Like most true flies, mosquitoes require a great deal of protein in order to breed. This is particularly true of females.
However, since the species has a short life, much of it aquatic, evolutionary processes took a side step. Rather than developing complex systems for taking in plant or decaying animal matter and designing ways of digesting it, mosquitoes have developed the means to digest blood in order to acquire the proteins they need to reproduce.
This is quite a time and effort saver, since mammalian blood has a large protein component. It means that with many species of mosquitoes, a female needs to only have a single large feeding in order to produce her eggs. This is important because the chances of any single female finding a suitable host for a meal are actually pretty slim. She makes up for it by laying a very large number of eggs when she does get such a meal, which is a further reason for the increased need for protein.
Mosquitoes are not alone among the insects, arachnids, and other small creatures in consuming blood for the protein. Ticks, chiggers, tsetse flies, and fleas do the same thing. There are even mammals that consume blood for the purpose of survival or reproduction. An example is the vampire bat, which ingests large quantities of blood relative to body weight.
The itching irritation that arises from the mosquito’s bite is a reaction to anti-coagulants that are injected into the host during feeding. This is necessary because without the anti-coagulants, a mosquito would need to puncture the flesh numerous times in order to receive a full meal.
It is the injection of the anti-coagulants that makes the mosquito dangerous, and not just an irritating pest. Like fleas, mosquitoes can and do pick up viruses and bacteria when feeding, and mosquitoes often inject the viruses and bacteria into other hosts during the initial injection.
The question then arises; why does the male not require the additional protein? This is a reasonable question, and it isn’t as complex as one might think.
Both male and female mosquitoes feed as aquatic larvae. But while the female must lay self-contained eggs, each with its own food source so the larvae can develop, the male needs only fertilize the female. This requires far less energy, and thus, less protein. So while a female gets nowhere nearly enough food while living as a larva, the male receives all he needs for his life’s purpose; fertilizing the female.
Unlike the males of other species, since their adult life is so short, basically neither male nor female mosquito needs the added protein to survive to the end of its lifespan. However, the female must have more protein in order to produce eggs and to furnish enough energy for the eggs to develop into new mosquito larvae.
The basic reason why only female mosquitoes suck blood is because only they are required to do so, for the perpetuation of the species.
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