Blood-sucking insects take huge meals. Temporary ectoparasites such as the tsetse fly typically ingest more than their own unfed body weight in blood. The reasons are twofold. First, taking a blood meal is a very dangerous activity and taking huge blood meals minimizes the number of times an insect must associate with the host. Second, locating the host is often difficult and huge blood meals are a way of making the most of each encounter. Mouthparts are adapted to the blood-feeding habit. Typically, they are either of the piercing kind seen in mosquitoes, bugs, lice, and fleas or the cutting kind seen in tabanids, black flies, and biting flies.
The host usually responds to feeding activity, particularly the injection of saliva, by mounting an immune response that includes pruritis (itching). Typically this begins to occur about 3 min after feeding commences. Thus, there is a selective advantage in completing the blood meal within this 3-min "safety period" after which the host will be alerted to the presence of the insect. To help achieve this, bloodsucking insects have produced a range of antihemostatic molecules in the saliva, one of the major functions of which is to minimize host contact time.
Antihemostatic molecules produced by the blood-sucking insect include anticoagulant molecules working variously, for example, on thrombin or factors VIII and X. However, platelet plugging of small wounds is probably of more importance to blood-sucking insects than blood coagulation. Consequently, they also produce anti-platelet aggregating factors such as apyrase. These are used to impede the plugging of the penetration wound in capillaries and to prevent clogging of the insect mouthparts. The insect saliva also contains powerful vasodilatory substances to increase blood flow to the wound and anti-histamines that will minimize inflammation and itching, possibly extending the "safe period." Salivary components are also important as they can facilitate the transmission of arthropod-borne pathogens. For example, the production of Leishmania-enhancing factor in the saliva of the sand fly Lutzomyia longipalpis enhances the establishment of the parasite Leishmania major in the vertebrate host.
It has also been shown that such effects may be limited to naive hosts, suggesting that the history of exposure to vector saliva may influence the outcome of potentially infectious inoculations. Parasites can also manipulate the salivary glands to their own advantage. Thus, malaria sporozoites damage the salivary glands of mosquitoes, reducing antihemostatic effectiveness, and thus extend probing time and increase the chances they will be transmitted to a new host.
Some blood-sucking insects feed only on blood during their entire life. Examples include the tsetse flies, streblids, hippoboscids and nycteribiids, triatomine and cimicid bugs, and lice. Blood is deficient in certain nutrients such as the B-group vitamins and pantothenic acid, and the insect cannot make these itself. To make up for this deficiency, these obligate hematophages harbor symbiotic microorganisms that produce these extra nutrients. These symbionts are often housed in a specialized body compartment, traditionally called a mycetome or, more recently, a bacteriome. For example, the tsetse fly Glossina harbors three symbiotic microorganisms, including Wigglesworthia glossinia, which is from the y-subdivision of the Proteobacteria, in the bacteriome of the anterior gut.
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