Insects are the pre-eminent form of metazoan life on land. The class Insecta contains over three-quarters of a million described species. Estimates for the total number of extant species vary between 1 and 10 million, and it has been calculated that as many as 1019 individual insects are alive at any given instant (McGavin, 2001). That gives about 200 million for each man, woman and child on Earth! It is estimated that there are 14 000 species of insects from five orders that feed on blood (Adams, 1999) but, thankfully, only 300 to 400 species regularly attract our attention. These blood-sucking insects are of immense importance to humanity.

Humans evolved in a world already stocked with blood-sucking insects. From their earliest days insects would have annoyed them with their bites and sickened them with the parasites they transmitted. As humans evolved from hunters to herders, blood-sucking insects had a further impact on their wellbeing by lowering the productivity of their animals. It is reasonable to assume that, because of their annoyance value, humanity has been in battle with blood-sucking insects from the very beginning. In recent years this battle has intensified because of an increasing intolerance of the discomfort they cause, our fuller understanding of their role in disease transmission and the demand for greater agricultural productivity. But despite considerable advances in our knowledge of the insects and improvements in the weapons we have to use against them, there is still no sign of an eventual winner in this age-old battle.

Probing stimulants

Probing occurs in response to the quality and quantity of host-related stimuli (Friend and Smith, 1977). As in the other phases of host location, the response is not performed in a completely stereotyped way. Anyone who has slept under a mosquito net is likely to have had firsthand experience of this flexibility of responsiveness. If the skin becomes pressed against the net, mosquitoes are quite happy to probe and feed through it. The set of stimuli received in these circumstances must be quite different from the range of host-related stimuli that an insect landing directly on the skin would normally receive. The new set of stimuli received after landing can still influence host choice even at this very late stage, with insects choosing to leave rather than feed (Gikonyo et al., 2000). Post-landing responses can also vary with internal changes of circumstance such as the insect's degree of hunger (Brady, 1972; Brady, 1973; Friend and Smith, 1975) or water deprivation (Khan and Maibach, 1970; Khan and Maibach, 1971), feeding experience (Mitchell and Reinouts van Haga, 1976) or reproductive state (Tobe and Davey, 1972). Even if internal and external factors are carefully controlled, different individual insects still show a considerable degree of innate variation in their response to a host (Gatehouse, 1970). This flexibility of close-range responsiveness allows the insect to make the most of the differing circumstances in which it contacts hosts.

Like all other organisms, blood-sucking insects have their own array of parasites. Many of these parasites are common to a range of different insects, blood-sucking and non-blood-sucking alike. Others are transmitted between the vertebrate host and the insect and so are peculiar to blood-sucking insects, and the parasites normally depend on the blood-sucking habit for their existence. In line with the approach taken in this book, it is the relationships between blood-sucking insects and this latter group of parasites on which I will concentrate in this chapter.

Transmission routes

Table 8.1 shows that blood-sucking insects are responsible for the transmission of many important disease-causing organisms. At its simplest, transmission may involve the insect as a mechanical bridge between two vertebrate host species. At its most complex, transmission involves an obligatory period of replication and/or development by the parasite in the vector insect. A division is often drawn between ‘mechanical’ and cyclical or ‘biological’ transmission.

Mechanical transmission is said to occur when the blood-sucking insect is no more than a flying pin, transferring pathogens from one vertebrate host to another on contaminated mouthparts. Relatively little work has been carried out on the relationships grouped together as mechanical transmission and the possibility of more complex interactions should not be ignored. For example, it has been suggested, on the basis of epidemiological evidence, that some mechanism for the concentration of parasites in the vector's mouthparts may occur in the mechanical transmission of trypanosomes such as Trypanosoma vivax viennei and T. brucei evansi (Wells, 1982).

Every animal needs to maintain a steady internal environment in order to carry out the various physiological processes that together allow life to continue. This maintenance is known as homeostasis. One of the key organs in homeostasis is the animal's surface covering, the skin, cuticle or other tegumentary substance. In mammals and birds the surface covering is extended to incorporate an outer insulating layer of hair or feathers. In many such animals this layer has proved to be an excellent home for permanent and periodic ectoparasitic insects, providing many different species with a relatively constant environment in which to live.

Vertebrate skin is formed of an inner dermis and an outer epidermis (Fig. 7.1). The thickness of these two layers, and the ratio between them, varies considerably between different parts of the body. The dermis is a connective tissue layer containing blood and lymphatic vessels, and nerves. Embedded in the dermis are the acini of the gland systems that open onto the skin's surface. There are two basic types of skin-associated gland in mammals. The sebaceous glands produce an oily secretion called sebum, which helps prevent the skin and hair from drying out and possesses anti-microbial components. The sweat glands (which are not present in carnivores or rodents) produce a watery secretion used in temperature regulation and in maintaining water and salt balance in the body.

