ratu

2 Review of Literature

Ever since the role of mosquitoes as carriers of certain human

diseases was established, they have been studied in detail a l l over the

world. These studies have generated immense literature dealing with

taxonomy, biology, ecology, disease rdationshi ps, etc., of mosquitoes

which constitute an integral part and a vital component in the

epidemiology and control of mosquito- borne diseases.

Mosquitoes occur practically in every region of the world except

Antarctica (Foster and Walker, 2002; Service, 2004). Mosquitoes are

small winged insects belonging to the family Cuticidae of the order

Diptera (two-winged flies) which comes under the class Insectu.

Mmquitoes are distinguished from all other insects by the following

characters: a long proboscis; the presence of scales on the thorax, legs,

abdomen and wing veins; a fringe of scales along the posterior margin of

the wings; and a characteristic wing venation, the second, fourth and

fifth longitudinal veins being bifurcated (James et al., 1911; Goma,

1966; Sewice, 2004).

There are over 3,300 recognised species of mosquitoes all across

the world and they are grouped into 41 genera under the famiiy

Culicidae (Service, 2004). The family Culicidae is divided into three sub-

families: Toxorhynchitinae; Anophelinae (anophelines) and Culicinoe

(culicines). The subfamily Toxorhynchitinae i s represented by a single

genus, Toxorhynchites which includes 76 species, All the species of this

genus are non-blod suckers and hence they are of little medical

importance. Culicinae i s the largest subfamily and it comprises of about

2,700 species grouped under 37 genera. Mosquitoes of medical

importance come under t h e following genera: Aedes, Ochlerotatus,

CuIex, Mansonia, Haernagogus, Sabethes and Psorophora. The subfamily

Anophelinae consists of three genera: Chagasia, Bironella and

Anopheles. Of these, the genus Anopheles i s of considerable medical

importance (Service, 2004).

The genus Anopheles comprises of six sub-genera: Stethomyia,

Lophopodomyia, Kerteszio, Nyssorhynchus, Anopheles and CeIlia (Knight

and Stone, 1977). Though there are over 430 species under the genus

Anopheles, only about 70 are malaria vectors, out of which 40 are

important malaria vectors (Sewice, 2004). In India, 58 species of

anophelines have been recorded, of which 6 are primary vectors:

An, culicifacies, An. stephemi, An. flwjatil is, An. dims, An. sundaicus

and An. minimus, and 4 are secondary vectors: An. annularis,

An. philippinensis, An, jeyporiensis and An. varuna (Nagpal and Sharma,

1995). The subgenus CeIIia is divided into six series: Neomyzomyia,

Pseudornyzomyia, Paramyromyia, Myromyia, Neocellia and Cellia

(Christophers, 1933 and Reid, 1968).

Anopheles stephensi belongs to the subgenus CelIia and series

Neocelliu. It i s a medium sized mosquito and can be distinguished from

all other Indian anophelines by having two equal bruad apical and sub-

apical pale bands, speckled palpi and legs, tarsomers of foreleg without

broad bands and thorax with broad scales. The Larva can be

distinguished by the absence of palmate hair on thorax, inner suturai

hair-simple, outer with 2-3 branches and outer clypeal hairs always

simple (Christophers, 1933; Puri, 1960).

Anopheles stephensi i s one of the major vectors of malaria in

India, Pakistan, Iran and Iraq (Christophers, 1 933; Krishnan, 1 961 ;

Manouchehri et a!., 1976; Ramachandra Rao, 1984; Nagpal and Sharma,

1995). This species was first identified and descrfbed by Liston in 1901

from Ellichpur in Vidarbha area of Maharashtra State, India. It was first

incriminated as a malaria vector from Bombay (now Mumbai),

Maharashtra by Bently in 191 1 (Covell, 1927). Subsequently it has been

incriminated in different parts of India by several workers. Some of the

important places where this species was incriminated as vector are

Delhi , Lucknow (UP), Calcutta (now Kolkata), Madras (now Chennai ),

Kutch (Gujarat), Hyderabad, Visakhapatnam, Ahmedabad, etc.

(Ramachandra Rao, 1984; Nagpal and Sharma, 1995).

Anopheles stephensi had also been found susceptible to experimental

infectjons of Wuchereria bancrofti and Brugia malayi (Rao and lyengar,

1932; Raghavan and Krishnan, 1949). Recently this mosquito was found

to be capable of transmitting Chikungunya virus under laboratory

conditions (Yadav et al., 2003).

