Insects, commonly called “bugs,” are invertebrates with external skeleton (exoskeleton) made of chitin, three body segments (head, thorax, and abdomen), six legs, one pair of antennae, and compound eyes. Though there are probably 6 to 10 million species on earth, only 1 million have been identified. Even so, that’s more than half of all known living organisms! Insects can crawl, fly, or swim, and all undergo some form of metamorphosis. As a group, they feed on other insects, pollen, plants, trees, and various liquids including blood and nectar.
So what do insects eat? Like us, insects must have a balanced diet containing carbohydrates, proteins, lipids, nucleic acids, water, vitamins, and minerals. These nutrients provide energy, promote growth and development, and keep an insect’s exoskeleton sufficiently tough. Insects obtain water from their food or from droplets and pools.
The structure of an insect’s mouthparts indicates how it will feed: chewing, piercing and sucking, siphoning, or sponging.
Chewing insects have two primary jaws (mandibles), one on each side of the head, positioned between the upper lip (labrum) and secondary jaws (maxillae). The mandibles open outwards and snap together in the center, enabling the insect to bite, grind, and cut materials. If a chewing insect is carnivorous (i.e. eats other insects), the mandibles will be knife-like; if it is herbivorous (i.e. eats plants), these will be broad and flat. Underneath the mandibles, a pair of hairy, sometimes toothed maxillae helps the insect manipulate food. The maxillae also have palps that give the insect sensory feedback about potential foods. In addition, the lower lip (labium) assists the maxillae in manipulating food, and the hypopharynx at the base of the labium helps the insect swallow food. Examples of chewing insects are grasshoppers, ants, cockroaches, white grubs, katydids, termites, and caterpillars.
In piercing-sucking insects, the mandibles and maxillae are modified to form a feeding tube (proboscis). The proboscis contains a needle that pierces plant and animal tissue so that the insect can suck out the liquids. Some piercing-sucking insects (e.g. plant bugs, cicadas, aphids, and leafhoppers) will feed on the fluids of plants, while others (e.g. assassin bugs, predatory stinkbugs, and mosquitos) will pierce the hard skin of captured prey. Female mosquitos inject saliva into their victim to keep the blood flowing, and suck out the blood through another tube.
Siphoning insects have no needle to pierce tissues, so they must find food that is easily accessible. For instance, butterflies, skippers, and some moths use their long proboscis to draw up nectar and water. Other moths have no mouthparts at all, feeding only in the caterpillar (larval) stage.
House flies and blow flies have sponging mouthparts, which sop up liquids and secrete saliva to dissolve and collect solid food particles. Such flies usually cannot bite, but some species, such as the tsetse fly, have sharp teeth on the end of the sponge to scrape flesh and draw up blood.
An insect’s feeding behavior also depends on the kind of metamorphosis it experiences, and it may have different mouthparts in immature and adult stages.
Insects such as dragonflies, leafhoppers, cicadas, aquatic bugs, and stinkbugs undergo incomplete metamorphosis. This means that after hatching from the egg, the baby insect (nymph) looks like a small, underdeveloped version of the adult. Think of a human child. He has almost all the body parts and features he will have as an adult; he just needs to get bigger. The only thing a child doesn’t have at first is hair and teeth. These have to grow in. Well, with an insect nymph, the wings have to develop. And just as a child has to eat every day in order to get big and strong, an insect nymph has to start feeding right away and keep on doing so until it becomes an adult. In the meantime, the nymph will shed its exoskeleton several times to accommodate its increasing size. Remember when you grew out of your clothes as a kid? Since these insects eat throughout their immature (nymphal) stage, they are considered to be quite destructive to plants. Nymphs can have chewing or sucking mouthparts. They may feed on leaves, flowers, or fruit, sometimes injecting toxic digestive enzymes or bacteria into the plant. This can cause leaf discoloration, fruit injury, and overall poor plant health.
Other insects undergo complete metamorphosis. This means that they look entirely different from one stage to the next. After hatching from the egg, the baby insect (larva) has six main legs, but sometimes extra ones (as in the case of a caterpillar). The larva usually has chewing mouthparts, and it will feed on vegetation and other insects until it is large enough to pupate, i.e. to create a protective cocoon in which to develop into an adult. You’re probably most familiar with the cocoons of butterflies and moths, but ants, bees, wasps, flies, and beetles also experience this stage. Some larvae, such as caterpillars, will shed part of their skin as they consume food, but these “molts” do not represent specific stages in development, only in size. Insects that live underground as larvae (e.g. white grubs, pillbugs, strawberry root weevils, root maggots, and corn rootworms) will feed on roots, potentially damaging grass and plants.
Choosing and Finding Food
So how do insects know what to eat? Scientists believe there are several factors. First of all, the “parent” insect will often lay her eggs either on or near a particular plant species. For instance, some moth and butterfly caterpillars will only eat one or a few different kinds of plants and trees. At the same time, a plant must be healthy in order to provide the nutrients an insect needs. For herbivorous insects, the environment of the plant also affects selection, as they will compare neighboring plants in terms of quality. This quality factor applies to predatory insects too. They need to collect sensory information to determine if the potential prey is poisonous, contains parasites, or may attack them. Additionally, insects must be able to catch the prey without expending too much energy.
In conclusion, insects eat a variety of foods, and their feeding behavior is influenced by a combination of anatomy, life cycle, and environment. While scientists have been able to classify most known insects into groups based on feeding behavior, who knows how many different ways the other 5-9 million species eat their meals?