Respiration in Insects

Above: the basic insect respiratory system consists of a series of rigid tubes, called tracheae (singular trachea), connected to the outside via pairs of pores called spiracles (typically one pair per segment on the sides of the thorax and abdomen, lacking on certain segments). Air enters the system via the spiracles and the tracheae are air-filled. The spiracles can often be opened and closed and lead into short tracheae that enter a pair of longitudinal tracheal trunks, which are the main tracheal tubes. From these lateral tracheae branch smaller tracheae that supply the tissues with air. This supply is especially rich in the more active tissues, such as muscles, nervous tissues and the gut. Tracheae also extend into the wings, running inside the wing veins. The tracheae branch until they reach a diameter of 2 to 5 micrometres (2-5 thousands of a millimetre) and then often enter stellate tracheole cells (transition cells) from which they emerge as finer branches called tracheoles, with diameters less than one micrometre. These tracheoles terminate inside the tissues, almost always as open-ended or blind-ended tubes about 200 nanometres in diameter (200 millionths of a millimetre).

Click images to enlarge.

Respiration and Circulation in Insects

moth Bombyx mori. Bottom left: a
trachea from the same insect. Note
the spiral ridges (taenidia, singular
taenidium) lining the inside of the
trachea, this ridge is formed of
cuticle and prevents the trachea
collapsing. These ridges may be
rings (annular) or spirals.

Right: a branch in a trachea from
Bombyx mori.

The outside cuticle of the insect extends inwards through the spiracles to line
the inside of the main tracheae and in the smaller tracheae this cuticle is
reduced to a thin membrane lining the lumen of the tracheae and forming the
taenidia. The tracheoles are often said to lack this tough inner lining, but
end. Apart from this lining, the walls of the tracheae consist of a single layer
of flattened epithelial cells.

A cross-section through a trachea (TR) in the
antenna of the rove beetle Aleochara
bilineata, illustrating the taenidia (TE) and a
small tracheole (Tr) branching off. The
trachea is supplying nerves and muscles and
is bathed in haemolymph (HE).

A tracheole (TR) in the antenna of
Aleochara bilineata, supplying a
muscle (MU) and a nerve (NE). M:
mitochondrion.

Click images to enlarge.

The tracheoles may terminate on the surface of a cell, such as a muscle cell, or they may penetrate inside the cell,
either part way or even forming an extensive network inside and also covering the outside of the muscle. The supply
is generally greater to flight muscles, especially of the fibrillar type (see insect locomotion). Even individual
The Role of Fluid in the Tracheoles

When an insect is at rest the ends of the tracheoles are filled with fluid. Textbooks sometimes state that this fluid is
needed to dissolve the oxygen. However, oxygen diffuses faster in air than it does in water, and the fluid is actually a
barrier to oxygen diffusion. Thus, when an insect exercises the fluid gets 'sucked' into the muscle cell until oxygen
reaches the ends of the tracheoles. (This happens, at least in part, as the concentration of solutes build up in the
exercising muscle, drawing in the water by osmosis). The fluid is there at rest because the tissues are bathed in fluid,
though it may serve to reduce water-loss and dehydration through the tracheal system. When an insect hatches from
its egg, its tracheal system is initially filled with fluid, but this fluid is actively absorbed by the tracheole cells until the
system fills with air.

The Transport of Air through the Tracheal System

In the basic system described so far, air simply diffuses in through the spiracles and along the tracheae. Diffusion is
rapid over a millimetre or even a centimetre, but is very slow at greater distances. This diffusion-driven system occurs
in some small or not very active insects. In large and active insects, such as moths, butterflies, bees and wasps,
diffusion alone is insufficient. These latter insects show breathing movements - that is they actively pump air
through the tracheal system. This is why the abdomen pulses in these insects. Sometimes only the tergum of each
abdominal segment moves up and down, as in beetles, or both the sternum and tergum, as in flies, or the side-walls
(pleura) may be very flexible and also move in and out, greatly changing the internal volume of the abdomen, a sin
moths and butterflies. In this way a rapid stream of air flows through the tracheal system. Certain spiracles may be
used to take air in, others to expel air, e.g. air may be drawn in through the thorax and expelled through the
abdomen. However, these circuits are not hard and fast and occasionally the direction of flow may be reversed.

