in the perineural sinus. It is composed of a thin layer of muscle that often actively contracts.

Alary muscles

Alary muscles are fan-shaped, paired, and segmentally distributed. They are sometimes called aliform muscles (e.g. Chapman).
Composed of muscle and connective tissue, neurally evoked potentials. Their contractions don't necessarily match those of the heart.

Accessory Pulsatile Organs

Tibial pulsatile organ in the legs. Septum runs the length of the leg.
Antennal pulsatile organ. Pulsatile organs commonly found at wing bases.

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Circulation

Hemolymph enters dorsal vessel through incurrent ostia on relaxation and exits through excurrent ostia or aorta on contraction. Heartbeat usually peristaltic in nature, i.e. it proceeds in a wave, usually from posterior to anterior but frequent reversals are seen, especially in adult insects. Some insects have synchronized (non-peristaltic) heartbeat, often seen in orthopteroid insects such as cockroaches and grasshoppers.

In one case (Periplaneta) the excurrent ostia are composed of an unusual muscle cell type with low density of myofilaments oriented in many directions. They are innervated by neurosecretory cells. They go through different phases to the dorsal vessel and incurrent ostia, i.e. are under separate control. Open for several contractions and close for several (Miller in CIPBP).

Alary muscles play a support role to some extent: they are not absolutely required for relaxation and filling of the heart.

Compensatory changes in the tracheal volume, caused by haemolymph oscillation between anterior and posterior body in Calliphora vicina. (a) During forward pulse period of the heart, haemolymph (Hl) leaves the aorta in the head. The tracheal volume in the anterior body decreases. (b) During backward pulse periods, haemolymph enters the anterior heart ostia in front of the pair of large abdominal air sacs and leaves the caudal heart through a pair of excurrent ostia. The increasing haemolymph volume partly displaces the volume of the air sacs (from Wasserthal, 1982b, modified). Expiration/inspiration of anterior and posterior body alternate due to periodic haemolymph shift, because the tracheal systems are separate. (From Wasserthal, 1996.)

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Synchrotron x-ray phase-contrast video of heartbeat in the grasshopper Schistocerca americana.

Double click on the screen to play the video; single click to pause.

Movie depicts the grasshopper's 8th abdominal segment in lateral view, with dorsal oriented upward and anterior to the right. Field of view is 1.3 × 0.9 mm. The dorsal edge of the abdomen can be seen at the top. The diagonal lines running from upper left to lower right are margins of the x-ray transparent Kapton tube. In this sequence, pulsatile movements in the dorsoventral axis associated with heartbeat can been seen, but flow within the heart is not apparent due to lack of contrast between hemolymph and the surrounding tissue. Large collapsing circular structures are air sacs. Tracheal tubes are also visible, particularly the two main tubes running horizontally at the bottom of the frame.

Typical hemolymph flow in the heart of the grasshopper Schistocerca americana.

Double click on the screen to play the video; single click to pause.

Movie depicts the grasshopper's 3rd abdominal segment in lateral view, with dorsal oriented upward and anterior to the right. Field of view is 1.3 × 0.9 mm. The dorsal edge of the abdomen can be seen at the top. The diagonal line in the lower left corner is a margin of the x-ray transparent Kapton tube. In this video, bubbles can be seen in three locations: within the perivisceral sinus, collected en masse and abutting the dorsal diaphragm (lower third of image); within the heart, moving rapidly left and right; and surrounding the heart in the pericardial sinus, either static or moving less rapidly than within the heart. The dorsal diaphragm moves dorsoventrally in association with heartbeat. A main tracheal trunk running antero-posteriorly is also compressed in association with these movements, but not strictly so. Within the heart, flow is complex and non-uniform, as evidenced by the trajectories of the bubbles. There is net transport of the hemolymph anteriorly, but the bubbles can be seen moving posteriorly as well. Additionally, vortical trajectories of bubbles can be seen. Due to bubble buoyancy, most bubble trajectories in the heart occur in dorsal margin.

These two videos from the article: Wah-Keat Lee and John J Socha.(2009) Direct visualization of hemolymph flow in the heart of a grasshopper (Schistocerca americana). BMC Physiology 2009, 9:2 doi:10.1186/1472-6793-9-2

 

Respiration

Insects do not have respiratory pigments such as haemoglobin (except for a few rare cases). Abdominal pumping (muscle mediated expansion and contraction of the abdomen) is generally seen to play a role in respiration but it could also be involved in circulation. In a recent review (Wasserthal (1996) Advances in Insect Physiology 26: 297-351) demonstrates that there is a close link between respiration and circulation. Haemolymph movements associated with changes in heartbeat direction cause collapse of airsacs and changes the direction of airflow between the spiracles—ventilation by periodic haemolymph shift, shown in a fly in the figure above.

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Periodic heartbeat reversal and pulsed CO2emission in the resting blowfly Calliphora vicina (mature female of 0.08 g). Temperature measurement at the level of the 3rd heart segment at T 21•C. A small soldering bit serves for fixation of the fly and heatmarking of the thorax haemolymph. There is one CO2emission peak during backward pulse period and another one in the course of the forward pulse period of the heart. The lowest COzemission is at the beginning of the forward pulse period.

