The Ecology of Malaria Vectors

Jacques Derek Charlwood

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000012187
• Edition: 1
• Topic: Médecine
• Subtopic: Maladies infectieuses

This practical book covers all aspects of the biology of malaria vectors, with notes on the vectors of dengue. It is the first work in this field to concentrate on mosquitoes, rather than covering all disease vectors. Authored by renowned field entomologist Jacques Derek Charlwood, it disseminates his vast experience working on mosquito biology, ecology and the evaluation of new vector control tools across five continents over the past 40 years.

Covering all aspects from classification and systematics, population dynamics, vector control, to surveillance and sampling, epidemics, and a selection of case histories, the book also considers genetics and resistance, Aedes biology, and malaria and dengue models. It is designed to fill the gap between very specialized texts and undergraduate books on general disease vectors, and is ideal as a textbook for postgraduate courses in entomology and mosquito vectors of disease.

CHAPTER 1

Classification and Systematics

The following comes mainly from the ‘Introduction to Insects’ by George C. McGavin in The Insects: Structure and Function, Fifth Edition:

It is, I think, both informative and entertaining. If you haven’t thought much about the importance of insects before, you may do so after reading this.

Stephen J. Simpson and Angela E. Douglas

The ancestor of the Arthropoda was in all probability a segmented worm-like marine creature that lived in oceans during the late Precambrian era. By the early to mid-Cambrian (540–520 million years ago) the early arthropods had already evolved into a range of clearly recognisable groups with distinct body plans. Arthropods are characterised by a number of features: the possession of a periodically moulted, chitinous cuticle that acts as a rigid exoskeleton for the internal attachment of striated muscles; segmental paired legs; and the aggregation and/or fusing of body segments into discrete functional units, of which the most universal is the head. The exoskeleton is the key to their successes and limitations. It has several functions – to support the body, protect the animal from the external environment, reduce water loss, store energy and (through the development of limbs and wings) assist in locomotion.

Flexible cuticle between sections of the limbs and body segments forms joints and allows movement by muscles attached to the cuticle. The exoskeleton also extends into the gut (foregut and hindgut) and lines the tracheae, the tubes used in respiration by the myriapods and insects. The combination of hardened plates with soft membranes between them gives both strength and rigidity to the body as well as flexibility. Thus, the construction of an arthropod as a series of jointed tubes gives it considerable mechanical advantage; the skeleton has greater power of resistance to bending than the endoskeleton of vertebrates. For the same cross-sectional area of muscle and skeleton, a solid endoskeleton would be nearly three times weaker than a hollow exoskeleton. Similarly, to have the same strength as an exoskeleton, an endoskeleton would need to be considerably thicker, leaving little additional space for musculature. Because of its mechanical efficiency, and being composed of remarkably flexible material, the exoskeleton of arthropods has been expressed in an astonishing range of body forms and structures unrivalled anywhere in the animal kingdom. However, the presence of an exoskeleton has a number of limitations: it puts a maximum limit on the physical size of an organism; it limits growth, thereby necessitating the shedding of the outer covering for any increase in size; and the skeleton needs to be perforated with sensilla to monitor the outside world. Growth can only be achieved in organisms with a hardened outer covering by the process of moulting. The inner layers of the cuticle are digested by a series of enzymes to separate the old ‘skin’ from the newly formed cuticle. After moulting, the arthropod swallows water or air to inflate its flexible cuticle until this hardens. The process of moulting is controlled by a complicated interplay between special hormones. The process of respiration has been solved in different ways by the various groups of arthropods. Insects and myriapods have small tubes, tracheae, through which oxygen diffuses to all parts of the body. The physical constraints governing the diffusion process are such that an increase in size of an insect or myriapod is not accompanied by a proportional increase in the rate at which oxygen is delivered to the tissues; this is one of the principal factors governing the maximum size of insects. However, respiration via tracheae can be very efficient and is able to operate with a very small difference in the partial pressure of gases between the tissue end of the tracheae and the outside atmosphere. Tracheal respiration delivers oxygen to insect muscle, which is the most active tissue in the animal kingdom.

A little over 1.5 million species of living organism have been scientifically described to date. The vast majority (66%) are arthropods such as crustaceans, arachnids, myriapods and insects. Insects represent 75% of all animals, and one insect order – the beetles (Coleoptera) – is famously species-rich (J. B. S. Haldane once famously said ‘If God loved life he loved insects and if he loved insects he loved beetles’). One thing is clear, however – the full extent of Earth’s biodiversity remains a mystery. From attempts more than 30 years ago to estimate the number of extant species to the present day, we still only have a rough idea of how many species live alongside us. Estimates range from as few as 5 million to perhaps as many as 10–12 million species. The task of enumerating them may become substantially easier as the loss and degradation of natural habitats, especially the forests of the humid tropics, continues unabated. It is certain that the majority of insect species will become extinct before they are known to science.