It i s widely distributed in India, Pakistan, Afganistan, Iraq, Iran,

Bahrain, Oman, Saudi Arabia, Bangladesh, China, Thailand, Myanmar,

Nepal and Taiwan (Knight and Stone, 1977; Rarnachandra Rao, 1984;

Nagpal and Sharma, 1995). This mosquito has also been found in Egypt,

the first record on the Afn'can continent (Gad, 1967). It i s distributed

throughout India except Andaman and Nicobar Islands (Ramachandra

Rao, 1984; Nagpal and Sharma 1995). However, it i s scarcely seen in the

North-East (NAMP, 2002). It was responsible for the transmission of

near[y 12% malaria cases in India, mostly i n the urban and industrial

areas (Tyagi, 2003).

In Kerala, An. stephensi was f i rs t detected from Kochi in 1992

(Mariappan et al., 19921, Subsequentty this species was collected from

Thiruvananthapuram, Kollam, Thrissur, Malappuram, Kozhikode,

Kasaragode and even from Thodupuzha in ldukki District (DHS, Kerala,

1994-2002).

2.1 Egg rnorphometry and variants of An. stephensi

The existence of two forms of An. stephensi was first detected

by Knowles and Basu (1 934). Ramsay and McDonald (1 936) and Mulligan

and Baily (1936) also reported the possibility of the existence of two

forms within the species. In India, Sweet and Rao (1937) showed that

significant differences existed in certain measurements of the eggs of

the two forms in this species. The measurements mainly included the

length and breadth of the egg, number of ridges on the egg-float and the

proportion of the total length covered by the float. Accordingly, they

classified the two forms into two varieties: An. stephensi type form and

An. stephensi var. mysorensis. Sweet et al., (1938) retabulated egg

measurements and reported that a natural barrier to successful cross

breeding of An. stephensi type and var. mysorensis seemed to exist. Rao

et al., (1 938) done the egg measurements of the two forms and reported

as follows: An. stephensi type form: length - 548 p, breadth inciuding

floats - 200 p, length of the float - 290 p, number of rfdges on one side

of the float - 18 and Proportion of egg covered by float - 0.53%. The

corresponding measurements of An. s tephensi var. mysorensis: length -

477 j ~ , breadth including floats - 161 p, length of the float - 218 y,

number of ridges on one side of the float - 13-5 and proportion of egg

covered by float - 0.46%.

These two forms were further compared with respect to the

length and breadth of the wing (Subba Rao et al., 1938) and atso on the

basis of the differences in the maxiilary indices (Senior White, 1937).

Puh (1949) raised the two forms to sub-specific status which was

accepted by Stone et al., (1959). However, Rutledge and Ward, (1970)

found the two forms sympatric and considered them as 'variants' and

not sub-species. Accordingly, Knight and Stone (1977) listed

An. stephensi var. mysorensis as a synonym in their 'A catatog of the

mosquitoes of the world'. Ramachandra Rao (1984) considered it as a

variety in view of the well recognised differences in the

behaviaur. Subbarao et al., (1987) reported yet another form with ridge

number modes of 13-16 and I t was designated as 'intermediate'.

According to them all these three forms are 'ecological variants'.

Recently, Nagpal et at., (2003) found that the adults of these two

ecological variants, type form and var, mysorensis could be separated on

the basis of the thoracic spiracle length and i t s indices.

-

2.2 Prevalence

Anopheles stephensi type form i s predominantly urban, whereas

var. mysorensis mainly inhabits rural areas (Viswanathan, 1950). The

type form i s considered an efficient vector of malaria especially in urban

areas but var. mysorensis i s a poor vector (Krishnan, 1 961 ). All the three

forms of this species occur in semi- urban areas while only 'intermediate'

and var. mysorensis are seen in nrral areas (Subba Rao et al., 1987).