Air sacs may facilitate this movement of air through the tracheae. Air sacs can occur in almost any part of the
system, and in rigid structures like the head and thorax they may be permanently expanded, acting as reservoirs of
air, whilst in the abdomen they may greatly inflate and contract (flatten and empty). The diagrams below illustrate the
air sacs in the abdomen of a honey-bee worker. You may have noticed how the abdomen of a honey-bee pulsates as
the muscles of the abdomen expand and contract the abdominal segments to fill and deflate the air sacs. If you have
ever chased a bee or wasp, then you may also have noticed that the abdomen pulsates harder and faster with
exertion! As we shall see below, these breathing movements are intimately connected to the circulatory  system of the
insect also.

Control of Spiracle Opening and Closing

Insects lose most of their water through the spiracles. Being small they do not have much water to lose! It is not surprising,
therefore, that insects typically only open their spiracles when they need more oxygen. An increase in carbon dioxide, a
product of respiration, causes the spiracles to open more. Starvation and reduction in metabolic rate causes them to
opening at the end of the trachea, opening at the end of the trachea, rather than the outermost opening of the rather than
the outermost opening of the that is the internal opening of the atrium, that is the internal opening of the atrium, atrium (a
naming convention I personally find unhelpful since in insects lacking atria, the spiracle opens directly to the outside and in
diagrams the external openings are generally labeled as 'spiracles' regardless and so I find myself referring to the most
external opening, or even the whole structure, as a spiracle, atrium or not).The external opening of the atrium is often
screened by spines, meshes and similar structures may cover the atrial openings in some insects, especially those from
dry conditions, though both the spines and the atrium may trap water moisture, they may actually be more important in
preventing dust from entering the tracheae. An internal spiracular valve often occurs between the spiracle and atrium,
which can open or close the spiracle.

Temperature is an important factor. At low temperatures, when metabolism may be low, the spiracles are largely closed but
open occasionally. At higher temperatures they may open and close periodically, and at still higher temperatures they may
open continuously.

Above: air sacs in the abdomen of a honey-bee worker. The tubes (or circles) indicate the main
tracheae entering/leaving the air sacs.

Respiration in Aquatic Insects

Insects have to be able to obtain oxygen if they are to survive when submerged under water. Some insects have what is
called a plastron mechanism - the hairs on their bodies are specially modified to trap a film or bubble of air when they
dive (these hairs are designed to resist collapse under high pressures). In Dytiscus (Great Diving Beetle) and Notonecta
(Water Boatman) air is trapped beneath the elytra (wing covers) and this air communicates directly with the insect's
spiracles (openings to its airways or tracheae). These plastrons and sub-elytral air-spaces can function as what are called
'physical gills' - trapped pockets of air that can actually absorb more oxygen from the surrounding water. Oxygen diffuses
across an air/water interface some three times faster than nitrogen, so as the insect takes up the oxygen from the trapped
air bubble, lowering the partial pressure of oxygen and raising that of nitrogen within the bubble, oxygen diffuses in (when
the partial pressure of oxygen in the water exceeds that in the air space) faster than nitrogen diffuses out. This maintains
the air bubble for longer and in experiments in which the air was replaced by pure oxygen, the insect actually had to
resurface for fresh oxygen sooner than when air was used, since with oxygen the partial pressure of oxygen in the bubble
is always greater than in the surrounding water and although the insect takes more oxygen with it to begin with, it is unable
to extract any from the surrounding water. In Notonecta (the Water Boatman) the hindlegs are used to drive water currents
over the physical gills to irrigate them with fresh oxygenated water. Eventually all the nitrogen in the air space dissolves
and then the insect must resurface to replenish its supply of air. In Dytiscus and Notonecta the trapped air volume can be
regulated when under water, acting to regulate buoyancy.

