The mosquito is one of the most loathed of all insects, partly because of the itchy bites they cause, but also because of their deadly role as vectors for disease, especially malaria.  In 2021, according the the WHO World malaria report, there were 247 million cases and 619,000 deaths from malaria.  In the past I painted a quick  (slightly unsatisfactory) sketch of a mosquito.  However, it’s a subject I’ve been wanting to return to for many years, and recently I’ve managed to find time to illustrate a life cycle based on Anopheles gambiae, one of the most efficient spreaders of the most lethal strain of malaria.  Throughout researching this blog, I found the pages on Malaria from the Centres for Disease Control and Prevention (CDC) invaluable.

Previous mosquito sketch

Problems and caveats

I’m not an epidemiologist, or even an entomologist.  I found, during research, a lot of  information on Anopheles mosquito species, although not an enormous amount on differences between the species. Much of it  is very technical.  So although the life cycle I’ve illustrated is accurate for an Anopheles, I can but hope it’s more relatable to Anopheles gambiae than to other similar species.  I also flirted with the idea of including the life cycle of the Malaria-giving parasite, Plasmodium falciparum.  But the amount of detail required to do it justice (it has two hosts and goes through a whole gamut of different life stages) means I’ve had to shelve it for a rainy day.  (For more on the Plasmodium parasite life cycle from Research Gate, click here.)

What is a mosquito?

Mosquitoes are flies, members of the Diptera. They’re cousins of the midges (Chiromonidae) and crane flies (Tipula).  Visually, they differ from the similar-looking gnats and midges in the angle at which they hold their legs, Chironomids holding them forward, mosquitoes holding them outward.

Chironomid midge adult Micropsectra radialis

In defence of mosquitoes

Please bear in mind that the vast majority of mosquitoes are not killers.  Only some of the known 3,500 plus species of Culicidae (Mosquitoes) feed on blood, of those only some target mammals (including us humans).  Most blood-feeders take blood from other mammals, some from birds, and even snakes can be the chosen prey.  In fact, some of the large Toxorhynchites mosquitoes feed on other mosquito larvae!

Only female mosquitoes take blood meals.  Males feed on nectar from flowers, and fruit juice.  So do females, unless preparing to lay eggs.  Although you may encounter male mosquitoes, buzzing and whining, they are incapable of, and have no interest in biting you.  (You can tell male mosquitoes as they tend to have far more elaborate and feathery antennae than the females.)

In defence of  female mosquitoes

Even female mosquitoes need to be better understood.  As with every living thing, they’re not out to get us, or planning to give us disease.  A female needs a blood meal in order to fully develop and lay her eggs.  She may well only take one big blood meal, and that may be at one sitting.  Repeated bites happen when she’s consistently interrupted, having to return over and over again to her meal.  It’s rather like a human being taken away from a pizza mid meal.  Then having to start the whole procurement process from scratch each time you try to have another bite.

And as for being a vector of disease, again, this is highly unfortunate for us, but in no way an evil master plan of the insect.  In fact, the Falciparum parasite damages mosquitoes too, compromising their gut lining and having to evade the immune system of these dipterans (PNAS Vol. 112 No. 5)

I know I’m fighting a losing battle here, and the havoc, damage and heartbreak caused by Malaria and other mosquito-borne diseases is absolutely massive.  I’m just pointing out that only some mosquitoes cause these illnesses, and many are harmless pollinators.  I think that as a whole, they deserve respect, and a kinder reputation.

Pen and ink diagram showing the amazing feeding equipment of a female mosquito

To see footage of the mouthparts of a feeding mosquito in action please check out this NPR video.

Anopheles: One Genus of Mosquito

Anopheles, the genus I’m illustrating and the one responsible for malaria, are Marsh mosquitoes,  They are one type of these biting fly; others are the Culex and Aedes.  Although all three are able to carry disease, injecting directly into the bloodstream, only Anopheles can transmit malaria.  Even more specifically, of the 460 known Anopheles species, only 100 can give humans malaria.  Of these, only 30 or 40 carry the most lethal form of the disease, Plasmodium malaria.  And of these, it’s only the females who can infect you, as they’re the only ones that feed on blood. (Centres for Disease Control and Prevention (CDC).)

