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Culicine Classification Essay

In this article we will discuss about Culex:- 1. Habit and Habitat of Culex 2. External Features of Culex 3. Life History.

Habit and Habitat of Culex:

Culex pipiens is found in temperate regions all over the world, and Culex fatigans throughout the tropics and sub-tropics. Culex lives in houses, in cities and farms, and is abundant also in rural areas. They are most abundant during spring, but hibernate during un-favourable climatic conditions, the adults hide in hollows of trees, caves, crevices, barns, etc.

The life span of male mosquitoes is seldom more than three weeks, they die after fertilizing the females. The females live from four weeks to several months, but they die when all their eggs are laid. Culex has several generations in a year.

External Features of Culex:

Shape, Size and Colouration:

The body of Culex is small, soft, and covered with small scales. Culex measures about 3 to 4 mm in length. The body colour is grey-black.

Division of Body:

The body is divisible into head, thorax and abdomen.

1. Head:

Head is globular and highly mobile on a slender neck. There are two very large black compound eyes, there are no ocelli. The top of the head has an epicranium below which is a clypeus which is thick and projects in front.

There are two filiform antennae, each with 15 joints, the basal segment is the scape which is concealed by a very large globular second segment, the pedicel containing a Johnson’s organ which is auditory in function, the other 13 joints form a flagellum having many bristles lying in rings.

The bristles are longer and much more numerous on the antennae of males giving them a bushy appearance. In the female the antennae have rings of few, short bristles, thus, sexes can be distinguished readily by the antennae. The head bears two maxillary palps and a proboscis.

The maxillary palps are stiff and have many bristles, the palps in the female are short and three-jointed, but in the male they are as long, or even longer than the proboscis, they are five-jointed.

Mouth Parts:

The proboscis is a straight, long tube formed by a fleshy ventral labium which has a deep groove on its upper side, in this groove is a long pointed and ventrally-grooved labrum epipharynx. At the distal end of the labium is a pair of small tactile labella which are reduced labial palps.

The groove of the labium also contains five needle-like stylets in a female Culex, they are two mandibles, two maxillae, and a hypo pharynx. The mandibles are finer than the maxillae, but both have saw-like edges on their tips. The hypo pharynx is also needle-like and has a fine salivary duct running through it and opening at the tip, through this duct saliva is poured to prevent coagulation of blood of the victim.

In the male the labrum-epipharynx and the labium are the same as in the female, but the mandibles and maxillae are very short and functionless and the hypo pharynx is fused with the labium.

Feeding:

The normal food of both sexes are nectar of flowers and juices of plants, but the female has its mouth parts modified for obtaining additional meals of blood of vertebrates. A female mosquito sits on a vertebrate and presses its labella against the skin, they act as a guide for the piercing mandibles and maxillae which are sunk into the flesh; the en-sheathing labium bends back to allow the needles to go in.

The labrum-epipharynx and hypo pharynx together act as a tube forming a food canal through which blood is sucked up from the wound; the suction is caused by the pharynx by which blood comes into the mouth.

Thus, the mouth parts are for piercing and sucking. Mosquitoes have three oesophageal food reservoirs in addition to the stomach, the reservoirs are used for storage of food, such as plant juices, but not for blood which passes directly to the stomach.

2. Thorax:

Thorax is arched, it has mesothorax which is very large and its tergum has three sclerites, a scutum, a trilobed scutellum and a post-scutellum. Prothorax and metathorax are very small. On the thorax there are two pairs of spiracles. From the mesothorax arise a pair of membranous functional wings which are long and narrow.

The nervures of wings are beset with scales, and the posterior margin of the wings is fringed with bristle and scales. The wings of metathorax are reduced to form a pair of small halteres, each of which has a swollen base orscabellum, a narrow stem or pedicel, and a distal swollen knob or capitellum.

Halteres vibrate 300 times per second during flight, they probably act as balancers, but their function is doubtful, however, if halteres are removed flight becomes difficult or even impossible.