Structures that have a higher affinity for air than for water, like plastron hairs, are called hydrofuge structures. Hydrofuge
hairs also exist on the siphons of mosquito larvae. Some aquatic insect larvae, like mosquitoes, breath through a siphon
- a tube with a spiracle at its tip that leads straight into the tracheal airways inside the insect. The spiracles are
surrounded by hydrofuge hairs which repel water and prevent it from entering the system and drowning the insect. The
water trough in the neighbouring meadow has dozens of these larvae that wriggle for cover whenever one's shadow
passes over them. The hydrofuge hairs cannot repel oil, which is used to control mosquitoes by applying a film of oil to the
water's surface; when the larvae try to breathe the oil enters through the spiracle and they drown. Eristalis, a type of
hoverfly, has aquatic larvae that are called rat-tailed maggots because of the long tail-like extensible breathing siphon.

Some aquatic insects have a closed tracheal system which does not open to the outside air via spiracles. These include
the dragonfly nymphs which have 6 double rows of lamellae lining the rectum and forming the branchial basket. Water is
pumped over these tracheal gills, mostly in and out through the anus, and oxygen diffuses across to the trachea which
fill the gills. Some aquatic insects have spiracular gills, like Simulium (black fly) pupae.

Bloodworms are red worm-like aquatic insect larvae that are often found at the bottom of ponds. These are the larvae of
midges, like Chironomus, and certain flies (belonging to the true-flies or Diptera). They are red because they contain a
form of haemoglobin (which contains 2 haem groups per molecule instead of 4 as are found in vertebrate haemoglobin)
in their haemolymph ('blood plasma'). This haemoglobin is used to store oxygen, enough for two days in low oxygen
conditions (as might occur in warm stagnant water). When oxygen is adequate they return to tracheal respiration and
replenish their oxygen reserves. The larva of Chironomus has a closed tracheal system and absorbs oxygen across its
'skin' (cuticle) and also has so-called 'blood gills'. These are regions of the body wall that are very thin and project from
the body surface as blood-filled sacs that are more-or-less devoid of tracheae. However, these do not seem to have a
normal respiratory function, but may assist in recovery from oxygen starvation. The larvae of some mosquitoes have long
anal papillae (projections) filled with tracheae and which are held in a current of water created by mouth brushes.
However, the respiratory role of such tracheal-filled appendages is hard to determine experimentally and a normal
respiratory function is doubted, though they may serve to excrete carbon dioxide.

Aquatic larvae may have closed tracheal systems, with no spiracles, or open tracheal systems, connected to the outside
via spiracles. In the former, oxygen is absorbed through the cuticle, into the tracheal system (cuticular respiration). This
may be facilitated by tracheal gills - structures filled with highly-branched, often feathery, tracheae that absorb oxygen
across the cuticle. Often these gills are folds in the wall of the rectum (rectal gills) in which case the rectum may actively
pump water across them, or they may be external appendages filled with tracheae. Sometimes these appendages are very
long and bizarre looking, but have sometimes been shown to have no gill function, as their removal may have no effect on
oxygen uptake, in which case they may function only in emergencies, or as an oxygen store, or they possibly act as
flotation devices. In those larvae with functional spiracles, often at one end of the animal, the fluid is only actively removed
from the system once the insect reaches the surface and starts taking in air, often through a snorkel-like siphon bearing
the spiracles, as in mosquitoes.

Cuticular respiration is often still significant in terrestrial insects. Some butterflies absorb a significant proportion of their
oxygen through the large surface area of their membranous wings. Generally, however, little oxygen is absorbed across
the skin of insects.

Circulation

Below: layout of some of the main structures in a large lepidopteran such as Attacus atlas, the Atlas Moth (Giant Silk Moth). PO = pulsatile organ. The dorsal vessel and associated structures are shown in orange; the gut in blue, the dorsal diaphragm in magenta and the ventral diaphragm in green.

The key component of the insect circulatory system is the dorsal vessel (DV), The anterior part of the dorsal vessel is
the aorta, the posterior part the cardiac vessel or heart. In some insects this is the circulatory system, more-or-less, but larger and/or more active insects have much more elaborate circulatory systems.

