Insect Life Cycles

Skyrail Nature Diary: January 2015

This month we will be looking at the life cycles of insects. Insects are a special kind of arthropod (animals with jointed legs) and are interestingly most closely related to crustaceans (e.g. crabs, prawns and crayfish). Insects are characterised by having an exoskeleton (on the outside) that is divided into three distinct parts; the head, thorax and abdomen. The head has the antennae, eyes and mouthparts. The thorax has the wings (when present), usually two pairs, and three pairs of legs. The abdomen contains most of the vital organs. As insects have an exoskeleton the only way they can grow is by regularly shedding the exoskeleton and growing a new one. Each time an insect changes its exoskeleton, they are said to be an instar.

Insects have three types of life cycles. The most primitive, flightless ones have offspring that are identical miniatures of the adults. An example is the silverfish, a book eating pest. The second type of life cycle is the insects with nymphs. Nymphs are generally miniature version of the adults without wings. The more primitive of these insects (dragonflies, mayflies, stoneflies and dobsonflies) have aquatic nymphs that breathe using external gills the adults don’t have. The dragonfly nymphs also have a modified lower jaw similar to a spring with two grabbing hooks on the end. The final type of life cycle is the insects with larvae and pupae. Here the offspring are so different from their parents that you wouldn’t think they were the same creature. Caterpillars are a classic example of this kind of offspring. Larvae eat and grow until they reach their maximum size. That’s when the larva finds a good hiding place, sheds its skin and becomes a helpless bag known as a pupa. Inside the pupa the larva is transformed into the adult, known as an imago. The purpose of this bizarre life cycle is thought to be avoidance of competition between the adults and their larvae. The larva’s main job is to eat and grow while the adult’s job is to mate and produce new larvae somewhere else (increase range and abundance). As a result of this the imago is very short lived.

Silverfish are one of the most primitive surviving groups of insects today. They have long thin antennae and three prong shaped threads on their abdomens. The threads on the abdomen are probably involved in gathering sensory information. All silverfish are nocturnal and prefer moist areas. Silverfish consume matter that contains polysaccharides, such as starches and dextrin in adhesives. These include book bindings, carpet, clothing, coffee, dandruff, glue, hair, some paints, paper, photos, plaster, and sugar. Silverfish can also cause damage to tapestries. Other substances they may eat include cotton, dead insects, linen, silk, or even its own moulted exoskeleton. During famine, a silverfish may even attack leather ware and synthetic fabrics. Silverfish can live for a year or more without eating.

Dragonflies and Damselflies are the most primitive living insects with nymphs. Their four wings are large and delicate and yet these beautiful insects are impressive aerial acrobats. They all have huge eyes that cover most of the head, giving them almost 360 degree vision. Each eye consists of numerous smaller units known as ommatidia. The jaws are quite strong and all dragonflies are predators. The six legs are arranged in a basket shape designed to capture prey on the wing. A single dragonfly can consume hundreds of mosquitos in a matter of minutes. 

Butterflies are quite possibly one of the best known insects with larvae. Where the caterpillars have normal jaws for consuming plant material, the adults have mostly lost their chewing mandibles and the rest of the mouthparts have been turned into a curled up double straw. Adult butterflies usually only eat nectar to fuel their flight. Butterfly wings are an engineering marvel. They appear to have only two wings but the two wings on either side of the thorax are connected with special hooks. The surface of the wings is covered with numerous tiny scales that reflect light, thus creating the unique colour patterns on the wings. These delicate wings are easily damaged and adult butterflies only live long enough to mate and lay eggs. Caterpillars often have extra false legs on the abdomen. Apart from occasionally being pests caterpillars are popular prey items for birds.

by Tore Lien Linde