The Ethology of Anopheles gambiae, the Vector of Malaria

ABSTRACT 

In choosing an insect subject for this full review article on its' behavior, I took into consideration which species 
have been the most devastating to Homo sapien as the criteria in my selection. Grasshoppers, locusts, fleas, 
and killer bees quickly come to mind, but the annoying mosquito became the subject of my study. Specifically, 
the Anopheles gambiae of tropical Africa, known as the vector of malaria. Much has been written on this 
member of the order of Diptera. In this article I will attempt to describe in detail the identification, lifestyle, 
lifecycle, habits, and behavior of this mosquito both male and female, although it is only the female that will 
attempt to draw a blood meal from Homo sapien. This particular insect is infamous throughout tropical Africa 
as a prevalent killer and debilitater of human life, specially to the Caucasian Europeans who have numerous 
times tried to successfully colonize western and central Africa in failure; due to the disease of malaria the 
insect transfers with her bite. I will also describe the general ecosystem in which A. gambiae thrives, comment 
on the patterns that evolve when nonresistant Homo sapiens enter into that system, and relate the 
consequences of A. gambiaes' ethology clashing with the ethnology of H. sapiens. History has shown that 
millions of people have died due to malaria, and that countless others have suffered from the effects of the 
disease. This insects' role in the the course of human events, and its' influence in the natural selection process 
of H. sapien commands my visitation, not only into the workings of its' behavior, but also to options and ideas 
of controlling this infectious vector. The reader can take this review on the ethology of Anopheles gambiae as a 
general guide for identification and a detailed description of its' behavior, especially when interactive with 
Homo sapien.


KEY WORDS: Anopheles gambiae. Homo sapiens. behavior. malaria. vector. 

INTRODUCTION:

1. Historical Background of A. gambiae: 

Throughout history descriptions of malaria appear in early medical texts of various civilizations. For the 
majority of this time, it was unknown that mosquitoes were the carriers of this disease. Instead, for lack of 
knowledge it was blamed on "bad air". Hence it was named malaria by the Romans, which in Latin means bad air. 
The Greek physician Hippocrates, also remembered as the Father of Medicine, wrote about the disease in the 
fifth century, dividing its associated fever into different types. Malaria is suspected to have contributed to the 
decline of both the Greek and Roman empires, sapping the vitality of the people and destroying their stamina. 
The Pontine Marshes southeast of Rome were notoriously known to travelers as a place of the bad air. It was 
more likely a large breeding ground for the vector of this debilitating disease; Anopheles gambiae. Also, when 
the Europeans tried to settle on the west coast of Africa in the seventeenth century they suffered high 
mortality rates, so much so to name the whole region "The White Mans' Grave". Specific examples can be found 
in the annuals of the Tuckey and Parks' expeditions of the early 1800's, where the effects of malaria from the 
bite of A. gambiae prevented successful occupation of tropical Africa at that time.

2. General Identification of Anopheles gambiae: 

Anopheles gambiae is of course, a mosquito belonging to the order Diptera which means "two wings" in Greek. 
This order is considered one of the most important to humans, disease-wise. Among the 90,000 species are 
other important vectors of disease: the Tsetse fly; the carrier of the deadly sleeping sickness, and Aedes 
aegypti; the insect responsible for the fatal yellow fever. An A. gambiae adult is usually 4-5 mm. in body 
length colored dark brown to nearly black. Its wings have blackish scales and are usually spotted lighter. These 
are characteristically folded back parallel to the body when at rest. Anopheles mosquitoes can be differentiated 
easily from other types by the way it positions its body tilted at an angle to the surface when at a landing 
pause, i.e., it almost stands on its head. In the remarks of my professor, "you can tell its a bad dude" because 
of this unique positioning. The males have a feathery antennae resembling Christmas trees which enable them to 
sense females, but the females only have a few short hairs for antennae. The long maxillary palpi are distinct 
for the species also, extending into a three pronged pitch forked arraignment forward from her face. Another 
less distinctive feature is that the scutellum is usually rounded as opposed to trilobed in other species.