From the thorax arise three pairs of legs which are very long and slender, they are fragile and have the usual parts of an insect leg, but the coxae are short and tarsi long with five joints ending in a pair of simple claws, below each claw is a pad-like pulvilus. The legs also have many scales and bristles.

3. Abdomen:

Abdomen consists of 10 segments of which the first is vestigial and fused to the metathorox; the second to the eighth are clearly seen, each has a pair of spiracles; the ninth and tenth segments are partly telescoped into the eighth. In the female the 10th segment is blunt and bears a pair of cerci, between them is a small post-genital plate which is part of the tenth sternum.

In the male the 9th and 10th segments are complex, they undergo a torsion of 180° as soon as the mosquitoes are born, so that the terga and anus become ventral and the sterna dorsal. The ninth segment in ring-like with a bilobed ventral tergum, it bears a pair of large claspers, each with a broad basal coxite followed by a narrow style which ends in a claw.

The 10th segment has a bilobed dorsal sternum from which project two processes with curved and toothed tips, the male intromittent organ or aedeagus projects posteriorly, it is formed by fusion of gonapophyses of ninth segment. The ejaculatory duct opens into the aedeagus. During copulation the male holds the female by its claspers and the aedeagus is inserted into the vagina.

Life History of Culex:

(i) Eggs:

After mating the female lays eggs on still water, the eggs may be laid on ponds, or pools, or rain-filled containers.

The eggs are cigar-shaped, tapering at one end. The eggs are laid at night and one female may lay up to 300 eggs. The eggs are laid side by side standing erect, and glued together by the legs to form boat-shaped rafts which float on water. The eggs hatch in 1 to 3 days and a larva emerges from the lower end of each egg.

(ii) Larva:

The larvae are called wrigglers because of their wriggling movements, they are microscopic on hatching. The larva leads an active life, it swims about, feeds and grows, and the larval life lasts from 3 to 14 days according to temperature. During this period it moults four times and grows larger after each moulting.

The larva has a large chitinous head which is flattened dorsoventrally, it has compound eyes developing, and closely behind each is a larval ocellus, it has a labrum, small toothed mandibles, a pair of maxillae with feeding bristles lying internally, labial plates, and a pair of jointed antennae.

It has a mouth over which is a pair of rotary feeding brushes, formed of stiff bristles, the brushes cause a current of water by which small particles of food are wafted into the mouth.

Food consists of algae and small organic particles, the larva feeds on these below the surface of water. Thorax is globular, its segments are fused together. On the head, thorax and abdomen are paired bristles, some of them forming bushy tufts, especially on the thorax. Abdomen is slender and has nine segments, on the first seven abdominal segments are tufts of bristles.

The eighth segment has a chitinous and tubular respiratory siphon, at the tip of the siphon are two spiracles leading into tracheae. Around the spiracles are five leaf-like lobes which can close over the spiracles to prevent water from entering. The respiratory system is metapneustic in which only the last pair of abdominal spiracle is open.

The larva though aquatic breathes air through the siphon and comes to the surface to take in air. When resting the larva pierces the surface film of water by its siphon which projects just above the surface and draws in air, and it hangs by the siphon with its head downwards, but it is inclined at an angle.

The siphon on its ventral side has two tufts of bristles, and two rows of flat spines called pecten. On the eighth segment is a patch of small scales in one or two rows forming a comb. In some species of Culex the comb has scales in several rows. The ninth segment of the abdomen is slender and covered by a chitinous dorsal plate.

At the end of the ninth segment is an anus surrounded by four leaf-like tracheal gills which differ from true gills in having tracheae instead of blood vessels. The ninth segment has a tuft of dorsal bristles at its tip, and ventrally a bushy tuft of bristles called ventral brush. The larva sinks in water being heavier, and it rises by wriggling movements of the abdomen. After the fourth moult the larva changes into a pupa.