Anopheles stephensi type form does not show definite seasonal

variations in most parts of the country. On the contrary, var. mysorensis

has seasonal prevalence (Ramachandra Rao, 1984). Bhatia et al., (I 958)

observed erratic behaviour in the seasonal prevalence of this species in

Delhi during 1953-1957, Russell et al., (1941) in a study in South India

found that there was no definite seasonal prevalence for this species

even though breeding places were available throughout the year. In

Bengal, An. stephensi i s more prevalent in the seasons with moderate

rainfall, relative humidity of about 85% and temperature between 77"

and 89" F (Chowdhury, 1936). In Iran, the peaks of the An. stephensi

populations are noticed in July, August and early September (Eshghy et

al., 1977). Menon and Rajagopalan (1 979) observed in Pondicherry that

breeding occured in most months of the year with more number of wells

being positive dun'ng monsoon, According to Viswanathan (1950), in

Deccan p tateau only var. mysorensis occurs. Senior White (1 940)

reported that in Calcutta type form and var. mysorensis were present.

The prevalence of An. stephensi was found throughout the year in peri-

urban areas of Delhi with higher densities during post-monsoon (Sharma

et al., 1993).

2.3 Breeding behaviour

Anopheles stephensi breeds in different types of water

collections such as wells, overhead tanks, cisterns, roof gutters,

ornamental tanks, fountains, barrels, buckets, etc. in urban areas. It i s

found breeding in pools, riverbeds, seepages, irrigation channels, wells,

etc, in rural areas (Christophers, 1 933; Krishnan, 1961 ; Ramachandra

Rao, 1984; Nagpal and Sharma, 1995). The breeding of this species in

wells, cisterns, fountains, etc., has been described as early as 1904 in

Bombay (Bentley, 1911). lyengar (1920) was the first investigator to

point out that the overhead cisterns harbour An. stephensi larvae and

that the s i l t in the water i s not inimical to i t s development. Covell

(1928) has reported that man-made containers are the favourabie

breeding places of this vector species. Dhir (1969), following an

outbreak of malaria in Delhi observed prolific breeding of this mosquito

in the water flooded over newly laid concrete floors for curing purpose.

Russell and Ramachandra Rao (1 940) observed that An. stephensi does

not breed in natural habitats in all areas. In an intensive study in

Pattukottai in Tamil Nadu, they found the breeding of An. stephensi in

37 wells out of a total of 1240 wells searched and this species was not

seen breeding in any of the other types of water collections. Tyagi

(1997) has made an interesting observation in the Thar Desert, India,

that An. stephensi breeds in well-like earthen pots locally called 'tanka

and beri'.

Though this species prefers clean water collections, it may

breed in polluted water as well, Breeding of this mosquito in waters

contaminated with sewage has been reported by Roy (1931a). In a study

of the effect of the chemical composition of water on the susceptibility

of this species to plasmodial infection, Russell and Mohan (1940) could

rear larvae in the water with sullage having 5 ppm of ammoniacal

nitrogen and 0.6 ppm of albuminoid nitrogen, The larvae of this species

have the abitity to survive in brackish or salt water (Challam, 1926).

Afridi and Majid (1938) have found breeding in slightly saline water in

pits with salinity ranging from 0.13 to 0.27%. Bana (1943) in Bombay city

found the larvae in salt pans, tanks, drums, etc., filled with sea water

which had become diluted with rain water. Saxena et al,, (1992)

demonstrated that slightly alkaline pH, lower salinity and higher amount

of free ammonia in water are essential for higher population density of

this species. In a study in Pakistan by Reisen and Siddiqui (1978) made

an observation in the laboratory that An. stephensi femahs laid more

eggs on water with increased ammonia content. The larvae are shade

lovers and prefer to breed in deep wells or shaded water collections

(Ramachandra Rao, 1984). The larvae of this species like that of other

anophelines, feed on the surface and have t he abllity to sink into depths

when disturbed.

The larvae are usually found alone in the breeding places, but

several studies have revealed their association with other species.

Russell and Ramachandra Rao (9940) have found in Madras (now Chennai)

that it breeds with , An. varuna, An. vagus, An. subpictus and

An. cilicifacies. Rajagopalan et al., (1 979) observed in Pattukottai,

Tanjavur, Tamil Nadu that thts species was found breeding in wells atong

with An. culicifacies. This species was also found to breed along with

An. subpictus, An. vagus, Ae. aegypti, Ae. vittatus and Cuiex

quinquefasciatus (Ashwani Kumar and Thavaselvam, 1992; lndranil et at.,

1 996).