Life cycle of Anopheles gambiae

Other mosquito genus: Culex

Culex are also vectors for disease, although not malaria.  They can carry West Nile virus, Japanese Encephalitis, and Elephantisis.  None of these is pleasant, and all can be fatal.  Unlike our Anopheles, their eggs are without floats (although laid singly). Larvae hang down at an angle from the water surface with their long breathing tufts breaking the meniscus.  Pupa are similar to Anopheles and Aedes species.  The adult females hold their proboscis at an angle to their bodies (rather like in the first illustration in this blog).

Other mosquito genus: Aedes

Aedes mosquitoes carry Yellow fever, Chikungunya, Zika fever, and Mayaro.  They’re easy to spot thanks to very striking black and white striped legs.  They also have a marking a bit like a harp on their thorax.  Again, only the females are disease vectors.  Aedes lay their eggs in a mass on the water surface, not singly.  Their larvae are like Culex, hanging down at an angle from the water surface and breathing with a tufted breathing tube (longer than the Culex species).  Pupa are similar to Anopheles and Culex.  Adults hold their proboscis at an angle to their bodies, which is similar to Culex species.  However, they tend to stand higher on their legs than Culex.  They may also lift their rear legs up.

Anopheles gambiae

This mosquito forms the basis of my mosquito life-cycle.  According to the CDC,  “Anopheles gambiae is the most efficient vector of human malaria in the Afrotropical Region (CDC 2010). Thus, it is commonly called the African malaria mosquito.”  And this is why it’s my mosquito of choice.  It lives in a band across the African continent, throughout sub-Saharan Africa and up to areas of southern Arabia.

Unfed female mosquito with paints and reference

The main features that distinguish it from other Anopheles relate to placement of hairs on the larva, and a greenish hue to the pupa.  Adults have pale bands on their palpus (mouthparts), spotted wings, and sparse speckling on their legs.  The abdomen doesn’t have little hair tufts on its sides (postlateral scale tufts).  There are often no abdominal scales on the mosquito back; if they are present they’re small.  For a splendid overview of mosquito morphology, please see the pdf from Nathan Burkett-Nadena of Florida University’s Entomology department.

Details of the life cycle of  A. gambiae come from the University of Florida’s “Featured Creature: African Malaria Mosquito”; and the Walter Reed Biosystematics Unit.

Life cycle of A. gambiae: Eggs

Eggs are laid on the surface of clean water, in batches of 100 – 250.  Each one is laid singly, not as a raft.  Each is boat shaped, slightly domed, and supported by a system of lateral floats.  They’re brownish in colour and are about 0.5mm in length.  Eggs hatch after 1 or 2 days into free swimming larva.

Eggs of A. gambiae, seen from above

Life cycle of A. gambiae: larva

Larva grow from their first stage, after hatching, into a final size of 5 – 6mm long.  They moult repeatedly as they grow, passing through 4 instar stages before pupating.  Lying just below the water surface, they get oxygen through a rudimentary breathing tube which sits just before their tail end and a brush of hairs.  This is true for all Anopheles species; their short breathing tubes mean they all assume this distinctive posture, parallel to the water surface.  Other abdominal hairs help aid in floating.   They feed on algae and organic matter, and can be seen wriggling in still water with the naked eye.

Larva of A. gambiae

Life cycle of A. gambiae: Pupa

Comma-shaped and tending to rest at the surface of the water, all mosquito larva look similar.  They have a large cephalothorax (combined head and thorax) which is clear, or slightly coloured.  Their abdomens are segmented and thin, terminating in a tail fin.  They use this to rapidly swim away and escape when disturbed, which is an unusual ability in the (normally static) pupal form of an insect.  A tuft of hairs at the beginning of the abdomen helps the pupa stick to the water surface.  You can see the antennae and limbs growing within the cephalothorax, and eyes are large and obvious.  Pupa breathe with a pair of respiratory trumpets which project from the back of the cephalothorax, into the water meniscus.  They do not feed in this stage, which lasts another 2 – 3 days.

Pupa of A. gambiae

Life cycle of A. gambiae: Adult female

I’ve chosen to illustrate the female in unfed and in feeding poses.  This is to show the massive difference that a good blood meal can make to the mosquito’s appearance.  After a meal, the abdominal plates are stretched and expanded by the contents of the gut.  Before, the grey mottled markings are far easier to see.  The mosquito is grey to pale brown, dotted with pale yellow areas, cream-coloured scales.

Unfed female African Malaria mosquito

The unfed female has the typical angled pose of a resting Anopheles, with body held in a diagonal in line with the mouthparts.  As with many mosquitoes, her legs are held aloft.