(iii) Pupa:

The pupa is comma- shaped and is called a tumbler. It has a large cephalothorax formed by the head and thorax.

On the mid-dorsal side of the cephalothorax is a pair of tubular respiratory trumpets which are broader at the distal end, they communicate with an anterior pair of thoracic spiracles. By means of the trumpets the pupa hangs from the surface film of water and takes in air through their distal ends which project slightly above water.

Inside the cephalothorax may be seen cases containing compound eyes, one pair of ocelli, antennae, wings, and legs of the adult. Behind the cephalothorax is a ventrally flexed abdomen formed of nine segments of which the first is very small but segments 2 or 9 are distinct and movable. On the abdomen are tufts of bristles.

The last segment bears a pair of chitinous leaf-like paddles by which the pupa swims. The pupa is a resting stage, during this period it does not feed, but the pupae of mosquitoes are peculiar in not being quiescent, they are active and can swim about. Unlike the larva the pupa is lighter than water and requires a muscular effort to sink down.

The pupal period lasts from two to seven days depending upon the temperature. During this time remarkable changes occur in the pupa while the adult insect called imago is being formed. When the imago is completed the skin of the pupa splits mid- dorsally along the back between the trumpets and the imago emerges, first the head, then body and appendages are extricated.

The imago rests for some time on the pupal skin, it stretches and dries its wings, then flies off. It can start laying eggs in a week’s time and, thus, repeat the life history.

(iv) Metamorphosis:

The young one that hatches from an egg is quite different from the adult insect in structure and mode of life, it is known as a larva. The larva feeds, moves, moults, and grows, then it passes into a quiescent stage, the pupa which is different from both the larva and the imago. Finally the adult is formed in the pupa.

This form of development is termed complete metamorphosis or holometabolus metamorphosis. It occurs in higher insects as seen in the mosquito. Growth and moulting up to the end of the larval period are controlled by the juvenile hormone of corpora allata.

The pupa though quiescent undergoes very great internal changes in order to form the imago. Most of the larval organs in the pupa, except the central nervous system and developing genital organs, are broken down, the process of breaking down and disintegration of larval organs is called histolysis.

The process of histolysis is brought about largely by blood corpuscles called phagocytes which feed upon the tissues of disintegrating organs, and their products of digestion pass into the blood to form new tissues.

For the formation of organs of the imago groups of formative cells are set aside in the larva, they are called imaginal buds or histoblasts. The imaginal buds are found all over the body of the larva, close to its internal organs or in invaginations of the epidermis.

The imaginal buds are the rudiments of future organs. The process of formation of new adult organs inside a pupa is known as histogenesis. Imaginal buds are dormant, they are stimulated by a hormone of prothoracic endocrine glands, these glands become active only during metamorphosis secreting a pupation hormone which causes imaginal buds to develop.

By this process the imago develops inside the pupa; when development is completed the pupal covering splits and a perfectly formed imago emerges. The final moulting into an adult is also controlled by the hormone of prothoracic glands, it occurs only after the juvenile hormone of corpora allata is not being produced.

Thus, in holometabolus metamorphosis the stages in the life cycle are, egg → larva → pupa → imago and the adult wings develop from inside from imaginal buds and are not visible externally. In hemimetabolous or heterometabolous metamorphosis, as seen in a cockroach the stages are, egg → nymph → imago, and the adult wings develop externally from the integument.

1. Introduction

Mosquitoes are one of the most notorious and influential insects in the public health field [1]. Even though only a few species among the 3000 mosquito species identified in the world are known for feeding on human blood, mosquitoes act as virus vectors for human-related diseases such as malaria, dengue fever, yellow fever, and West Nile virus, as well as animal diseases such as equine encephalitis and canine heartworm [2–6]. For example, Culex pipiens (C. pipiens) complex including C. pipiens pallens and C. pipiens molestus, which primarily occur in urban areas, are the primary vectors of West Nile virus [7], and Aedes aegypti (A. aegypti) is a key vector for the arboviruses causing dengue and yellow fever [8,9].