2.4 Resting behaviour

Since mosquitoes are feeble insects and subject to dehydration,

they tend to rest in places where air i s static, humidity i s high and

temperature i s ideal. Most of the species prefer to rest in dark places in

the indoor or outdoor situations during daytime (Goma, 1966). The habit

of mosquitoes to rest indoors i s known as endophily and the tendency to

rest outdoors i s called exophily. Even in the indoor situations, some

species prefer to rest on or near the floor, others prefer places higher up

on the walls or in the ceiling and s t i l l others hang on the underside of

other objects (Boyd, 1949). Most of the anophelines in India are

nocturnal in their activities and spend the day time in suitable shelters

(Ramachandra Rao, 1984).

Anopheles stephensi usually rests in houses, cattle sheds,

barracks, etc., (Ramachandra Rao, 1984; Nagpal and Sharma, 1995). In

Bellary in Karnataka state, Bhasker Rao e t al., (1946) could co(iect large

numbers from human dwellings and cattle sheds. Afridi and Majid (1938)

found that this species was mainly resting in houses close to breeding

ptaces. Reisen and Emroy (1976) observed in Lahore, Pakistan that

adults of this species were resting in cattle sheds. 8atra et al., (1 979a)

showed that in Salem, An. stephensi adults were resting in human and

animal dwellings. The information on the outdoor resting behaviour of

this species i s scanty. Knowels and Basu (1934); Ganguli (1935);

Strickland et al., ( I 936); Senior White (1 940) have observed the scarcity

of this species in the eastern region of India. Hati (1 987) could cotlect

An. stephensi from a variety of daytime resting habitats in Calcutta.

Chatterjee et at., (1993) reported that hanging objects seemed to be the

preferred resting sites of the species in Calcutta. Batra et al., (2001)

reported that day-time resting preferences of the species in human

dwellings and cattle sheds did not differ significantly. Quraishi (1965)

could collect fairly gmd numbers in a pit sheker in Iran suggesting the

tendency of this species resting outdoors at ieast in that country.

2.5 Swarming

The swarming of mosquitoes i s a conspicuous flight activity

generalty performed by adult males (Goma, 1966). A swarm i s an

aggregate of flying male mosquitoes that as a mass moves littie in space

(Daschaudhuri, 1974). The size of the swarms and their locations as well

as period of swarming vary widely from one species of mosquito to

another (Goma, 1966). Swarms are formed in relation to a certain point,

the swarm marker. Swarms are generally formed close to a prominent

vertical object such as tree-top, above a contrasting dark and light areas

on a more or less horizontal surface, over a flat surface, a large animal

or man (Nielsen and Haeger, 1960). Swarming i s influenced by many

environmentai factors such as light intensity, temperature, re\ative

humidity, wind velocity, rainfall and sound (Roth, 1948; Daschaudhuri,

1974; Venkat Rao, 1961). Swarming is chiefly induced by the light

conditions especially a t twilight.

The swarming behaviour of An. stephensi has been thoroughly

studied. The swarms of An. stephensi are composed of 500-600 males

(Reisen et al., 1977). The swarms reach their maximum size in about 1 0

minutes and the male-female ratio i s found to be 3:5 (Quraishi, 1965).

The swarming starts 7.7 minutes before sunset with an illumination mean

of 437 lx at 27" C. After 2 to 20 minutes duration, the swarms diffuse

and the males remain outdmrs for the rest of the night or return to their

resting sites (Ramachandra Rao and Russell, 1938; Russell and

Ramachandra Rao, 1942). Swarming of males occurs regularly at about

6:30 pm and continues for about 30 minutes in cages (Russell and Mohan,

1939).

2.6 Mating

The mating habits of mosquitoes vary considerably from one

species to another (Horsfall, 1972; Nielsen and Haeger, 1960). Some

species mate as soon as the femaies emerge from the pupae while some

others start mating only after several hours. Males do not mate until

their genetalia (hypopygium) have rotated through I 80" . This

phenomenon is known as 'hypopygiai rotation'. After the emergence of

the male mosquito, the genetalia and the adjoining eighth abdominal

segment rotate through an angle of 180" and thereafter remain fixed in

this position for life. The duration to complete this process varies from

species to species and usua tly takes 6 to 24 hours (Marshall and Staley,

1932; Clements, 1963; Goma, 1966; Sewice, 1993b; Kettle, 1995; Foster

and Walker, 2002).

Mating usually occurs in flight. Copulation frequently lasts from

5-60 seconds, but in some species such as Deinocerites cancer and

Cuiiseta inornata, it may last for 45 minutes or more (Service, 1993b).