The circulatory system of insects is open. This means that the circulating fluid is not confined to definite vessels for its whole course (as it is in mammals, except in organs like the liver where the blood flows through channels called sinuses rather than capillaries). In a closed system, the blood flows away from the heart in the arteries and then into microvessels, such as capillaries, which then connect to veins which carry the blood back to the heart. In an open system, the blood is pumped out of the ends of open arteries, into blood-filled spaces called sinuses and then circulates
back to the heart, generally through the sinuses.

In the insect, the circulating fluid is strictly called haemolymph (hemolymph), rather than blood, as it functions as both blood and lymph. The dorsal vessel is pulsatile and pumps blood to the front of the insect, where the aorta opens in haemolymph sinuses in the head. Often the aorta ends in one or more contractile sacs (frontal aorta sac) and pulstile organs which act as additional pumps. Often a pair of antennal arteries, each with a pulsatile organ at its base, supplies haemolymph to the antennae. The antennal arteries are open-ended, expelling the haemolymph into the haemolymph sinus which fills the central axis of the antenna. A pair of optic arteries (which maybe funnel-shaped as in the diagram above) also supply the compound eyes.

The aorta may also supply paired pulsatile organs in the mesothorax and metathorax (middle and last thoracic segments, also called the pterothorax). These accessory hearts pump haemolymph into the wing sinuses - blood sinuses that travel along the wing veins (see insect locomotion for a description of circulation in the wings).

Structure of the dorsal vessel

The dorsal vessel consists of a single layer of muscular cells, forming the tube, sandwiched between two membrane and covered on the outside surface by a connective tissue coat. (In insects connective tissue such as this generally consists of tough sheets or membranes formed by the trachea and tracheolar cells). It is perforated by pairs of slit-like pores, the ostia (singular ostium), one pair per body segment, especially in the abdomen, though the heart may extend into the thorax where they may also be a few pairs of ostia. There may be as many as 13 pairs of ostia, just one, or any number in-between, depending on species. The ostia are equipped with ostial valves - the rims extend into the heart lumen as lip-like valves which close when the heart contracts to prevent blood leaking from the heart. The heart is attached to various structures by radiating fibres (ligaments). It may be attached directly to the dorsal wall (back of the body wall) or via filaments and likewise fan-shaped ligaments or msucles may attach it to the sides of the body wall (terga) or to the pericardial septum (see next section). The fact that the dorsal vessel is dorsal, and close to the body wall, is probably important in thermoregulation. The haemolymph in the dorsal vessel transports measurable heat energy around the insect body. If the insect is basking in the sun, then this heat will be absorbed through the back and then circulated to the muscles of the thorax, brain and other parts, by the circulation. Circulation through the wings is important for absorbing heat in some basking insects, warming themselves up in the morning. Indeed, the wings were preceded in ancestral forms by flat extensions of the top of the thorax which may have served to absorb heat and transfer this heat to the leg muscles. In some insects ostia may also occur at the rear end of the dorsal vessel, though in general only the anterior end is open.

Septa (diaphragms)

In the abdomen is a muscular sheet, the dorsal septum or dorsal diaphragm or pericardial septum, which stretches from side-to-side (connecting to the terga or sides of the insect body wall) and either beneath the heart or joining onto the sides of the heart below the ostia. This sheet may be perforated, allowing haemolymph to pass across, or its may be open only at the posterior end of the abdomen. It may extend into the thorax, though generally in a reduced form. Often, fan-shaped muscles, called alary muscles, fan from the dorsal septum to attach along the sides of the heart (cardiac vessel). The haemolymph-filled cavity (haemocoel) above the pericardial sinus, and bathing the heart, is called the
pericardial sinus.

An additional septum may also be present, the ventral septum/diaphragm which is a muscular sheet covering the
nerve cord (which is ventral in insects). Beneath this diaphragm is the perineural sinus (PNS), bathing the nerve cord. In-between the ventral and dorsal diaphragms is the perivisceral sinus, bathing the gut and other organs.

Pattern of Circulation

The basic pattern of haemolymph circulation, which as we shall see is an oversimplification, is as follows:

• Haemolymph flows forwards along the dorsal vessel and aorta, as they contract by peristalsis (with the ostial
  valves closed) and then squirts out of the front end of the aorta (which may be through the antennal and optic
  arteries or some other arrangement) into sinuses. The peristaltic waves travel from the rear to the front of the
  dorsal vessel.
  
