Aphids
Family Aphididae or Aphid Family
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Small
insects, less than a quarter inch long. They come in a variety of
colors, white, green, orange, red and black, sometimes in the same
species. They are soft-bodied, normally wingless, with long legs
and small eyes. Nearly all aphids have two tubes, called
cornicles, projecting from their hindquarters, as seen in the large
aphid at lower right in the righthand photograph. These tubes
produce odoriferous chemicals that signal between the aphids, and also
a waxy substance that can gum up the mouths of predators. Aphids have needle-like mouthparts that are inserted into a plant and tap into the tissue called phloem, which carries sugar-containing sap. Phloem sap is under positive pressure in the plant so it is forced right through the aphid and collects as a droplet at the aphid's anus. As it passes through, the aphid's digestive system absorbs as much sugar and amino acids as it possibly can. Often, ants, called dairying ants, will be found protecting aphids and harvesting the sugary sap that collects on their bottoms. Aphids generally live in familial colonies. Once the colony's founder has found a good host plant, she -- and it is always a she -- will begin to produce offspring. These offspring, nourished within her body and born live, are genetically identical to her and hence female. The offspring then produce grand-offspring; in fact the offspring are born pregnant. The colony very quickly grows. If the food source begins to fail, some species can produce winged daughters that fly off to start new colonies. In the tropics aphids can keep on cloning themselves indefinitely. However, in colder climates the females will switch, in autumn, to producing sexually capable male and female offspring. These mate, and the females lay eggs that overwinter and hatch in the spring to start new colonies. Some aphid species specialize in sucking sap from only one species of plant. Others can feed from many different, unrelated plants. It is usually easy to identify an insect as an aphid; just look for the cornicles, which are unique to aphids. However, to identify the aphid to the species, it is often necessary to clear it -- that is treat it with chemicals that make it transparent-- and mount it on a microscope slide for examination under the compound microscope. Aphids are likely very common in the Park, but rarely noticed because of their small size. The ones in the photographs were found on a plant growing along Connelly's Run near the South Bridge. You can often find them in gardens as well, where they can do serious harm. The rapidly burgeoning population can suck so much sap that the plant is weakened, and the aphids often transmit viral diseases, just like mosquitoes do to humans. Furthermore, the excess sap oozing out their hind ends can foster mold growth that can further harm the plant. |
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