The duration of mating of one pair of An. stephensi was found to be 10

seconds (Reisen et al. , 1977). One male mosquito can mate with more

than one female and a single mating will provide sufficient sperms for

the functional life of the female (Goma, 1966; Service, 1993b). In most

species of mosquitoes, female are inseminated only once in their life

time (Gorna, 1963; Craig, 1967). Multiple insemination can also occur in

nature when the female mosquito is remated during the nuliiparous

period before the formation of the reinsemination barriers (Craig, 1967).

In Aedes aegypti secretions from the male accessory glands inhibit

further mating (Craig, I 967; Gwadz et al., 1 971 ).

2.7 Biting and feeding behaviour

The blood-sucking habit i s almost universal among mosquitoes

and knowledge on this aspect is critical in understanding the

epidemiology of disease transmission. Mosquitoes usually start biting

from dusk but some species especially of the genus Aedes bite during the

day also. Most anophelines are nocturnal and many major vectors such

as An. gambiae and An. minimus have their peak biting rate in the early

morning (Muirhead-Thorn pson, 1 95 1 ). Aedes africonus i s crespuscular

with i t s biting activity being largely concentrated during sunset (Gillett,

1971).

Anopheles stephensi is by and large a nocturnal feeder,

AnopheIes stephensi was found to be most active in biting before

midnight (Chowdhury, 1936). Reisen and Aslamkhan (1978) made similar

observations in Pakistan, They also made a critical study of the

differences in biting activity between seasons. The biting rates were

found to be higher during warmer manths especially in the second

quarter of the night. In January and February, there was practically no

biting after 1900 hrs and biting was virtually crepuscuiar, but in

November and December some biting took place in late night. These

observations were agreeing with Eshqhy and Janbakhsh (1977). Biting

activity is bimodal with peaks from 21:OO to 24:OO hrs and 04:00 to 06:00

hrs (Nursing et al, 1934; Shinkle, 1969). However, Bhatt et al., (1996)

reported that An. stephensi had unimodal biting rhythm and feeding

mostly occurred during the early night with occasional increase duting

pre-dawn hours. According to Reisen and Astamkhan (1 978) this vector

had unimodal biting rhythms but occasionally had multimodaI rhythms or

were arrhythmic.

For most mosquitoes blood as a f o d is obligatory for the

development of eggs. It is well known that it i s only the females that

feed on blood. However, in a few groups of mosquitoes, the blood

sucking habit i s totally absent. Mosquitoes under the sub-family

Toxorhynchitinae and genus Malaya under the sub-family Culicinae are

not known to suck blood. The males and females of Toxorhynchitlnae

feed on honey while that of the Malaya feed on the regurgitated

stomach contents of certain Cremostogaster ants (Christophers, 1933;

Goma, 1966; Service, 1993b; Kettle, 1995). Blood feeding can take

place before or after mating depending on the species and

circumstances. In An. stephensi, the first blood meal is taken on the

first or second night of a female life irrespective of whether or not i t is

inseminated (Sewice, 1993b). The amount of blood ingested by a fully-

fed female varies according to the size of the species, but there are also

some variations between individual females (Bruce-Chawatt and Gockel,

1960). Briegel and Renonico (1 985) observed that An. stephensi

ingested 2 to 10 ~1 of blood during feeding.

Mosquitoes take their blood meat from a variety of hosts which

include warm-blooded and cold-blooded animals such as birds, cattle,

pigs, man, frogs, snakes, insect nymphs, etc. Many species of

mosquitoes show a marked predilection for a particular type of host

which they select if reasonably available, taking another only in the

absence of their preferred hosts* Many of the major malaria vectors

such as An. minimus, An. culicifacies, An. maculatus, An. stephensi and

An. albimanus are attracted to man or to animals according to their

respective availability (Laarman, 1955; and Macdonald, 1957).

Mosquitoes feeding predominantly on humans are referred to as

anthropophagic and those feeding on non-human hosts are called

zoophagic. Species that feed on birds are sometimes called

ornithophagic instead of zoophagic (Senrice, 2004).