3.Ecosystems Fertile for A. gambiae- Africa: 

A. gambiae, as well as other mosquitoes, live and die near water, preferably clean, still water. Marshes serve 
well as do anything that can hold water after a rain. It has been shown that water-filled footprints and old 
discarded automobile tires have been utilized by the species. Generally, Anopheles' distribution is mainly in the 
humid tropics of the world between 40 degrees N. to 45 degrees S.; this includes the African continent. 
Habitats more near to median of this range allow the species to breed year-round, accentuating its debilitating 
effects to humans existing in the area. They are especially prevalent in the central part of the continent near 
the rain forests fed by the tributaries of the Congo and Niger rivers, although mosquitoes in general exist in 
almost every habitable place on Earth. The male of the species A. gambiae, which is yet unable to blood feed due 
to its lack of suitable mouthparts for that purpose, instead feeds on nectar and plant juices. As one would 
summize, the rain forest is abundant in both those sources.

DISCUSSION

A. Lifecycle Overview: 

A. gambiae undergoes the process of complete metamorphosis from egg to adult. the female deposits her brood 
of eggs in numbers from 30 to as many as 500, and she'll lay from 8 to 10 broods before she dies. Laying her 
eggs singly in the water, these floating eggs hatch into larval wrigglers within a few days to lie parallel to the 
surface, breathing under the water through rudimentary breathing tubes located at the tip of their abdomen. In 
about a week the larvae have gone through four instar stages and pupate resembling the an adult. the pupae are 
very active and are sometimes called "tumblers" due to their characteristic movements. After a few more 
days, the pupae develop into flying adults, which live, eat and mate relatively near the water they've emerged 
from. All in all, the mosquitoes' entire lifecycle encompasses about 26 days. In her short lifespan, a female 
producing an average of 200 eggs per brood, and laying 9 broods per life, can give rise to 1800 offspring 
before she dies.

B. Foraging Behaviors- Larvae: 

The larvae are called wrigglers. They have large heads with long antennae, a large swollen thorax, and a 
cylindrical abdomen with a dorsal breathing tube tube or plate on its eighth segment. Most larvae feed on 
organic debris, algae, and other larvae coexisting within their watery habitat. This variety of diet is a key 
factor in the species ability to survive; the feeding on different foods precludes intense competition. The 
dominant microorganisms of the surface microlayers are bacteria. These organisms, and also algae, secrete 
exopolymers and other dissolved organic matter which the larvae ingest (Wotten,1996). Anopheles larvae 
often occur around fallen vegetation, algal filiments, cyanobacterial mats and where surface microlayer 
bacteria are especially plentiful (Wotten, Chaloner, Yardley, Merritt 1997). The presence of these components 
in the larvae's' diet substantially effect its growth. Larval feeding patterns involves a complex pattern of 
behaviors, including the generation of water currents by rapid movements of the lateral palatal mouth brushes 
(Merritt et al 1992b). In 5 to 7 days the larvae pupate their morphology resembling the adult. In this phase 
commonly coined the tumbler, the pupae make characteristic tumbling movements through the water, being 
free-living and active. These pupae have their breathing tubes located on the thorax as opposed to the larvae, 
and within a few days they emerge as flying adults with very different foraging strategies.

C. Foraging Behaviors- Nectar: 

The male Anopheles gambiae exists on nectar and plant juices, and these are, but not entirely included in the 
diet of the female. Mosquitoes in general, commonly feed on nectar and other sugar sources, which they locate 
chiefly by visual and chemical cues (Foster and Handcock 1994). A summery of current evidence indicates that 
the nectar sources are not as attractive as blood sources to the females at specific times in their life, but that 
sugar feeding is usually necessary and more frequent than bloodfeeding. Sugar meals provide the necessary 
energy for both survival and flight during the mating period and until the female needs to find a vertebrate host. 
The sugar sources of the central African A. gambiae includes: flowers, extrafloral nectaries, homopteran 
honeydew, aging or damaged fruits, healthy and damaged plant parts, and sometimes the regurgitates from 
ants. This wide variety of food sources is yet another key factor in the species successful survival. Studies 
indicate that both sexes are nearly equal in their utilization of sugar and that the male take it moderately more 
often; a consequence of their intense swarming. The males emerge from the waters first, staying closer to the 
breeding site, foraging for sugar to survive, which is abundant in the dense rain forest, particularly after the 
rains. It is interesting to note that the most commonly searched for flowers are pale colored or white 
(Gadawski and Smith 1992), due to the fact that pale objects reflect more light at night in contrast with the 
green vegetative background.