• Haemolymph flows back from the head to the abdomen through interconnected sinuses, passing from the thorax
  into the abdomen through the PNS, and then into the perivisceral sinus and then into the pericardial sinus.
  
• Haemolymph enters the heart via the ostia, when the heart expands (with the ostial valves open).
  

Generally, the heart is closed at its posterior end, delivering blood forwards. However, the nymphs of ephemerids (such as mayfly nymphs) a pair of caudal arteries emerges from the rear of the heart and supplies the three caudal filaments ('tails') and blood in the back chamber of the heart is pumped backwards along these (the direction of flow being controlled by valves).

Role of the Septa

The ventral diaphragm undulates and this is
generally thought to propel the haemolymph
backwards and sideways in the PNS.
However, in some lepidopterans (moths and
butterflies) it has been shown to have at most
only a secondary effect on circulation on the
PNS, but serves primarily instead to mix the
haemolymph in the perivisceral sinus. The
main role of the ventral diaphragm likely
depends on species. (Indeed it is absent in
some insects).

In each leg, a horizontal septum, which may
be continuous with the ventral diaphragm,
extends into the legs, such that haemolymph
circulates into the leg beneath the septum,
reaching the end of the septum in the leg tip,
and then away from the end of the leg above
the septum, having turned the corner at the
end of the septum in the leg tip. (See
diagram opposite).

The heart draws in haemolymph from the
pericardial sinus, though in some insects
there are also ostia underneath the heart
which draw blood from the perivisceral sinus
(with the dorsal diaphragm attached to the
sides of the heart).

The dorsal vessel in most insects is
myogenic, meaning that it beats of its own
accord even without nervous input and so is
automatous (so it will keep beating for a time
even when removed from the animal),
although in the cockroach Periplaneta it is
apparently neurogenic, meaning that
impulses from the nervous system cause it to
beat. However, when left to its own devices it
may beat at the wrong rate. The heart of the
diving beetle, Dytiscus, beats at 30-70 beats
per minute (bpm) when intact and in situ, but
this reduces to 15 bpm when its attachments
to the alary muscles are severed. Tension in
the alary muscles pulling on the dorsal vessel
thus alters and regulates its pulsation rate.

In some butterflies and moths, at least, circulation has been shown to help drive breathing by affecting air sac expansion
and contraction. Circulation and breathing are therefore synchronised. Here we look at this mechanism in detail.

Generally, although all parts of the heart can act as a pacemaker when dissected out, the rear-end is dominant and sets
the overall pace as waves of contraction spread from the rear of the dorsal vessel forwards, along the aorta which is also
contractile. In many insects, however, the heart may periodically reverse its direction of beat, with contractile waves
passing from the front backwards, expelling haemolymph through the ostia. Although this has often been thought of as
pathological, or a means to clear obstructions, in moths and butterflies, at least, this phenomenon has been shown to be
a normal part of the circulation pattern and to be tied to breathing movements. In adults of the giant silk moth, Attacus
atlas, for example, the heart beats in the forwards direction for 4-5 minutes, at a rate of about 30 bpm, and then reverses
direction for about 3 minutes at 18 bpm. This periodic pattern repeats throughout adult life, with some changes in the
timing associated with age. This defines the forward pulse period (FPP) when the heart beats forwards and the reverse pulse period (RPP) when the heart beats backwards.

The FPP/RPP cycle is synchronised with breathing movements of the abdomen, which contracts and expands
periodically, shortening at the beginning of the FPP and then beginning to expand (lengthen) about 30% the way through
the FPP and expanding more rapidly during the end of the FPP (when the heart stops beating for about 10 seconds) and
continuing to expand throughout the RPP. In total the length of the abdomen changes by 1-2 mm. Contraction of the
abdomen pumps air out of the tracheal air sacs and the expansion of the abdomen sucks air into the air sacs. This
movement is mainly brought about by telescoping of the abdominal segments, especially on the underside.