Anopheles stephensi is basically a zoophilic mosquito (Roy et ai.,

1938; Reisen and Boreham, 1979)& According to them, more than 95% of

the blood meal tested were bavid positive. The anthropophilic indices of

An. stephensi observed in some earlier studies in India are as follows:

Roy et al., (1938) - Bengal 3.4%

Afn'di et at., (1939) - Delhi 1.4%

Bhaskar Rao et a(., (1 946) - Karnataka, Deccan 0.8%

Precipitin tests done in Iran showed a possitivity of 15.7% for

human btood (Manouchehri et at., 1976). Nair et al., (1967) observed a

human index of 37.5% in Broach town in Gujarat where cattle population

was very Low. Batra et al., (1979b) found that due to the low ratio of

cattle to people in Salem town in Tamil Nadu, An. stephensi mainly fed

on man. Chandra et al., (1996) reported that out of 225 samples from

human dwellings, 213 (94.6%) were found positive for human blood. In a

recent study in Delhi, Kaushal Kumar et al., (2002) found that the overaI1

anthropophitic index of An. stephensi was 16.93%.

2.8 Oviposition and biology of eggs

Mosquitoes have two distinct methods of egg lying; the eggs may

be laid singly or they may be glued together to form a ' raft'. In general,

Anopheles, Tuxorhynchifes, wsomyia, Aedes, Psorophoru, Haemogogus,

etc., lay eggs singly, whereas Culex, Culiseta, Mansonia, CoquilIettidia,

Uronotaenia etc., lay eggs in raft or cluster (Gillett, 1971 ; Kettle, 1995;

Foster and Walker 2002). The eggs may be laid directly on water or out

of water. According to Bates (1940) many anophelines will lay eggs

while resting on water surface. Russell and Rarnachandra Rao (1942)

made the first direct observation on the ovipositing dance performed by

An, cuIicifucics gravid females in natural conditions. The female

performed a 'havering dance' a little above the water surface while

dropping eggs. Culicines also lay their eggs directly on the water

surface. The eggs are laid vertically in several rows and held together to

form an egg raft which floats on the water (Service, 2004). Aedes,

Haemagogus and Psorophora lay their eggs on moist substratum while in

contact with it (Gorna, 1966). The process of oviposition in Mansonla

species i s unique among mosquitoes. The eggs are laid in a sticky mass

that i s glued to the underside of floating plants (Gorna, 1966; Service,

2004).

The time of egg laying i s very important to understand the

influence of other factors on egg laying in different types of habitats.

The time of egg laying varies from one species to another. Many species

apparently lay their eggs mainly at night as in Mansonia fuscopennota or

a t dusk as in An. gambiAe. Aedes mosquitoes generally lay their eggs

during day time (Goma, 1966). Russell and Ramachandra Rao (1942)

noticed that though An. cuticifacies laid eggs throughout night, greatest

number was laid in the first and third of the night under natural

conditions. Pat (I 945); Muirhead-Thomson (1 940) also observed similar

habit of earty oviposition.

The shape and size of the mosquito eggs vary characteristicalLy.

The eggs of most species of mosquitoes are elongate, ovoid or spindle

shaped; others are spherical or rhomboid (Service, 1993b; Foster and

Walker, 2002). Anopheline eggs are generally boat-shaped and in most

of the species there i s a pair of conspicuous lateral air-filled chambers

called the floats which prevent them from sinking. Eggs of a few

anophelines, such as the desert-inhabiting An. cinereus of East Africa

and the palaearctic An. plumbeus lack floats and hang perpendiculariy in

water. On the contrary, eggs of the species under genus Chassis have

rnultipie floats (Sewice, 1993b).

When the eggs are first laid they are soft and white, but

subsequently turn black and become hard (Patrick Hehir, 1927; Crawford

and Chalarn, 1927; Coma, 1966; Roy and Brown, 1970; Service, 1993b).

This phenomenon is called 'tanning'. The duration of tanning varies in

different species. In Ae. aegypti it is found to be 240 minutes

(Christophers, 1960).

The eggs of Anopheles usually cannot tolerate dryness for more

than a few hours or at most a day, but their survival depends on the

severity of the desiccation. In tropical region, where embryonic

development is rapid, eggs usually hatch within about 2 to 3 days; but if

they become stranded on wet surfaces, hatching can be delayed for a

few days (Ramachandra Rao, 1984; Service, 1993b). Horsfall and Porter

(1946) found that the eggs of An. punctulatus and An. farauti remained

viable as (ong as 14 days on a moist surface in the laboratory. According

to Roy (1931 b) the eggs of An. stephensi can't withstand drying except

for a short period. He observed that eggs dried at 1 1 " C hatched when

floated after five days but not after seven days. Horsfall (1972) i s of the

opinion that the eggs of An. stephensi may withstand dryness for several

days under certain conditions. The eggs cannot withstand long

submergence as well. Bhatia and Wattal (1958) studied the viability of

submerged eggs of An. stephensi, An. annularis, An. culicifacies and

An. subpictus. They found that the eggs submerged over 92 hours failed

to hatch out. The eggs of An. stephensi and An. culicifacies die when

temperature fa l l below 10" C, but eggs of many temperate Aedes species

such as Ae, stfmulans hatch a t temperatures as low as 5" C (Service,

1993b).