The occurrence and transmission of mosquito-borne diseases can be strongly related to the abundance of the host vector (i.e., mosquito) [10,11]. Thus, for effective mosquito control, there have been several approaches related to quantifying and/or predicting the occurrence of mosquitoes within various spatial and temporal ranges. Udevitz et al. [12] predicted the occurrence of four mosquito species (Anopheles punctipennis, Culex territans, Aedes atlanticus, and Psorophora ferox) based on physico-chemical factors using stepwise logistic regression; Hales et al. [13] predicted the global distribution of dengue fever under current and future climates based on regressions with macroclimatic data; Peterson et al. [14] used a machine-learning approach to describe patterns of mosquito occurrence through space and time; Kearney et al. [15] predicted climate impacts on the potential range of the dengue fever vector, A. aegypti, based on biophysical models of energy and mass transfer; and Ruiz et al. [16] evaluated the impact of temperature and precipitation on West Nile virus infection in Culex species using random forest.

These approaches have also highlighted the roles and importance of meteorological factors, natural predators, competitors, prey, and mosquito density [17,18,19]. High tide frequency and low rainfall in the late dry season and early wet season led to higher population growth rates in Aedes vigilax, which facilitated mosquito breeding and subsequent abundance peaks, especially if low rainfall occurred during favorable tides [20]. Precipitation can support and maintain a “comfort zone” for mosquito egg laying as well as growth during the immature larval phase even though it can also be detrimental depending on the strength, intensity, and amount of precipitation [6,21,22]. Warm temperatures can trigger peak occurrences of mosquitoes [16,23,24,25,26,27], whereas winter freezing can cause high mortality of mosquito eggs, larvae, and adults [28].

Changes in land use (e.g., urban expansion, industrialization, etc.) have influenced the spread and dispersal of mosquitoes. For instance, increased soil moisture due to irrigation development and/or the construction of dams promoted a rapid density peak of Culex quinquefasciatus [29]. The continuous increase in artificial ecosystems as a result of industrialization and urbanization has enhanced successful introduction and establishment of vector mosquitoes. Generally, urban mosquitoes breed in open water habitats and stagnant water with high organic content such as sewage ditches, leading to successful establishment of mosquito populations [30,31,32,33,34,35,36].

However, to date, despite several studies on the relationships between environmental variables and mosquito occurrence [16,37], there are insufficient studies on various scaled environmental factors (e.g., meteorological and land use) and/or their combined influence on the prediction of occurrence patterns of mosquitoes in Korea. Thus, we analyzed the occurrence patterns of urban mosquitos based on meteorological and land use types. We characterized the environmental conditions that cause higher probabilities of mosquito occurrence. We also investigated the importance of meteorological factors in predicting the occurrence of mosquitoes according to the land use type. Our study can contribute to effective control and risk assessment of mosquitoes.

2. Materials and Methods

2.1. Mosquito Data

The data on mosquito abundance were obtained from a public health center in Yeongdeungpo-gu, Seoul, Korea. In total, the data were collected at 12 sites using digital mosquito monitoring systems (DMS, Environmental Technology & Development: E-TND) [38] from May 2011 to October 2012. The system attracted female mosquitoes by diffusion of CO2 during the night when mosquitoes are active (6:00 p.m.–7:00 a.m.), and then the number of mosquitoes attracted to the system were counted using an infrared ray LED sensor. After counting the attracted mosquitoes, the system sent the collected data to a data server by a CDMA module in real-time. The monitoring system showed high efficiency (R2 > 0.85) between manual observation and automatic observation [38,39]. During the data preprocessing procedure, we excluded extreme and unrealistic values due to mechanical errors in the mosquito monitoring system (daily maximum observed value in waterfront area: 2000 individuals/day; daily maximum observed value in non-waterfront area: 200 individuals/day), based on consultation with a mosquito expert in the study areas. Missing data (e.g., extreme data, unrealistic data, etc.) were calculated with average value of observations before and after a day, then daily monitoring data were summarized to weekly data at each site, and then used in further analyses.