Peristaltic abdominal contractions also occur, superimposed on the steady abdominal expansion just described, at a rate
of about 10 per minute, when the heart is beating backwards in the RPP (see graph below). This occurs in the later part
of abdominal expansion, when the abdomen is fully expanded with air, and gives rise to volley-like pulsations of the
abdomen. Each peristaltic wave causes the abdomen to 'shorten' by bending each segment downwards slightly and
begins at the tail and moves forwards along the abdomen. These peristaltic movements coincide with expiration through
the abdominal spiracles. As each segment of the abdomen shortens there is a delayed closing of the spiracle pair on that
segment, allowing air to be expired at the beginning of the contraction. Expiration through these spiracles does not occur
during the initial shortening of the abdomen, but only during these volley-like movements. Expiration then proceeds from
the rear to the front of the abdomen.

Breathing in the thorax is regulated differently. The metathoracic spiracles (those in the hindmost or third thoracic
segment) do not close for prolonged periods, but show fluttering movements, and expire air during the FPP and inspire
air during the RPP. It is thought that pressure resulting from haemolymph accumulation in the head and thorax, during
the FPP, squeezes the air out of the thoracic tracheae, thoracic air sacs and thoracic spiracles. Similarly, thoracic
inspiration occurs during the RPP when blood is moved from the head and thorax into the abdomen. The thorax is rigid (it
has to be for the attachment of leg and flight muscles) and so removing haemolymph from it causes a suction which fills
the thoracic tracheal system with air. This is aided by the tracheal air sacs, which inflate during inspiration when
haemolymph pressure drops during the RPP, and contract when haemolymph pressure increases during the FPP and
expiration. Thus, periodic changes in heart beat direction fill and empty the air sacs, which act as bellows to bring about
inspiration and expiration. Thus, haemolymph circulation helps drive breathing in the thorax.

A valve and vertical septum, consisting of fat cells (independent of the main fat body) separates the abdominal and
thoracic haemolymph cavities. This is the valve plate labeled on the diagram, and occurs in at least some moths and
butterflies. This probably means that the abdominal contractions affect pressure changes only in the abdomen and
probably restricts haemolymph exchange between the thorax and abdomen to the PNS.

Two pairs of accessory pulsatile organs (PO) one in each of the mesothorax and metathorax (see diagram) occur in
butterflies, such as Papilio machaon. These pulse at their own rates, not in synchrony with the heart, but pulse mainly
when ventral flow of haemolymph from the thorax into the abdomen has paused.

Backwards (and sideways) flow of haemolymph in the PNS, and crossing from the thorax into the abdomen, peaks during
the last third of the FPP. This makes sense, since the dorsal vessel is delivering haemolymph to the head, which flows
back filling the thorax and then flows back into the abdomen through the PNS. This backwards flow slows during the RPP
and may not resume until part-way through the FPP, when sufficient pressure has been generated by the forward-flowing
haemolymph. Thus haemolymph transport in the PNS is periodic.

The frontal air sac in this insect beats in synchrony (though with only 10% the magnitude) with the heart, during the FPP,
expelling haemolymph into the head sinuses, antennal and optic arteries. It appears to stop beating during the RPP.

Circulation in the Wings

Circulation in the wings of insects has only been demonstrated in a few species. However, Wigglesworth (1972) noted: 'Circulation in the wings of insects was observed by Henry Baker in the grasshopper as early as 1744, and is probably universal. ... This circulation is necessary for the normal sclerotization of thw ing and for maintaining the wings in a healthy condition; parts deprived of circulating haemolymph become dry and brittle and often crack away.'

The haemolymph flows between the tracheae and the walls of the wing veins that enclose them. As a general rule, haemolymph enters the wing by the leading costa vein (see below) and leaves via the posterior margin. the pattern of flow in the larger veins is fairly constant, but flow varies in the smaller veins. This flow is driven mainly by pulsatile organs in the thorax but may be assisted by further pumps in the wing itself.

