Family Aphididae - Aphids

224 genera, 40 subgenera, 1351 species of aphids currently recorded in the US and Canada, of which about 80 species are pests of food crops and ornamental plants. (1)

Aphids may be identified by two tubelike projections on the posterior, called cornicles. These appear to function as a means of chemical defense, emitting pheromones to alert other aphids about a predator nearby. They also offer mechanical protection, as the fluid emitted can gum up the mouthparts of the predators.

Although many different aphid species are known, they may sometimes be identified by the host plant upon which they are found. However, several different species of aphid may infest a single plant species.

Aphids suck juices from plants and may be quite damaging. Some are restricted to a single plant species or group of related plants. Others may alternate between two entirely unrelated host plants - for instance, Rosy Apple Aphid (Dysaphis plantaginea) begins the year on apple, then migrates to narrow-leaved plantain for several generations before winged adults return to apple trees where they produce eggs that will overwinter.

Over-wintering eggs hatch in the spring into wingless females. These wingless females are parthenogenetic (they can reproduce asexually without fertilization) and hold eggs in their bodies to give birth to living young. Their offspring are similar to the females, but some develop wings. Near autumn male and female wingless forms are born. These mate and the females lay fertilized eggs for over-wintering. Males can be winged or wingless and parthenogenetic females are usually wingless. In warm climates, living young may be produced continually with no over wintering egg stage (description adapted from University of Georgia website).

For live birth, see this image

In the course of feeding, aphids excrete a sweet liquid known as honeydew. This provides food for ants, which are known to tend aphid colonies and protect them from predators in exchange for the sweet liquid. Ants have been observed herding aphids, and stroking them to "milk" them. Some species of ants have also been known to move aphids from one plant to another when the food supply is insufficient, and some even take aphid eggs into their underground nests to help them overwinter.

Predators of aphids include (left to right, below) lacewing larvae, ladybird beetles and their larvae, harvester butterfly caterpillars, and the larvae of syrphid flies.

Aphids are also attacked by parasitic wasps, which lay eggs inside them. Parasitized aphids swell and become lighter in color and are eventually killed by the developing wasp inside. These are referred to as "aphid mummies."

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The taxonomy of aphids is not very clear yet: See some comments here