A. gambiae have distinctive dark wing patches.  These occur at specific areas along each wing, which is tricky to capture in illustrations showing the wings folded and overlapping.  Wing margins are fringed with small hairs.  Being a smallish species, A. gambiae have wing span of about 3 – 4.5 mm.

Distinctive mottled wings of A. gambiae

  Another feature that helps identify this species is the striping of the palps, which are banded in a specific pattern and have white tips.  These organs sense odour, so are pretty prominent in mosquitoes who use smell as one way to find their prey.

Palps of A. gambiae

Life cycle of A. gambiae: Adult female and feeding

Mosquitoes are crepuscular and find their prey by sensing exhaled CO2, body temperature, and vibration.  Evidence suggests humans with blood type O, who breathe heavily, are pregnant, or who have a heavy load of skin bacteria may be more prone to mosquito bites; although there are genetic factors at play too.

Anticoagulants prevent the blood from clotting as she feeds, and the itch we feel comes from out our immune system’s response – the production of histamines which lead to inflamation and itching.

Feeding African malaria mosquito

The fed female holds the same pose as an unfed female at rest, although they often lower their bodies closer to the horizontal when feeding or when weighed down with blood.  The droplet at the tip of her abdomen occurs when excess water is exuded to make way for more solid parts of the continuing blood meal, but also has a role to play in reducing body temperature via evaporation (Current Biology Lahondere & Lazzari 2012)

Droplet of exuded water

Once fed, the female mosquito goes to digest her meal, laying eggs a couple of days later and thus continuing the cycle.

Conclusion

It’s no surprise that there’s an enormous amount of information on mosquitoes and malaria online.  Wading through it has been a delight as well as a headache. and the temptation to fall down endless fascinating rabbit-holes relating to disease prevention and control, or to the mechanisms of the mosquito mouth parts, has been hard to resist.  I nearly got diverted by Plasmodium falciparum, which I swear I remember illustrating for a project at college.

Completed mosquitoes with some reference and art materials

However, I’ve tried to retain focus, and stick to Anopheles gambiae.  How it lives, what it looks like in its’ different life stages, and why  it matters to us as humans.

For a good spring board, if you want to learn more about mosquitoes, you could do worse that check out the Wikipedia entry on mosquitoes.  And that’s not something I say every day.

I hope you’ve enjoyed visiting these biting flies with me, and haven’t ended up feeling itchy or listening out for that high-pitched and un-mistakable whining sound…

Illustrating the mosquito compound eye

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Introduction to the Euglena

It is found in fresh water (often in puddles or ponds) and differs from ameoba and paramecium in being able to photosynthesize, and so produce its own source of food.  The euglena appears green due to the chloroplasts in its cytoplasm, sites where photosynthesis occurs.  In reality, these can often appear clearer and larger than in my diagram, like green rods.

The euglena can also surround and take on food through phagocytosis (see my blog on ameoba) so is not entirely autotrophic (producing its own food).

Movement in Euglena

Euglena moves by thrashing its whip-like tail or flagellum.  In fact, it has two flagella, one far smaller (not shown).  These produce a spiral, helicoidal motion which casues the euglena to spin along as it moves forward.  To see euglena moving, and how bright green they are, have a look at Craig Smith’s euglena video.

The eye of the Euglena

The stigma is a visibly red eye-spot, its colour coming from caratenoid pigmentation.  This little spot will cover up a photo-receptive area (the paraflagellar body) at the base of the flagella periodically, causing the euglena to change position until the photoreceptor is exposed again.  This cunning process means the euglena can sense where light is, and move towards it.  Obviously, in an organism that requires sunlight to photosynthesize and thus feed itself, moving towards light is paramount.

As with amoeba and paramecium, the unicellular euglena needs to regulate water levels within its body.  It uses a contractile vacuole for this, which expells excess water and thus ensures osmoregulation.  As with paramecium, there are radiating canals, functioning like drainage paths, leading to the vacuole.  Without a contractile vacuole; euglena, ameoba, and paramecium would absorb too much water through osmosis and would explode.

Euglena store carbohydrates in their bodies in the form of paramylon or paramylum granules, similar to starch.  These reserves vary in size and act as food reserves.