Heartbeat reversals have been well documented in flies (above) and lepidopteran pupae (below). They correlate with the phases of CO2 outbursts occurring during discontinuous gas exchange in some species.

Heartbeat reversals coordinated with CO2bursts and pHchanges in the haemolymph of a 4dayold nondiapausing pupa of the papilionid butterfly Troides rhadamantus. Heartbeat direction was determined by thermistors upon the 5th and 7th abdominal tergites while a heatmarking resistor was arranged between them upon the 6th tergite. The beginning of the forward pulse period coincides with the onset of the CO2 burst. The pH becomes more acid during constriction and flutter phase of the spiracles, reflecting CO2 storage in the haemolymph. Male, 5.44 g, at Ta 20oC ( (Hetz and Wasserthal, 1993 and unpublished). (From Wasserthal, 1996.)

Thermoregulation

Circulation allows heat transfer between different parts of the body. Experimental localised heating of Manduca adults produced increased heart rate. Transection of the ventral nerve cord removed the response, providing evidence for nervous system mediated cardioregulation based on localised heating. Some winged insects elevate their temperature before flying by basking in sunlight with wings open. Wings act as heat collectors. Similarly, they could dissipate heat produced by flight muscles.
Haemolymph contains cryoprotectants for cold climate insects: allowing supercooling or freezing tolerance (covered later in “Thermoregulation”)

Haemolymph pressure

Haemolymph pressure can be used as an aid to movement. Role in ecdysis, unfurling of wings, oviposition. The ptilinum of Diptera is everted under haemolymph pressure, allowing the still-flexible and unsclerotised adult to crawl through the pupation substrate.

Larvae are generally pressurised. The less sclerotised, softer bodied insects use haemolymph pressurisation as a way of maintaining body shape.

The somatic (body wall) muscles are responsible for the generation of pressure. The dorsal vessel is probably primarily circulatory. In fact, the pressure exerted by abdominal muscle contraction can interrupt or attenuate the normal heartbeat.

Heartbeat: Myogenic or neurogenic?

Myogenicity is controversial due to different techniques. Definitely a rich supply of neurons. Convincing evidence from bathing heart prep in tetrodotoxin which blocks nervous activity. Heart kept beating, therefore myogenic.

Appear to be an anterior and posterior pacemaker.

Lateral cardiac nerve cords found in some insects. Neuronal cell bodies are found in these cords. Many shown to be neurosecretory: a neurohemal organ. Neurosecretory influence eg. during feeding and locomotion, is highly likely but the identity of the factors is not well known. Injection of extracts of various organs can produce artefactual changes in heart rates.

Haemocytes

Involved in immunological defence and wound healing.

Haemocytes are difficult to study because the haemolymph undergoes coagulation immediately upon exposure to the air or exposure to damaged tissues.

There are multiple types of cells, and different taxonomic groups and ages of insects have different complements of haemocytes. There is controversy over the number of categories. It is difficult to carry out "longitudinal" studies in which cells are followed through their development and life cycle. It remains possible that multiple "categories" represent the same cells in different states of response to extrinsic factors.

Haemocytes originate from embryonic mesoderm, and are produced in haemopoietic organs that are localised in different regions in different insects, e.g in caterpillars behind the prothoracic spiracles.

The classification according to Lackie (1988) is one of the simpler and more useful.

Prohaemocytes	Small round stem cells. Precursor cells.
Plasmatocytes	Also called macrophages, may be granular in appearance. Phagocytic and motile, involved in encapsulation (multilayered aggregates of haemocytes around, for example, parasites), nodule formation (clusters of cells around, for example, groups of bacteria) and wound healing. Perhaps the most common haemocyte type.
Granular Cells	Fine filopodia, rounded, bright under phase microscopy
Coagulocytes (cystocytes)	Lyse to produce a gel or coagulum: they coagulate. Involved in wound clotting.

Wound healing

rapid clotting, strengthening the clot, repair underlying cells, repair basal lamina. Epidermal cells regrow over clot.

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References Chapman, Chapter 32 Lackie (1988). Haemocyte Behaviour. Advances in Insect Physiology 21: 85-178 Lee, W. K. and Socha, J. J. (2009). Direct visualization of hemolymph flow in the heart of a grasshopper. Integr. Comp. Biol. 49, E99. Wasserthal, L. T. (1996). Interaction of circulation and tracheal ventilation in holometabolous insects. Advances in Insect Physiology 26, 297-351. Wasserthal, L. T. (1999). Functional morphology of the heart and of a new cephalic pulsatile organ in the blowfly Calliphora vicina (Diptera : Calliphoridae) and their roles in hemolymph transport and tracheal ventilation. International Journal of Insect Morphology & Embryology 28, 111-129.

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TOPIC REVIEW Do you know…? The difference between the open and closed components of the insect circulatory system? The names of the components of the circulatory system and their location in the insect body? How the circulatory system can interact with the respiratory system? What is the make-up of the haemolymph and the names of some of the different cell types in the haemolymph?

End of the Module: Circulation

Go on to the next Module: Metamorphosis