Insects are the dominant multicellular life form on the planet, ranging in size from minute parasitic wasps at around 0.2 mm to stick insects measuring 35 cm in length. Insects have evolved diverse lifestyles and although they are mainly terrestrial, there are a significant number of aquatic species. Insects have a versatile, lightweight and waterproof cuticle, are generally small in size and have a complex nervous system surrounded by an effective blood–brain barrier. Insects were the first creatures to take to the air and have prodigious reproductive rates. These factors, together with the complex interactions they have with other organisms, have led to their great success both in terms of species richness and abundance. The very high diversity of insects today is the result of a combination of high rates of speciation and the fact that many insect taxa are persistent – that is, they show relatively low rates of extinction. In comparison to insects, vertebrate species make up less than 3% of all species. As herbivores, vertebrates are altogether out-munched by the myriad herbivorous insects. In tropical forests, for example, 12%–15% of the total leaf area is eaten by insects as compared with only 2%–3% lost to vertebrate herbivores. Termites remove more plant material from the African savannahs than all the teeming herds of wildebeest and other ungulates put together. Vertebrates also fail to impress as predators. Ants are the major carnivores on the planet, devouring more animal tissue per annum than all the other carnivores. In many habitats ants make up one-quarter of the total animal biomass present. Indeed, it is estimated that the biomass of ants on the planet is the same as that of humans. Insects pollinate the vast majority of the world’s 250,000 or so species of flowering plant. The origin of bees coincides with the main radiation of the angiosperms approximately 100 million years ago, and without them there would be no flowers, fruit or vegetables. At least 25% of all insect species are parasites or predators of other insect species. Insects are also important in nutrient recycling by disposing of carcasses and dung. Insects are the principal food source for many other animals. Virtually all birds and a large number of other vertebrates feed on them. An average brood of great tit chicks will consume around 120,000 caterpillars while they are in the nest and a single swallow chick may consume upwards of 200,000 bugs, flies and beetles before it fledges.

Insects are also nutritious. They are often a prized part of the diet in many cultures. In Papua New Guinea, the larvae of wood-boring longhorn beetles are barbecued or roasted. The larvae are about the size of a person’s thumb and are very greasy (not to everyone’s taste). More appetising are the various beetles and orthopterans eaten in Cambodia and other Southeast Asian countries (copious small light traps are scattered throughout the rice fields to catch them). In Tanzania, the grasshopper (locally called senene) (Figure 1.1) is a seasonal delicacy caught in huge light traps.

Figure 1.1 ‘Senene’ are a delicacy in the Kagera region of Tanzania. Fried, they remain good to eat (and they are delicious) for a year – until the next short season of abundance.

Insects can also have a huge negative impact on humans. One-sixth of all crops grown worldwide are lost to herbivorous insects and the plant diseases they transmit. About one in six human beings alive today is affected by an insect-borne illness such as plague, sleeping sickness, river blindness, yellow fever, filariasis and leishmaniasis. About 40% of the world’s population are at risk of malaria.

Malaria transmission varies regionally, and sometimes over very short distances, as a consequence of factors such as transmission intensity, which vector species are dominant, and characteristics of the human populations. At a global level, there are important differences between sub-Saharan Africa and the rest of the world. The first is that the African vectors Anopheles gambiae, A. coluzzii and A. funestus are the most efficient vectors of malaria and the ones with the strongest preferences for humans. This is perhaps because these species evolved with humans. Indeed, to paraphrase Voltaire ‘If Anopheles gambiae did not exist man would have created it’. Africa has two other anopheline species, A. arabiensis and A. nili that are also very efficient vectors. All these species tend to bite indoors and at night, and because of these vectors, Africa overall has very intense transmission.

The other source for the following is from the Introduction to Medical Insects and Arachnids (Lane and Crosskey, eds., 1993).

TAXONOMY

Confucius said something along the lines that the first thing one must do is give something a name, for it is only then that one can proceed. Thus, biological classification aims to group and categorise biological entities that share some unifying characteristics. Identification is a precise term describing the allocation of an unknown specimen to a predefined group. In any system, it is important that it is easy to retrieve information, and this is best done if similar animals are grouped together. There is, however, no single ‘right’ classification; they are suggestions of how organisms are related.

Taxonomy is the arrangement of similar entities (objects) in a hierarchical series of nested classes, in which each, more inclusive, higher-level class is subdivided comprehensively into less inclusive classes at the next lower level. These classes (groups) are known as taxa (singular: taxon). The level of a taxon in a hierarchical classification is referred to as a taxonomic rank or category.

There are two basic stages in systematics: first, recognition of biologically operational units (species, subspecies, etc.), which are the basic units of any classification; and secondly, the ordering of these units into higher categories or levels (genera, families, etc.).

The familiar binomial system for naming animals and plants was devised by the Swedish naturalist Carl Linnaeus, and the tenth edition of his great work titled the Systema Naturae (published in 1758) marks the beginning of scientific biological nomenclature. The name of a species consists of two words, the first being the generic name (used for all member species of a genus) and the second word being the specific (species) name.

WHAT IS A SPECIES?

Taxa at all levels above the...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Foreword
• Preface
• Author
• Chapter 1: Classification and Systematics
• Chapter 2: Mosquito Life Histories
• Chapter 3: The Search for the Host
• Chapter 4: Dispersal
• Chapter 5: Population Dynamics
• Chapter 6: Mapping
• Chapter 7: Vectorial Capacity
• Chapter 8: Chemical Methods of Vector Control
• Chapter 9: Alternative Methods of Vector Control
• Chapter 10: Surveillance and Sampling
• Chapter 11: Epidemics
• Chapter 12: The Diseases – Malaria, Filariasis and Dengue
• Chapter 13: Sampling Techniques
• Chapter 14: Laboratory Studies
• Chapter 15: Global Heating – ‘The Future Ain’t What It Used to Be’
• Chapter 16: Some Case Histories
• Chapter 17: Some Useful Websites
• Glossary
• References
• Bibliography
• Index