Just as insect legs have been modified over evolutionary time, wings of different insect groups have also been adapted and specialized for purposes other than flight. In many insects, the forewings are modified for protection, which involves hardening so that the forewings are less fragile. Insects like grasshoppers and cockroaches have evolved thick leathery forewings known as tegmina. These wings are partially sclerotized to provide protection for the membranous hind wings, and are used for steering and flight. Depending on the species, the tegmina may also be important in camouflage, visual displays, and sound production. For example, some praying mantises have patterns on their tegmina that resemble eyes to scare predators away. You've probably heard of another use of tegmina before, even if you didn't realize it. The distinctive chirping sounds that crickets produce are actually created by rubbing their tegmina together. The forewings may be leathery in some hemipterans as well, but only on the proximal portion of the wings. The rest of the forewings and the entirety of the hind wings are membranous. The forewings of these hemipteran insects are called hemelytra. This type of wing modification is what gives the insect order its name, as hemi means half, and ptera means wing. The Hemiptera then are literally insects that only have half a membranous forewing. Don't be completely fooled by the name though. There are many insects in this order that have completely membranous forewings, like aphids and cicadas. A characteristic of all beetles is the presence of fully hardened forewings called elytra, which form a solid shield that covers and protects the hind wings and abdomen. Unlike the wings that we have previously described, the elytra are not leathery, but completely sclerotized. Recall that sclerotization is the process by which hardening of the cuticle is achieved through cross-linking proteins in the chitin protein matrix. The rigidity of the elytra can vary between species, and can be used to help distinguish specimens at the family level. In beetles, it is the membranous hind wings that power flight, though the elytra can help to stabilize flight. As beetles rely only on their hind wings to power flight, they are unsurprisingly not strong flyers. In the lepidoptera, the wings are modified not only for protection, but for other functions including mimicry, camouflage, mate attraction, insulation, and thermoregulation. This is achieved by the presence of scales that cover both the fore and hind wings. These scales are set in sockets in the wing membrane at an incline relative to the wing surface. They can be arranged either randomly or in rows, although they always overlap each other to form a complete covering, like the scales you see on a fish. The scales can provide some protection for the insect. If a butterfly is caught in a spider's web for example, the scales detach easily, allowing the insect to escape unharmed. The plethora of colours found on the wings of moths and butterflies is often the result of the presence of pigments in these scales, or even the structure of the scale itself. Some scales can be made up of reflective surfaces that are set apart at specific intervals. When particular wavelengths reflect off of these surfaces, it causes the overall surface to appear coloured. For example, if short wavelengths are reflected, a blue colour is produced. Colours produced in this way are called iridescent. Lepidopteran wing colouration serves many purposes, ranging from visual mate attraction to predator avoidance through camouflage, mimicry, or by startling predators. Dark colouration on broad lepidopteran wings is also important for thermoregulation, as it increases heat absorption. The scales also act as an insulating layer that helps maintain thoracic temperatures. It's a little like having a row of roof shingles on their wings. Many male lepidoptera have wings scales that contain glands, which produce and secrete aphrodisiac pheromones to attract mates. These scales can be elongated, with brush-like processes that provide a large surface area for pheromone evaporation. Finally, wings scales can also help smooth airflow over the wings and body, making flight more efficient. Again, similar to the function of scales in fish. One of the most extreme insect wing modifications occurs in the Diptera, in which the hind wings are modified into advanced balancing organs called halteres. This is a unifying characteristic of the speciose order. As these structures no longer support flight, flies effectively only have one pair of wings, and this is reflected in the order name Diptera, which means two wings. Halteres looked like knobbed stalks, and function as sophisticated gyroscopes to maintain stability during flight. This is achieved by concentrations of sensory receptors known as campaniform sensilla, that are positioned at the base of the halteres. These sensilla look like small flexible domes, since they do not have a hair, or seta, as part of the sensory structure. Campaniform sensilla allow a fly to sense minor distortions in the haltere's position during flight, which in turn informs the fly about the speed and direction of its flight. While winglessness is considered an ancestral trait, some groups of insects have secondarily lost both pairs of wings. In some insects like the termites, only the reproductive castes have wings. These wings are temporary, and are shed after their nuptial flight. In some insect species, such as water striders and aphids, winglessness varies between generations, and is determined by environmental conditions. In unfavorable conditions, adults produce offspring which develop wings with the moult to their adult stage, allowing the next generation to fly away and find a more suitable habitat. If conditions in the original habitat are stable, the offspring will develop through adulthood without wings, which are energetically costly to grow and maintain. Insect wings have evolved for many other functions besides flight, but the ability to fly is still a prime driver behind the success and longevity of the group. The wing modifications we have discussed so far, not only provide vital functions, but can also be fundamental for differentiating certain insect orders, as many of the wing modifications are specific to an order and are easily recognized. Join us in the next video as we explore the intricacies and peculiarities of how insects move their wings and take to the sky.