2.9 Fecundity

The number of eggs laid by a single female in a batch, i.e., in

one gonotrophic cycle varies greatly between species. Generally the

larger species lay more eggs than the smaller species. Even within the

same species, number of eggs in individuat egg batches vary

considerably . In the first gonot raphic cycle Anopheles maculipennis

melanoon lays up to 500 eggs (Shannon and Hadjinicalo, 1941). In

Malaya the average number of eggs laid a t one time by different species

of Anopheles has been recorded by Lamborn (1922). The number vanes

from 29 in An. tessellatus to 89 in An, vagus. Similar observations in

respect of Indian anophelines are meagre. It has been found to be an

average of 80 in An. stephensi, 118 in An. minimus and 147 in

An. maculatus (Roy and Brown, 1970). Factors affecting fecundity are

the following: 1 ) body size, 2) amount of blood ingested and 3) type of

blood (Reuben, 1987). Fecundity in An. stephensi is mainty influenced

by the source of blood meat (Roy, 1931 b; Stahler and Seelay, 1971 ),

amount of blood meal (Reisen and Emroy, 1976; Briegel, 1990) and larval

density (Reisen, 1975). Infection of larval stages with a protozoan

Nosema algerae may also reduce fecundity of An. stephensi (Haq et al.,

1981). The total number of eggs (aid by an individual mosquito during i t s

lifetime depends on the species, longevity, the avaitability of food, the

tatat number of eggs batches oviposited and the temperature. Many

species are known to lay from 800 to 1000 eggs during the lifetime of an

individual female. However, in many species, it will be a maximum of

four batches of eggs to be laid by a single female (Gama, 1966).

2.1 0 Gonotrophic cycle

The duration between the ingestion of blood and oviposition i s

known as the gonotrophic cycle, The entire period from one biood meai

to the next, generally consists of three phases: the search for a host and

taking blood meal from it; the digestion of blood and egg development;

and the search for a suitable breeding place and oviposition. The

frequency of blood feeding depends on the duration of the gonotrophic

cycle and the number of the gonotrophic cycles undergone by each

female mosquito represents i t s physiological age (Detinova, 1962). In

most species of mosquitoes, there will be strict alternation of blood

feeding and oviposition , The process of complete ovarian development

following a blood meal i s termed gonotrophic concordance (Kettle, 1995;

Service, 1993b), Sometimes, feeding may not result in egg development

and egg laying. This condition i s referred to as gonotrophic dissociation

(Swellengrebel and De Buck, 1938; Sewice, 1993b). Gonotrophic

dissociation i s known mainly in Anopheles but has also been recorded in

CuIex tritaeniorhynchus {Washino, 1977). Another condition,

gonotrophic discordance, describes cases where blood-feeding will lead

to maturation of egg but the gravid female wil l not oviposit and may

re-feed (Venkat Rao, 1947), This phenomenon is cornmonty seen in

Anopheles culicifacies and An. stephensi (Service, 1 993 b).

The feeding and oviposition cycles have a minimum duration of

2-3 days. The number of gonotrophic cycles a female mosquito can

generally undergo vary from one species to another. Reisen et al.,

(1986) observed 8 gonatrophic cycles in An. cuticifacies, 6 in

An. stephensi and 5 in An. subpictus. fn most species, the first

gonotrophic cycle i s longer than that of subsequent cycles. In the first

cycle, the ovary i s always in Stage I (Chn'stophers Stage) and hence the

ovary development has to start from this stage. But in the subsequent

cycles, the ovary wilt be in Sta$e II and further maturation will start

from this stage (Gillies and Wilkes, 1 965). Anopheles stephensi requires

4 and 2 days to complete the first and subsequent gonotrophic cycles

respectively (Reisen and Aslamkhan, 1979; Reisen et at., 1980) and

appears to oviposit and refeed on the same night (Reisen and Aslamkhan,

1978; Reisen and Mahmoad 1980). The duration of the first and second

gonotrophic cycles decreased with increasing temperature (Mahmood

and Reisen, 1981). Russell and Mohan (1939) reported that under

iaboratory conditions, the ~onotrophic cycle of An. stephensi took 2 to 3

days, the first cycle taking a day more.