D. Mating Behavior: 
Males tend to swarm near both the breeding waters and floral food sources, which usually go hand and hand, 
and probably never travel further than one kilometer from that site. In a group, the swarming males are 
thought to attract females to the swarm by their characteristic pitch created by wingbeats. The use of sound is 
an essential component in the mating behavior of most mosquitoes and many related flies with similar adapted 
antennae. The male A. gambiae beats his wings about twice the rate of the female of the species. This results in 
a clearly audible difference between the sounds of the sexes, an obvious adaptation to the use of sound of the 
female to attract males (Belton, 1994). Detecting the lower wingbeat of the approaching female, a male may 
fly towards her and clasp on to her in mid-air. Copulation takes only a few seconds or more and they release 
from their nuptial flight. As a fairly general rule, mating takes place before the female has had its first 
bloodmeal. and one male can fertilize more than one female. The single mating will provide sufficient sperm for 
the life of the female. Because the female will have several gonotrophic cycles, the insemination ordinarily 
takes place before the cyclic activity begins; sometimes within an hour after the pupae has emerged from its 
sunlit pool of water as a flying adult.

E. Bloodfeeding Behavior of Females: 

Once the female Anopheles has been inseminated, she will seek out a blood meal necessary for the development 
of her eggs. Much has been considered on the subject of bloodfeeding; once it was thought that the insect chose 
a certain bloodtype, or even a certain skin color of H. sapien as its preference. It has been accepted that the 
most successful vectors of malaria do preferentially feed on humans, and secondary on cattle and other 
domestic animals depending on host availability (Garrett-Jones et al, 1980).Similar more recent results were 
confirmed in Kenya (Mbogo, Kabira, Muiruri, Nzovu, Ouma, Githure, and Beier, 1993). The fact is, Anopheles 
gambiae likes to feed on H. sapien, and probably has for a long time. Odor may be an important guide for 
mosquitoes in searching out their prey, acting both as an attractant and an excitant. Anopheles, if given the 
choice will chose an unwashed person over a clean one, and be attracted to dirty clothes rather than clean 
clothes. The characteristic smell of human odor is likely to be a principle determinant in preferential host 
selection. Some studies suggest just that; that human sweat excites and attracts the mosquito (Cork and Park, 
1996). It is believed that mild air currents play an important role in carrying and directing these attractants. 
Heat and moisture are important also, inducing many mosquitoes to alight and to probe. A probable theory 
includes that blood from skin capillaries is more easily distinguished as a person sweats. Temporary increases 
in attractiveness also may occur with febrile persons. These feverish people are less able to defend themselves 
and may be more likely to attract mosquitoes because of the rise in body temperature which would increase the 
output of suspected attractants (Lindsey and Armstrong, 1993). Once bitten, H. sapien runs the risk of being 
infected, and if he or she hasn't killed the insect with a reactionary swat she may fly off to bite again.

F. Graviditv and Oviposition: 

Generally after bloodfeeding, the female A. gambiae usually produces a brood of eggs, the process known as 
gonotrophic concordance. Recently, it has been found that the smaller females require multiple bloodmeals in 
order to produce their first batch of eggs (Lyimo and Takken, 1993). These smaller females also take longer to 
achieve reproduction, produce fewer offspring, and run the risk of picking up and transmitting the deadly 
malaria parasite at an earlier age. The female A. gambiae prefers clean, sunlit pools of water to lay her eggs, 
usually after a rain. She may hover over the surface when laying, or even rest on top the water to oviposition. 
The temperature of the water in which this happens is, for the most part moderate, and extreme temperatures 
inimical to life are exceptional; for example, shallow pools exposed to constant sunlight. The lethal temperature 
of ambient air for A. gambiae is 104 degrees F. The favored habitats and breeding waters are therefore, those 
in which the insects are protected from extremes of heat.

