Fluid Feeding (haematophagy, haem detoxification)
Surface Nutrient Absorption (e.g. tapeworms)
Many animals, especially invertebrates, feed on fluids, or soft plant and animal tissues, by piercing and sucking. Some free-living flatworms seize their invertebrate prey by wrapping themselves around it. They then penetrate the body wall with a protrusible pharynx that penetrates and then sucks out the victim's body fluids and viscera. Mosquitoes have piercing mouthparts which the females use for feeding directly from blood vessels (the males feed on nectar and sap). Piercing mouthparts are also found in plant-feeding insects such as aphids. In aphids, the mandibles and maxillae have become modified into stylet-like structures that lie within a grooved labium. The stylets are extremely long to allow location and penetration of phloem vessels deep within the plant host. The labium does not penetrate the plant but bends or loops as the insect brings its head closer to the plant surface. We will consider haematophagy, blood-feeding, as an example of fluid feeding, in some detail.
Haematophagy - feeding on blood - has developed independently in many different organisms but through the process of convergent evolution many blood-feeding arthropods, and indeed other animals, including vertebrates, show similar mechanisms for locating, ingesting and processing their blood meals. One of the major problems for blood feeders is to overcome blood clotting and cessation of bleeding. It appears that the saliva of the vampire bat, hookworms (parasitic nematodes that live in the gut of mammals), leeches, ticks, and blood-sucking insects all contain various anticoagulant agents.
The size of the blood meal is affected by a range of factors (e.g. temperature, time since last meal, mating status, source of blood meal etc). Blood feeding insects often take huge meals. Adult insects commonly take in meals which are twice their unfed body weight, while the nymphal stages of the blood-sucking Hemiptera may take meals of ten times their unfed weight. These large blood meals impair the mobility of the insect, increasing the short- term chances of being swatted by the host or eaten by a predator. For example, the flight speed of one species of tsetse fly ( a vector of sleeping sickness) has been found to be reduced from 15mph to 3mph after feeding with the fly often only capable of a downward glide away from the host. However, as can be seen by its widespread adoption by so many different groups of blood-feeding insect, the disadvantages of taking such large blood meals must clearly be outweighed by the disadvantages.
The benefits are probably as follows:
1) The reduced risk of being swatted which goes with visiting the host as few times as possible.
2) The minimization of the number of visits that the insect must pay to the host. This could be advantageous in several ways:
a) If the hosts are difficult to find, then the insect must make the most of each encounter.
b) It would be selectively advantageous to take the largest meal possible if the extra…