General Morphology

The development cycle of mosquitoes includes two phases (see Figure 1.1): (1) an aquatic phase, with the succession of immature stages, such as eggs, larvae (four stages) and pupae, and (2) an air phase with the male and female adults.

The eggs are ovoid and measure approximately 0.5 mm. They are laid either on the surface of water (e.g. Anopheles and Culex) or near the surface of water (e.g. Aedes). The eggs may be laid separately (e.g. Anopheles and Aedes) or close together in the form of an "egg raft'' at the time of oviposition (e.g. Culex and Coquillettidia). The eggs are able to float due to side floats (e.g. Anopheles) or apical floats (e.g. Culex). The variations in egg ornamentation have been used to dismember the complex Anopheles maculipennis in order to understand the phenomenon of "anophelism without malaria'' in Western Europe.

This raises the concept of "species complex'', in which sibling species are important disease vectors whilst others are not involved in pathogen transmission at all, despite the fact that these species cannot be differentiated morphologically. Identification of the individual species must be based on more sophisticated techniques, in particular molecular ones. Many of the main vectors of pathogenic diseases belong to a species complex, such as Anopheles gambiae, An. dirus, An. farauti, Culex pipiens, etc.

The larvae which emerge from the egg evolve in four stages (L1, L2, L3 and L4), intersected with three moults which allow the larvae to grow from 1 mm (L1) to 15 mm (L4) in a week (longer in temperate regions). The four larval stages have a comparable general morphology. The larva is composed of three parts: head,

Figure 1.1 Mosquito cycle in its habitat.

thorax and abdomen. The 8th abdominal segment is modified (the "respiratory segment'') with two important structures: (1) on the lateral side, the "comb" (pecten) which is made up of spines and scales of different forms, sizes and numbers, according to the genus and species of the mosquito, and (2) on the dorsal face, the spiracular apparatus which is located either directly on the tegument for Anopheles, or at the end of a respiratory siphon for Culicinae. This is a useful characteristic with which to differentiate the position of the larvae, as Anopheles larvae (without siphon) stay parallel to the water surface, whilst Culex or Aedes larvae (with a siphon) have an oblique angle of suspension to the water surface. For Mansonia, the end of the siphon is modified in a hard organ used to bore plants. Mansonia larvae do not breathe like other mosquitoes but attach themselves to the roots, leaves and stems of aquatic plants in order to obtain their air supply.

The pupa's morphology is completely different from that of the larva, consisting of two parts: (1) a prominent cephalothorax equipped with two respiratory trumpets (the pupa does not have an oral apparatus as it breathes but does not feed), and (2) an abdomen made up of eight visible segments (the ninth segment is barely visible), the eighth segment carries a pair of swimming paddles. The pupa is quite mobile and dives when disturbed. Its lifespan is short (one to two days).

The adults. Male and female mosquitoes can be easily differentiated by observing the head and the end of the abdomen. The head comprises of two compound eyes made up of hundreds of ommatidia, and two antennae with 15 articles in the male and 16 articles in the female. In the male, there is a great number of large setae which allows for the easy recognition of the "plumose" antennae, whereas the female has "pilose" antennae. The oral apparatus is of the "sucker" type for the male and the "biter" type for the female which includes: (1) a labium folded up in a gutter and finished by two labellums; in this gutter, there are six piercing stylets which will penetrate the skin and search for a capillary for the intake of blood; (2) the labrum (or upper lip), which serves as the "roof" of the food channel; (3) the hypopharynx which is connected to salivary glands by the salivary channel and forms the floor of the food channel; and (4) two mandibles and two maxillae. On both sides of this female piercing apparatus, there is a pair of maxillary palps, which are as long as the proboscis for Anopheles but shorter than the proboscis for Culicinae. This difference allows for easily differentiation of Anopheles from other mosquitoes.

The thorax includes three segments: the prothorax, the mesothorax, and the metathorax. Each segment comprises a pair of legs made up of a hip (or "coxa"), a trochanter, a femur, a tibia and a tarsus with five articles; the last article carries at its end two claws which help the mosquito to hold on to the support. The legs carry more or less coloured scales which are used for mosquito identification. The second segment, or "wing segment", is the largest and carries a pair of wings (two wings = "Diptera") with veins and scales whose form and colour ("wing ornamentation'') are also used for species identification. The third segment carries halters which are used for balance during flight. The morphology of the lateral parts of the thorax called "pleurites" is very much used in systematics. There are two respiratory spiracles (on the second and third segments). The dorsal part of the second segment is called the "scutum" which is prolonged by a "scutellum". This is simple and rounded for Anopheles or trilobed for Culicinae. The abdomen is composed of ten segments of which eight are quite visible. Each segment comprises a dorsal chitinized part (tergite) and a ventral chitinized part (sternite), connected by a very extensible pleural membrane which allows for the swelling of the abdomen of the female after a blood meal or the maturation of the ovaries. Segments nine (genital segment) and ten (anal segment) are quite modified. The genital apparatus on the male is very complicated and its morphology is used in systematics (especially for Culex). Between 12 and 24 hours after the emergence of the adult, the male genital apparatus undergoes a 180° rotation and becomes ready for mating. The terminalia surround a complex penis (the "phallosome") which is located on the tenth segment. The abdomen of the female ends with two cercus.

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