The insect world is vast, but within the order Hemiptera, or “true bugs,” lies the suborder Sternorrhyncha. These insects are found across the globe and are distinguished by features that unite a diverse array of species. The name is derived from the Greek words “sternon” for chest and “rhynchos” for beak, referring to the distinctive placement of their mouthparts.
Defining Characteristics of Sternorrhyncha
The defining anatomical feature of Sternorrhyncha is the position of their rostrum, which contains their piercing-sucking mouthparts. This beak-like structure appears to arise from between the front pair of legs, a characteristic that separates them from other Hemiptera. These specialized mouthparts consist of needle-like stylets housed within a grooved labium, allowing them to penetrate plant tissues and extract fluids.
All members of this suborder are phytophagous, meaning they feed exclusively on plants. Their stylets probe plant tissues, often reaching the phloem to access sugar-rich sap. As the stylets penetrate the plant, a solidifying saliva is secreted, forming a sheath that helps stabilize the feeding site. This method of feeding has led to an association with endosymbiotic bacteria, which live in specialized tissues within the insect and supplement their nutrient-deficient diet.
Most Sternorrhyncha are relatively small, with sedentary or slow-moving lifestyles, and many species live in groups or colonies on their host plants. Another common physical trait is the structure of their feet, which have one- or two-segmented tarsi, the final segments of the leg.
Common Groups Within Sternorrhyncha
The suborder Sternorrhyncha includes some of the most well-known garden and agricultural insects, categorized into four major superfamilies.
- Aphids (superfamily Aphidoidea) are small, pear-shaped insects often found in dense colonies on new plant growth. A distinguishing feature of many aphids is a pair of tube-like structures called cornicles protruding from their abdomen, which can emit defensive chemicals.
- Whiteflies (superfamily Aleyrodoidea) are tiny insects that resemble miniature moths and are covered in a fine, white, waxy powder. When an infested plant is disturbed, they often fly up in a characteristic cloud and can form large populations on the undersides of leaves.
- Scale insects and mealybugs (superfamily Coccoidea) represent a highly diverse group known for extreme sexual dimorphism. Females are immobile, legless, and covered by a protective waxy or shell-like secretion. Mealybugs are a type of scale insect that retain mobility and are covered in a powdery wax, while adult males are tiny, winged, short-lived insects.
- Psyllids (superfamily Psylloidea) are often referred to as “jumping plant lice.” They resemble tiny cicadas and possess strong hind legs that enable them to jump impressively when disturbed. They feed on fresh plant growth and are often host-specific, meaning they feed on a narrow range of plant species.
Unique Life Cycles and Reproduction
Many species within Sternorrhyncha exhibit complex and efficient reproductive strategies. Aphids are well-known for a reproductive method called parthenogenesis, where females can produce offspring without mating. This process, known as thelytoky, involves unfertilized eggs developing into females, which allows for rapid population growth.
This process is often combined with telescoping generations, where a female aphid carries developing daughters that are, in turn, already developing their own offspring. This strategy allows multiple generations to exist simultaneously within a single aphid, contributing to their ability to overwhelm host plants. While asexual reproduction is common during spring and summer, many aphid species switch to sexual reproduction in the fall to create overwintering eggs.
Scale insects begin life as mobile nymphs called “crawlers,” which is their primary dispersal stage. Crawlers move to find a suitable feeding spot on a plant before settling down. Female crawlers then develop their protective waxy covering and often lose their legs. The life cycles of male scale insects are distinct, involving pupa-like stages before they emerge as winged adults.
Ecological and Economic Significance
The feeding habits and reproductive capabilities of Sternorrhyncha make them significant pests in agriculture, horticulture, and forestry. Their method of sucking sap directly weakens plants, leading to reduced growth, wilting, and diminished crop yields. As these insects consume large quantities of sap, they excrete a sugary, sticky waste product called honeydew.
This honeydew coats leaves and stems, promoting the growth of a black fungus known as sooty mold. The mold does not infect the plant, but its dark layer blocks sunlight, which can impede photosynthesis and further stress the plant. Many species of Sternorrhyncha are also effective vectors of plant viruses. As they move from plant to plant, their probing mouthparts can transmit pathogens, leading to widespread disease in crops.
However, some species have beneficial uses. The most famous example is the cochineal scale insect (Dactylopius coccus), which feeds on prickly pear cacti. For centuries, this insect has been harvested to produce a vibrant and durable red dye called carminic acid. This natural dye has been used to color textiles, cosmetics, and food products.