Blood-sucking insects are the vectors of many of the most debilitating parasites of humans and their domesticated animals. In addition they are of considerable direct cost to the agricultural industry through losses in milk and meat yields, and through damage to hides, wool and other products. So, not surprisingly, many books of medical and veterinary entomology have been written. Most of these texts are organized taxonomically, giving details of the life cycles, bionomics, relationships to disease and economic importance of each of the insect groups in turn. I have taken a different approach. This book is topic-led and aims to discuss the biological themes common to the lives of blood-sucking insects. To do this I have concentrated on those aspects of the biology of these fascinating insects that have been clearly modified in some way to suit the blood-sucking habit. For example, I have discussed feeding and digestion in some detail because feeding on blood presents insects with special problems, but I have not discussed respiration because it is not affected in any particular way by haematophagy. To reflect this better I have made a slight adjustment to the title of the book in this second edition. Naturally there is a subjective element in the choice of topics for discussion and the weight given to each. I hope that I have not let my enthusiasm for the particular subjects get the better of me on too many occasions and that the subject material achieves an overall balance.

The difficulty that hungry blood-sucking insects have in locating their next blood meal depends upon the closeness of their association with the host. At one extreme we have the permanent ectoparasites which are in the happy position of having food continually ‘on tap’. Only by accident will they find themselves more than a few millimetres from the skin of the host and the blood that it holds. At the other extreme are those temporary ectoparasites, such as blackflies and tabanids, that do not remain permanently in the vicinity of the host. When these insects are hungry their first problem is to locate the host, often a difficult and complex behavioural task. These differences in lifestyle are reflected in the number of antennal receptors different types of blood-sucking insect possess (Chapman, 1982). Not surprisingly the more independent, host-seeking insects possess the most receptors. Thus, lice have only 10 to 20 antennal receptors and fleas about 50, but the stablefly, which spends most of its time at some distance from the host, has nearly 5000 antennal receptors. Considering two bugs, we see that Cimex lectularius has only 56 antennal receptors compared to 2900 on the more adventurous Triatoma infestans.

The level of reliance on blood is also an important factor in host location. So for obligate haematophages such as the tsetse fly and triatomine bugs regular host location is absolutely essential.

It is believed that haematophagy arose independently at least six times among the arthropods of the Jurassic and Cretaceous periods (145–65 million years ago) (Balashov, 1984; Ribeiro, 1995). The very patchy nature of the insect fossil record means that discussion of the evolution of the blood-sucking habit has until now relied heavily on detective work, with the major clues lying in the diversity of forms and lifestyles seen in modern-day insects, and in some cases in the details of their relationships with vertebrates. From careful interpretation of this evidence quite credible accounts of the likely evolution of the blood-sucking habit can be made. From this starting point it has been convincingly argued that the evolution of the blood-sucking habit in insects has occurred on several occasions, in each case along one of two main routes (Waage, 1979), and these are discussed below. Insect molecular systematics is beginning to emerge from its ‘Tower of Babel’ stage (Caterino et al., 2000) and it will make a major contribution in defining the detail of the evolutionary routes taken by haematophagous insects (Esseghir et al., 1997; Hafner et al., 1994; Lanzaro et al., 1998; Mans et al., 2002; Sallum et al., 2002). The proposed population bottleneck suffered by phlebotomines in the late Pleistocene and the subsequent radiation of the species out from the eastern Mediterranean sub-region is a good example of what we can expect (Esseghir et al., 1997).

This section of the book gives an outline of the major groups of insect that feed on blood, concentrating on those groups that are habitual blood feeders. Detailed coverage of each blood-sucking group is not attempted; that would far exceed the space available in a book such as this. Instead, this chapter has been written as a quick reference section for those new to medical and veterinary entomology and for those who need a quick outline of a particular group in order to make most use of other parts of the book. A basic outline of each assemblage is given, covering the essential details of the group's medical and veterinary importance, morphology, life history and bionomics.

Insects are individuals, and even within a single species insects often do things in subtly different ways from each other; this is, after all, a prerequisite for evolution. Consequently, generalizing about a genus, family and whole class of insects is an imprecise business, but bearing in mind that there are many exceptions to these generalizations, this approach can still be a good way of introducing typical group characters. In discussing each group, quantitative values are often given for fecundity, duration of each stage in the life cycle, longevity, etc. These figures are good guidelines to expected values under optimum conditions, but in the field one can expect a good deal of variation.

Host choice

Blood-sucking insects feed from a range of different host animals; because of the bites we receive we are acutely aware of the fact that many of them feed from humans, but many other animals are also exploited, including other mammals, birds, reptiles, amphibians and fish, and even insects, arachnids and annelids (Hocking, 1971). Any one insect does not feed equally well from all of these potential resources; it displays host choice. For some insects, particularly some permanent ectoparasites, the host choice may be very specific. Occasionally, for example for human lice, just a single species. For other blood-sucking insects host choice is clearly not as restricted as introduced exotic hosts (e.g. those in zoos) quickly become incorporated into the diet of local blood-sucking insects.

Let us consider what is meant by host choice. In its main sense it denotes the species of host animal or animals from which blood-sucking insects obtain their blood meals. But host choice can go beyond particular species of host chosen. Insects often choose to feed on particular individuals from among preferred species, which may well have implications for disease transmission (Burkot, 1988; Kelly, 2001; McCall and Kelly, 2002).

Although most blood-sucking insects in their undisturbed, natural surroundings show a preference for feeding from a particular group or species, or even a primary cohort of their chosen species, the degree of host specificity shown varies greatly from one type of insect to the next.