2.1 1 Flight and dispersal

From the epidemiologicat point of view, ftight and dispersal of

mosquitoes assumes significance. These characteristics have relatian nat

only to the intensity and spread of the disease but aiso to the kind and

extent of control measures needed ta interrupt the transmission

(Ramachandra Rao, 1984). Most mosquitoes disperse only a few hundred

metres from their emergence sites. However, there are records of

mosquitoes dispersed up to 100 krn or more from their breeding sites by

air (Service, 2004). Mosquitoes generally disperse by flight for feeding,

resting, mating, oviposition, etc., (Provost, 1952). Each species of

mosquito has the physical ability to fly a certain distance. It i s known as

flight range. It i s not necessarily a Limit beyond which a species will not

fly. It i s an indication of the distance beyond which the species will be

present in negligible numbers (Kettle, 1995).

Anopheles stephensi i s generally considered as a moderate flier

with a flight range up to 1.5 km (Covell, 1944)- However, in rural areas,

it can fly up to a distance of about 5.2 kms (De Burca, 1946). Quraishi et

al., (1966) observed in an experiment in Iran that An. stephensi

mysorensis could fly up to 4.5 kms. Fleisen and Aslamkhan (1979) in a

release-recapture experiment in Pakistan found that the maximum

distance dispersed was 1.78 kms. Anopheles stephensi was found to

disburse up to 2.5 kms in Bahrain (Afridi and Majid, 1938). In Pakistan,

An. stephensi was found very scarce beyond a distance of 0.8 km from

the breeding site (Mulligan and Baily, 1936). Estimate of flight range by

indirect observations ranged from 0.8 km in an urban situation to 4.9 krn

in a rural environment (Krishnan, 1961 ).

2.1 2 Colonisation

Anopheles stephensi i s one of the species which can easily be

colonised. Rao et al., (1938) could establish a colony of An. stephensi

type form, Subsequently, Russell and Mohan (1 939) successfully

established colonies of An. stephensi var. mysorensis. Ansari et al.,

(1978) developed some newer methods of mass rearing of the species

using wooden cages (70 x 60 x 60 cms). They found that a cycling cotony

was better than a non-cycting colony for obtaining maximum egg

production per female. Anopheles stephensi i s a stenogarnous species

and can be reared in small cages. Egg production was found to be poor

when human blood was offered f Roy, 1931 b; Stahler and Seelay, 1971 ),

whereas, when fed on human blood the longevity was found to increase

(Gilmert et al., 1990).

2.1 3 Resistance to insecticides

The use of insecticides for the control of vectors continues to be

the mainstay of various vector-borne disease control programmes.

Therefore, monitoring of vector susceptibility to various insecticides has

become imperative to ensure judicious and effective use of chemical

insecticides, In India, the first report of DDT resistance in mosquitoes

was that of Culex fatigans (now Culex guinquefasciatus) in Delhi in 1952

(Pal et a{., 1952). Anopheles stephensi was first among the malaria

vectors to be noticed to have developed resistance to DDT in 1955 in

Erode town, Tamil Nadu (Rajagopalan et al., 1956), Subsequently the

resistance to DDT in this species was observed in Salem, Bhavani and

Kumarapalayam (Bhombore et al., 1964). Resistance of this species to

HCH was also reported from Calcutta and Kotah, Rajasthan (Bhatnagar

and Wattal, 1979). Bhaskar and Shetty (1 992) found that An. stephensi

was susceptible to DDT, dieldrin, malathion, fenitrothion and propoxur in

Bangalore. This mosquito is known to have developed triple resistance

to DDT, dieldrin and malathion in Goa (Thavasetvam et al., 1993).

Vector control by adulticides have several problems and in a

control programme involving integrated methods, only initial suppression

of the vector population can be achieved by anti-adult measures. In a

sustained control programme larvicides have an important role. For the

control of anopheline breeding temephos is generally used as an

effective Larvicide. Ternephos i s widely used for the control of breeding

of An. stephensi in urban areas in India. The Larvae of An. stephensi has

been found susceptible to this insecticide (Das et al., 1979; Batra et al.,

1981).