G. Aestivation: 

A number of Anopheles mosquitoes have a resting phase known as aestivation. This is similar to hibernation 
which enables them to survive adverse seasonal climatic conditions, and normally it is a fertilized female found 
in this phase. The site selected is usually a dark and cool place, like caves. The female becomes immobile and 
doesn't bloodfeed, her oviposition activity is completely suspended during this time. In this state A. gambiae 
may accumulate fat deposits within her to be used later as a source of energy in flying and oviposition. In 
Africa, females in this state are quite able to survive throughout the adverse dry season.

H. Natural Predators and Defense: 

Mosquito eggs appear to have few natural enemies, probably due to the relatively short period of the insects' 
lifecycle is taken up by this stage. Aquatic fowl are reported to dine on them, as well as small fish and snails. 
Larvae and pupae are another story. They 're being in the water a longer time than eggs, and being substantially 
larger, invite more predators. The Tiger Beetle has been recorded preying on larvae in Ghana, while Anthomyid 
flies of the genus Lispa attack pupae at the surface. Interestingly, many surface feeding minnows show a 
preference for larvae, and Gambusia is also used as a biological control in some areas. It appears the only 
defense for larvae and pupae is to allow itself to sink to the 
bottom of its pool until the danger has passed. Even a shadow will cause
this reaction. Adults are preyed upon by many natural enemies. When flying they are subject
to attacks from birds, bats, and dragonflies. At rest, the may become a lizard, bug, or a
spiders' meal. When ovipositing they can be attacked by toads and frogs. Pondskaters love to
eat both pupae and adults. As would be expected, most enemies seek out A. gambiae at dusk
when the mosquito reaches their peak in flying. Little is known of any avoidance strategies,
except maybe in the act of swarming. Safety in numbers is the theme, where an individual 
mosquito is less likely to be attacked, statistically.

REFERENCE CITATIONS:

1. Foster, N.A. and Handcock, R.G. 1994 Nectar-related olfactory and visual attractants for mosquitoes. 
Journal of the American Mosquito Control Association, Inc. 10 (2): 288-296.

2. Gadawski, R.M. and Smith, S.M. 1992 Nectar sources and age structure in a population of Aedes provocans 
(Diptera: Culicidae). Journal of Medical Entomology 29 879-886.

3. Belto, P. 1994 Attraction of male mosquitoes to sound. Journal of the American Mosquito Control 
Association, Inc. 10(2): 297-301.

4. Garrett-Jones, C., Boreham, F.L., and Pant, C.P. 1980 Feeding habits of anophelines (Diptera: Culicidae) 
Bull. Entomol. Res. 70: 165-185.

5. Mbogo, C.N.M., Kabiru, E.W., Muiruri, S.K., Nzovu, J.M., Ouma, J.H., Githure, J.I., and Beier, J.C. 
Bloodfeeding behavior of Anopheles gambiae S.L. and Anopheles funestus in Kilifi District, Kenya. Journal of the 
American Mosquito Control Association, Inc. 9(2) 225-228.

6. Cork, A. and Park, K.C. 1996 Identification of electrophysiologically- active compounds for the malaria 
mosquito, Anopheles gambiae, in human sweat extracts. Medical and Veterinary Entomology 10 269-276

7. Lindsay, S.W. and Armstrong J.R.M. 1993 Human attractiveness to mosquitoes. Journal of Medical 
Entomology 30(2) 368-373.

8. Lyimo, E.O. and Takken, W. 1993 Effects of adult body size of fecundity and the pregravid rate of Anopheles 
gambiae females in Tanzania. Medical and Veterinary Entomology 7 328-332.

9. Merritt, R. W., Dadd, R.H., and Walker, E.D. 1992 Feeding behavior, natural food, and nutritional 
relationships of larval mosquitoes. Annual Review of Entomology 37 349-376.

10. Wotten, R.S., Chaloner, D.T., Yardley, C.A.,and Merritt, R.W. 1997 Growth of Anopheles mosquito larvae 
on a dietary microbiota in aquatic surface microlayers. Medical and Veterinary Entomology 11 65-70.

11. Wotten, R.S. 1996 Colliods, bubbles, and aggregates; a perspective on their role in suspension feeding. 
Journal of the North American Benthological Society 15 127-135