For example, Dragonfly nymphs and some beetles have been shown to have such pumps in the middle and rear thoracic segments (meso- and metathorax) and in the mesothorax of the fly Musca, for example. These are shown as the 'mesothoracic PO' and 'metathoracic PO' (pulsatile organ) in the diagram of the lepidopteran circulatory system of a moth above. These thoracic pumps consist of a muscular membrane or plate attached by elastic fibers to the dorsal wall of the insect, enclosing a haemolymph space in between the membrane and the dorsal wall and is often situated in the scutellum of the insect.The haemolymph is pumped out from this space through a valved orifice and into the dorsal vessel via a connecting vessel (in the Spinx Moth, Sphinx convolvuli) or directly into a dorsal loop of the dorsal vessel as in Bombyx mori (or in some insects simply discharges into the body cavity as in Musca). The space also connects to the posterior wing margins and when the pump relaxes and expands it sucks blood through the venous network of the veins. Blood enters the anterior veins of the wing from intermuscular haemolymph cavities in the thorax.

As already stated, additional wing pumps may occur, there are four in each wing in the fly Musca, for example, along certain wing veins. There are five such pumps in the basal part of the wing of the Fruitfly Drosophila. It is not clear whether these wing pumps contract actively or whether they rely on elastic recoil. These various accessory pumps apparently work independently of the dorsal vessel. The dorsal vessel itself beats at different frequencies according to  temperature, level of activity, stage of development of the insect (beating slowly in the pupa for example) and may almost stop completely during winter dormancy. Thoracic pumps respond in the same way but may beat at a different rate to the dorsal vessel and may beat very fast when the insect is preparing to fly. Circulation of the wing is known to increase during flight in at least the German Cockroach, Blatella.

Circulation in the wing is intimately linked to respiration. The thoracic pulsatile organs are most active during the periods of reverse dorsal vessel beat. When the pulsatile organ(s) suck blood out of the wing veins the air-filled tracheae in the wing veins expand under the negative pressure, drawing air into the wing.

Thermoregulation

The presence of circulating blood in insect wings raises the obvious question of the role of such circulation in thermoregulation. Insects, especially large insects, require warmth in order to fly: if their muscles are too cold then they can not generate sufficient power. Insects can absorb heat from visible light and infra-red radiation. Although the body temperature of insects generally fluctuates in rapid response to changes in ambient temperature, insects can regulate their body temperatures to some extent. Aedes mosquitoes, for example, will bask in the parabolic flowers of Dryas (a dwarf shrub of the Rose family) which always face the sun. The coat of hairs or scales in moths increase the temperature of the thorax by a few degrees by improving insulation.

The Silver-washed Fritillary, Argynnis paphia, regulates its temperature by opening their wings to a varying extent with their surfaces directed towards the sun and can maintain an internal temperature up to 27^o C above ambient and they try to maintain an optimum body temperature between 32 and 37^o C.

Below: a male Orange Tip Butterfly (Anthocharis cardamines) resting and apparently thermoregulating by adjusting the spread of its wings.

Circulation in the legs

In the aquatic insect Notonecta (The Backswimmer) a vessel or dorsal blood sinus runs up each leg and just below the knee (at the top of the tibia) a muscle wraps around it. this muscle contracts at intervals, driving the blood along the vessel and towards the thorax, through a valve and into the main body cavity while sucking blood from the thorax into the ventral sinus of the leg. In at least some insects, movements of the legs themselves help to circulate haemolymph.

Further reading

Chintapalli, R.T.V. and Hillyer, J.F. 2016. Hemolymph circulation in insect flight appendages: physiology of the wing heart and circulatory flow in the wings of the mosquito Anopheles gambiae. J. of Experimental Biology 219: 3945-3951. doi:10.1242/jeb.148254

Wasserthal, L.T. 1981.  Oscillating Haemolymph 'Circulation' and Discontinuous Tracheal Ventilation in the Giant Silk Moth Attacus atlas L. J. Comp. Physiol. 145: 1-15.

Wasserthal, L.T. 1982. Antagonism Between Haemolymph Transport and Tracheal Ventilation in an Insect Wing (Attacus atlas L.) - A Disproof of the Generalized Model of Insect Wing Circulation. J. Comp. Physiol. 147: 27-40.

Wigglesworth, V.B. 1972. The Principles of Insect Physiology, Seventh ed. Chapman and Hall, London and New York; University Press, Cambridge.