2.2. Environmental Data

To clarify the relationships between land use types and the occurrence of urban mosquitoes, land use data are extracted from each monitoring site within a 250-m buffer on a digital map using ArcGIS 9.3 (ESRI, Redlands, CA; http://ww.esri.com). The main components of the land use types in our monitoring areas were residential areas (RESD), commercial areas (COMM), culture and sport areas (CULT), public facilities areas (PUBL), artificial grassland (GRAS), inland wetland (WETL), bare soil (BARE), inland water (WATE), industrial areas (INDU), and traffic areas (e.g., roadways) (TRAF) (Table 1). The land use data were obtained from the Ministry of Environment, Korea.

Table 1. Characteristics of land use types and meteorological factors at 12 monitoring sites. Land use data was extracted from each monitoring site within a 250-m buffer, and then the proportions (%) of land use were calculated.

CategoryVariablesAbbreviationMean (±SD *)
Land use typesResidential area (%)RESD28.4 (±23.8)
Commercial area (%)COMM16.2 (±13.2)
Culture and sport area (%)CULT7.9 (±13.5)
Public facilities area (%)PUBL8.5 (±11.8)
Artificial grassland (%)GRAS7.5 (±10.3)
Inland wetland (%)WETL0.0 (±0.1)
Bare soil (%)BARE2.6 (±4.1)
Inland water (%)WATE5.0 (±6.4)
Industrial area (%)INDU0.1 (±0.3)
Traffic area (%)TRAF23.8 (±6.7)
Meteorological factorsAverage daily temperature (°C)TempAVE21.4 (±4.7)
Maximum daily temperature (°C)TempMAX25.6 (±4.6)
Minimum daily temperature (°C)TempMIN17.7 (±5.2)
Average daily wind speed (m/s)WindAVE1.4 (±0.7)
Average daily precipitation (mm)PrcpAVE0.3 (±1.1)
Average daily humidity (%)HumiAVE70.6 (±13.3)
Total daily precipitation (mm)PrcpTOT5.7 (±22.6)

In addition, we used seven meteorological factors, which can influence mosquito occurrence, including temperature (mean, maximum, and minimum), precipitation (average and total), wind speed, and humidity (Table 1). These factors were then applied to predict the degree of mosquito occurrence (see Data Analysis Section for the detailed explanations). All the meteorological data were obtained from three automatic weather stations (AWSs), which were located near the research sites and operated by the Korea Meteorological Administration (http://www.kma.go.kr). Three AWSs were located within a 2 km distance, and meteorological factor differences among the stations were small (Figure 1).

2.3. Data Analysis

2.3.1. Classification of Habitats

We evaluated the relationships between the occurrence patterns of mosquitoes and land use types in two phases. First, a hierarchical cluster analysis was applied to classify the spatial difference in monitoring sites based on the proportions of land use types (Table 1) using Ward’s linkage method with a Euclidean distance measure. A multi-response permutation procedure (MRPP) [40] was applied to test whether or not there were significant differences among the clusters. Second, principal component analysis (PCA) was conducted based on mosquito abundance. Mosquito abundance at each site was transformed with natural log (ln (x + 1)). Then, the sites on the PCA ordination map were marked as groups from the cluster analysis extracted from the similarities of land use types. PCA is an indirect gradient analysis method for seeking the strongest linear correlation structure among variables [41], and it is a technique widely used for reducing the dimensions of multivariate problems. In PCA, eigenvalues, which explain a portion of the original total variance, are calculated. Each axis score using the eigenvector, which contains the coefficients of the linear equation for a given axis, was shown in an ordination [42]. Cluster analysis, MRPP, and PCA were performed using PC-ORD 5.0 [43].

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