Their nucleus controls all the life functions; feeding, growth, regulation, digestion and (within the nucleolus inside the nucleus) reproduction.  Euglena can only reproduce asexualy, through binary fission,  The nucleus divides mitotically and then the cytoplasm will split longditudinally.

I’m no expert on micro-organisms, and in fact when I was at University they were all still bundled together into the now obsolete phylum of “protozoa”.  Please do let me know if there are any mistakes that need fixing, and I’d be delighted to fix them.

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Introduction to the Paramecium

Like the amoeba, the Paramecium is an independent, free-living unicellular organism. Paramecium can be found in abundance in stagnant water where they feed off tiny bits of plant, and bacteria.  The largest are only 1/2mm long, so to examine them a microscope is required.

They’re part of a phylum called Ciliophora which refers to the cilia, or tiny “hairs”, found all over their bodies.  Each row of cilia beats in a wave-like metachronal rhythm.  They are used to propel the animal through water.  As these rows of hairs spiral around the body, the Paramecium spins as it swims.

The cilia protrude from a flexible yet stiff cell membrane, called a pellicle.

Anatomy of the Paramecium

At the top of the diagram you can see the contractile vacuole.   There’s one at either end of the paramecium; unlike amoeba, paramecium have a consistent body shape.  These vacuoles make sure there’s not too much nor too little water inside the animal’s cell membrane.  Spreading out from these vacuoles are radiating canals which provide paths to the vacoule.

A paramecium body is full of cytoplasm which maintains organelles.

Paramecium feeding

Paramecium eat through an oral groove, which directs food into the paramecium’s mouth (or cytosome).  Food is brought to this area by ciliary action; once at the mouth it’s packaged into food vacuoles.  Here, the food is digested as it circulates in the animal’s body.  See this process in action below.

Nucleus of the Paramecium

The macronucleus programs and runs the paramecium; it controls digestion, growth, movement, and other cell functions.

The micronucleus is separate and controls reproduction.  Paramecium can reproduce asexually ( up to two or three times a day) by dividing with binary fission.  Less usually, they reproduce sexually through conjugation. This involves fusing material from two micronuclei, which undergo meiosis before three of the four resulting nucleii disintegrate.  The remaining nucleus undergoes mitosis. Daughter nuclei join before the cells split apart. The original macronucleus disintegrates, and is replaced with a newly formed one. After this, asexual reproduction normally occurs.

Paramecium can defend themselves!

Paramecium can defend themselves with the use of trychocysts, similar in function to those found in the jellyfish and sea anenomes.  These trychocysts are needle llike projections that can be ejected out from the cell mebrane.  Below are some trychocsyts in action:

More can be found at 101Science’s site where I did some research.

Next week: the Euglena!

Although I love micro-organisms I am certainly no expert, so please get in touch if you spot any errors, I’d be more than happy to fix them.

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Amoeba: an introduction

Amoeba are free-living aquatic creatures, and can be seen in any drop of standing fresh water, if you put it under a compound microscope.  They also occur in salt-water, and as parasites within other animals (such as humans).  The largest is a little under 1mm in size. They change shape by oozing outwards into pseudopods, and use these to engulf food.  So these pseudopods surround foodwith a water droplet, then take it into their bodies to digest. These packets of food are called food vacuoles. The engulfing process is known as phagocytosis. For a video showing this process please click on the link: Ameoba eats two paramecium.

Anatomy of the Amoeba

Amoeba aren’t blue.  (Interestingly, another micro-organism called Stentor certainly IS blue).  I used the colour to show the amoeba’s features.

Although they always change shape, amoeba seem to have a  definite end, which is known as its uroid.

Their contractile vacuole is used to regulate the amount of water in their body, and is near the uroid.

The nucleus is where information and genetic material is stored; it controls reproduction, eating, and growth.

Pseudopodium are organelles which “walk”.  They extend from the body, then cytoplasm flows into them, and the amoeba is propelled forward.

The amoeba body is full of cytoplasm.  This is clear plasmagel (seen toward the edges of the pseudopods, just below the cell membrane); or it is granulated endoplasm (further inside the body).

Amoeba breathe by diffusing oxygen through their cell membranes.  They reproduce asexually by dividing their cytoplasm and nucleus through mitotic division, in a process called binary fission.  This produces two identical daughters.

Although I love micro-organisms I am no expert, so get in touch if you spot errors, I’ll be more than happy to fix them.  Next week: the Paramecium! And for the third micro-organism, I’ll be looking at the euglena.

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