Aphidoidea: The Superfamily of Sap-Sucking Insects
Aphidoidea is a superfamily of insects that belong to the order Hemiptera and the suborder Sternorrhyncha. They are commonly known as aphids, plant lice, greenflies, or blackflies, and they feed on the sap of plants by piercing their phloem tissue with their mouthparts. Aphids are among the most diverse and widespread insect groups, with about 5,000 species described and many more undiscovered. They can be found on almost every kind of plant, from crops and ornamentals to weeds and trees.
Aphids have a complex life cycle that involves both sexual and asexual reproduction, as well as alternation of host plants in some species. Most aphids reproduce parthenogenetically (without fertilization) during the growing season, producing live young that are genetically identical to their mothers. Some aphids can also produce winged forms that can disperse to new plants or locations. In temperate regions, aphids undergo sexual reproduction in autumn, producing eggs that overwinter on the host plant or on another plant species. The eggs hatch in spring and start a new generation of parthenogenetic females.
Aphids are important ecological players as they interact with many other organisms. They are preyed upon by various predators, such as ladybugs, lacewings, hoverflies, and parasitic wasps. They also have mutualistic relationships with some ants, which tend them for their honeydew (a sugary excretion) and protect them from enemies. Aphids can also affect the health and growth of their host plants by sucking their sap, inducing galls, transmitting viruses, and altering their physiology. Some aphids are considered serious pests of agriculture and forestry, causing significant economic losses.
Morphology of Aphids
Aphids have a soft-bodied, pear-shaped or oval body that ranges from 1 to 10 mm in length, depending on the species and the morph. They have a pair of compound eyes, a pair of antennae, and a pair of cornicles (tubular structures) on the posterior end of the abdomen. The cornicles secrete defensive fluids such as waxes or alarm pheromones. Aphids also have a cauda (tail-like structure) below the cornicles that helps to direct the honeydew away from the body.
Aphids have mouthparts adapted for piercing and sucking plant sap. They have a stylet bundle composed of two mandibular and two maxillary stylets that form a hollow tube. The stylet bundle is enclosed by a sheath called the rostrum or beak. The stylet bundle can be retracted or extended by muscles attached to the rostrum. Aphids insert their stylets into the phloem sieve elements of the host plant and suck the sap by pumping their cibarial muscles.
Aphids have three pairs of legs that are usually yellow or pale in color. The legs have five segments: coxa, trochanter, femur, tibia, and tarsus. The tarsus has one or two claws and a pad-like structure called the pulvillus that helps with adhesion. The legs are used for walking, clinging, grooming, and fighting.
Aphids may have two pairs of wings or none at all, depending on the morph and the environmental conditions. The winged morphs are called alates and they have membranous wings with reduced venation. The forewings are larger than the hindwings and have a stigma (a thickened area) near the tip. The hindwings have a hook-like structure called the hamulus that hooks onto a fold on the forewing to keep them together during flight. The wingless morphs are called apterae and they are usually more numerous than the alates.