Aphid

Aphids are insects that make up the taxonomic superfamily Aphidoidea, an extremely large group containing about 4000 different species. Almost all of the species are harmless, very few growing longer than about 6 millimeters. However, there are roughly 250 species that are very harmful to agriculture. Their high reproduction rates and insatiable appetite for plant fluids makes them an almost unstoppable force, destroying whole crops and spreading plant diseases across the country. Although they are a huge danger to farmers, they are also very fascinating to study. The females, instead of laying eggs, can make miniature clones of themselves, as many as five a day! They can also produce carotenoids by themselves, something no other known animal is able to do.

Anatomy
The anatomy of the aphid varies from species to species, but some characteristics are common throughout the superfamily. The body of an average aphid is soft and about five millimeters in length. Most are green in color, earning them the nickname 'greenfly', but special nutrients that they produce can make them red, yellow, and many other colors. Their mouth is a combination between the maxillae and the mandibles, forming a sucking mouth part called the proboscis. The aphid possesses two compound eyes. Each have an ocular tubercle behind it, made up of three lenses. It also has two antennae, which can be made up of many segments. The aphid has six legs, each tarsi being double-jointed and double-clawed. On the abdomen lies a pair of cornicles. Some species of aphids are able to make these cornicles, or siphunculi, can excrete weak defensive juices. Some aphids can have wings on their backs, but these usually only appear when the aphids are moving plants or during mating season. At the end of the abdomen lies a tail-like protrusion called a cauda.

Reproduction
Due to the aphid's high vulnerability, it needs to reproduce extremely quickly and often in order to keep the species alive. An aphid can reproduce two ways: Through asexual reproduction and sexual reproduction.

Asexual reproduction
In order to reproduce more quickly, female aphids can reproduce without male fertilization by a process called parthenogenesis. Through this process, a female aphid can give birth to genetic clones of herself. Instead of being laid in eggs, the clones emerge as nymphs with a thin membrane surrounding them for protection until they can mature. In as little as 8 to 10 days, the nymphs mature into adults, and can begin to produce more females through parthenogenesis. Adult aphids can make up to five aphids a day for a month. If the population of aphids on one plant grows too large, or if the plant begins to die, then the females with wings begin to appear in the cycle. They then glide on air currents with their wings to different plants, where they start a new aphid population.

Sexual Reproduction
When the months start to get colder, some changes occur in the aphid life cycle. Males, for unexplained reasons, start to appear in the cycle. They fly to the plants with many female aphids, and when a new set of daughters are born, the males mate with them through internal fertilization. The females then lay the eggs, which are black with thick shells to protect them against the cold. The eggs remain unhatched until spring when wingless female nymphs emerge, ready to start the process over. Since asexual reproduction takes over during the warm months, all of the males disappear until sexual reproduction is needed again in the winter. Since sexual reproduction takes so much longer than asexual reproduction, no sexual reproduction occurs during the warm months when the nymph clones can survive on their own. If an area stays warm enough all year, then it is possible that no sexual reproduction will happen at all.

Diet
The entire diet of the aphid consists of the phloem found in plants. The aphid extends its proboscis into the plant, excreting a special liquid as it does so to digest the plant sap. The aphid then begins to suck out the fluids of the plant, which are composed entirely of sugar. The aphid spends almost its entire life feeding. An aphid sucks so much fluid out of a plant that an average colony can kill a plant in just a few days. When a plant dies, the aphid colony moves on to another plant, and immediately begins to feed upon the juices of that plant. Many aphid species are specialized, and will only suck the juices out of one particular plant species.

Habitat
Aphids live on the plant that they feed off of. It is entirely likely that the aphid will never leave the plant during the course of its life, but instead will always remain clinging to the underside of the leaf. However, if the plant dies, the aphid will move to a new plant. Aphids prefer warmer climates, as the warm weather means that they do not need to include sexual reproduction in their life cycle, but the aphid can thrive anywhere where plants exist.

Defenses
Aphids have very little defense against attackers, but they are not completely helpless. If the attacker is small enough, an aphid will attempt to kick it away with its legs. If this does not work, the aphid will usually try to just try to walk away from the fight. There are a few species of aphids that are able to excrete a sticky, waxy substance as a defense. This substance sticks to the attacker, possibly causing him to flee. Finally, if all else fails, the aphid will jump off the plant to escape. The aphid is so light that the fall does not hurt it, although it will have to climb all the way back up the plant.

Relationships With Other Organisms
Aphids share a very close relationship with ants. The diet of an aphid consists mostly of sugar, and since their bodies cannot process all of it, they excrete some of it as a sweet, sticky substance called honeydew. Ants love honeydew and use it as a food source. Ants harvest so much of the honeydew that some species capture and herd the aphids like [[cattle]. The ants become the caretakers of the aphids. They will bring the aphids into their tunnels when it is cold outside, store the aphid eggs in a safe location, take the aphids to new plants when one the one they are on begins to die, and will even clip the wigs of any aphids that grow them so that they cannot fly away. Some ants, in an effort to get more honeydew, will milk the aphids and make them excrete more. A few species of aphids have lost the ability to excrete honeydew on their own because of this, and require the ants to milk them so they can get rid of wastes.

Environmental Impact
Aphids are considered pests because of their heavy negative impact on all plant life. Because of how aphids feed, the plant can start to die. The lack of vital juices can kill the plant. Also, the hole left in the skin of the plant provides an entrance hole for plant diseases to enter into the new plant. The aphid can also transmit diseases across different plants, which causes even more destruction. Aphids also excrete honeydew, which is harmful to plants. The honeydew makes the plant leaf extremely sticky, and acts as a good substance for mold to grow on. The mold turns the leaves black, and can destroy the leaves.

Farmers and agriculturists have been trying for years to control the devastation caused by aphids. One of the favorable methods of aphid control is introducing a predator of aphids into the overpopulated area. Some predators of aphids include the ladybug, fly larvae, and parasitic wasps. The more common method used by farmers is the use of pesticides. The pesticides have to be absorbed into the plant itself, because if it is sprayed on the surface of the plant, it will just drip off without touching the underside of the leaves, where the aphids reside. Natural predators of the area can also keep the population down. Some larvae need to be laid inside of an aphid in order to survive, and when they hatch, the aphid dies. The many natural predators and natural hazards contribute to the suppression of the aphid population.

Carotenoid Production
Carotenoids are nutrients that are essential building blocks for vitamins and antioxidants. They are also responsible for the colors in fruits and the skins of some animals. No known animal has been able to produce these nutrients by themselves, instead, they have to get them by eating fruits and vegetables. The aphid is the only known exception to this. It is crucial that aphids have some method of producing carotene, since the get almost none of it from their diet. Without carotene, an aphid could not make essential vitamins such as Vitamin A.

The gene responsible for the production of carotenoids was thought to have been found only in plants, so it was very shocking when it was found in aphids. Scientists are not even sure how the gene for carotenoid production got into the DNA of the aphid. Some believe that at some point in time, a gene from fungi somehow got copied into the DNA of the aphid. Scientists claim that a long-term relationship between fungi and aphids could make this gene transfer possible. This theory has not been proven, however. It is more likely that God created the aphid with this gene already encoded into its DNA.