Insects can vary in shape and color within a single species—a phenomenon known as polymorphism. This may simply be a difference between males and females, but some have as many as twelve color forms.
Many animals and plants show polymorphism in some way or another. In the context of insects, the term is used when different shapes or colors are found within an equivalent stage of the same species. Thus there are polymorphic larvae, pupae, and adults.
Polymorphism is most familiar to us in the differences between the sexes of most insects. These polymorphisms exist for biological reasons, and the differences between male and female insects (sexual dimorphism) have great survival value.
Insects need to be visible to mate, but they also need to be concealed from predators, one of the main problems for insects. In general terms, the male has to find, court, and mate with a female without being seen by a predator to protect her future offspring.
Sexual dimorphism is often seen in the color, with the male generally being more brightly colored than the female. The most familiar examples are butterflies.
Within the blue family, many species have brilliant blue males and drab brown females. The purpose of these differences lies in the need for females to recognize males for courting and also help males recognize each other. The female’s dull colors enable them to escape the attention of birds.
Males can also be distinguished from females by systems associated with mating. Male dragonflies have a copulatory apparatus under the second segment of the abdomen, where sperm is passed into the female, while male water beetles have large pads on their forelegs to help them grip the female while mating.
For females, the biological function of producing and laying eggs means that they are often endowed with conspicuous ovipositors.
Colour polymorphisms may be found in almost any stage of an insect’s life. Insect larvae can be considered to show two phases of color polymorphism: the first is differences between larvae of the same instar, and the second is when instars differ in shape and color. Instar is the developmental stage of an arthropod between molts.
In most insects, the larval stages are uniform in appearance within any given instar. This is expected as these larvae are highly adapted to one way of life. The caterpillars of several moth species are exceptions to this rule and produce different color forms.
One example is the elephant hawkmoth, whose final instar caterpillars may be predominantly green or brown-grey. Similarly, the third instar larvae of the emperor moth show an incredible range of patterns, all based upon black, green, and orange. In this species, it is difficult to find two caterpillars alike.
Colour polymorphisms are not restricted to within instars. Many insects change color as larvae increase in size after each molt. Early instars of the swallowtail butterfly resemble bird droppings, but the later instars are black and yellow.
Transformations occur because each change in the caterpillar’s size requires a new protective strategy. While a small caterpillar has a better chance of surviving by resembling bird droppings, larger caterpillars can use warning colorations.
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For most insect species, the pupal stage is hidden inside a cocoon or buried in soil or wood. When a pupa is exposed, like with butterflies, there may be polymorphism in the color of the chrysalids.
A great example is the chrysalids of the swallowtail butterfly, which may be either brown, green, or brown-green, depending on where they are. Brown chrysalids are found on twigs and tree bark, while green ones can be found on vegetation.
Similar color polymorphisms are found in other butterflies, such as the small white. Such differences in chrysalids are directly related to survival and camouflage.
Colour polymorphism in adult insects has attracted the greatest interest among scientists. Some are determined by genetics, such as the peppered moth with its three different color forms.
Examples are the dark valesina form of the silver-washed fritillary and the pale helix form of the clouded yellow.
The value of adult polymorphism in the peppered moth is well known, but that behind the silver-washed fritillary and clouded yellow is less obvious. The valesina forms of the female silver-washed fritillary may be directly linked to temperatures as huge numbers of this normally rare variety are often seen in years of drought and high temperatures.
An amazing adult polymorphism is found in the two-spot ladybird. The adult occurs in the well-known red with black spots version but also in a black with red spots. This is thought to be an adaptation to help the ladybirds warm up quickly in early spring, the black individuals heating up quicker.
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Among the bug order is a common insect called Philaenus spumarius, one of the spittle-bugs whose larvae live inside the frothy blobs on grass stems. Adult Philaenus have a very wide range of color varieties, from entirely black to pale brown. Rarely do two adults look alike, and at least 24 different patterns have been recognized.
A common polymorphism in some groups of insects is the degree of wing development. Greenflies have both winged and wingless forms. Their food quality determines whether the greenflies are present as winged or wingless forms, adapting to environmental conditions.
Winged adapt to different environmental conditions, and food availability has been taken to extremes in the parasitic Hymenoptera.
Gall wasps, particularly those which attack oak, usually have two parts to their life cycle. In Neuroterus quercus-baccarum, known as the spangle gall wasp because of the spangle galls on oak leaves, the overwintering grubs inside the spangle gall produce females.
These lay their eggs in oak catkins which, when they develop into larvae, give rise to currant galls. The grubs inside these currant galls emerge as a sexual generation, with male and female adults, whose larvae give rise to spangle galls, completing the cycle.
Bees, wasps, and ants (Hymenoptera) have evolved a remarkable form of polymorphism. Queen, workers, and drones of the same species all perform different functions within the colony and are different structurally.
Polymorphism is one part of the puzzle that allows insects to survive in a world where they are the smallest. Within the natural world, there is an infinite variety of strategies for survival. Polymorphism is simply a piece of the jigsaw enabling insects to continue to reproduce and thrive.