Midges Flies and Mosquitoes: Diptera

Adult dipterans have large compound eyes, or eyes with multiple lenses, that often meet over the top of the head in males but are usually separated in females. There are two basic kinds of antennae (an-TEH-nee), or sense organs. Dipterans that have long antennae with six or more segments include mosquitoes, crane flies, midges, punkies, and no-see-ums. Those with short antennae include bee flies, flesh flies, horse and deer flies, house and stable flies, hover flies, and robber flies. The mouthparts are long and are used for sucking liquids. In predatory and blood-sucking species, such as robber flies and mosquitoes, the jaws form stiff, needle-like structures that pierce the exoskeleton or hard outer covering of other animals. They then form a straw to draw bodily fluids from the prey. In houseflies and others, the mouthparts are soft and fleshy with sponge-like structures at the tip and are used for sopping up liquids. Some flies with short antennae have a special air-filled sac. This sac is inflated only once and is used to help the young adult burst out of the pupa (PYU-pah).

An important feature that distinguishes adult flies from all other insects is the presence of only two wings. They are attached to the middle section of the thorax or midsection, which is enlarged to hold the flight muscles inside. The second pair of wings is reduced in size and resembles small clubs. They are used as balancing organs during flight. Flies that live on the bodies of animals, such as bat flies, as well as some other species, are wingless. In winged species, the bases of the wings may or may not have flaplike lobes at their bases.

The legs are variable, depending on the habits of the fly. Some are spiny and are used to capture insect prey while flying. The legs of some males are used to grasp females while mating or in elaborate courtship behaviors. Parasitic species have legs that help them cling to feathers or hair. Many species of dipterans have legs equipped with combs and brushes for grooming. The feet of all species are five-segmented and are incredibly sensitive. Some flies actually taste their food with their feet. House flies have oily and bristly pads on their feet allowing them to walk upside down on ceilings or climb smooth surfaces such as glass.

The fly abdomen has eleven segments and may be long and slender or shorter and thicker. The last two or three segments vary considerably in different species and are used for mating and egg laying.

The legless larvae (LAR-vee), or young, never resemble the adults. They are long and are either nearly cylinder-shaped or tapered at both ends. In mosquitoes the thorax is much larger than the head or abdomen. Across their bodies are swollen regions usually covered with short spines. These bumps and ridges are rough to help the larvae to get a grip and move through soil, mud, wood, water, or flesh. In species with adults that have long antennae, the larval head is distinct and nearly round and has jaws that chew from side to side. In all other flies the small pointed head is less distinct or not distinct at all and has jaws that move up and down. In these species most of the head can be withdrawn inside the thorax. Simple eyes, or eyes with only one lens, and antennae of fly larvae are greatly reduced in size or absent.

The thorax and abdomen are soft. They may or may not have spiracles along their sides, breathing holes that connect to the respiratory system. Aquatic species have only a single pair of spiracles located at the tip of the abdomen. These are sometimes mounted on a long, snorkel-like extension, allowing them to remain underwater as they breathe air directly from the surface. For example, rat-tailed maggots, larvae of drone flies, live in the bottom of ponds and breathe through a long tube resembling a rat's tail. Other aquatic species have snorkel-like extensions fitted with tiny saws used to tap into air pockets in the tissues of underwater plants.

The pupae of flies with long antennae show hints of adult features. Their legs and wing pads are clearly visible and are not completely attached to the body. In other groups of flies the pupae are smooth and resemble seeds because they are wrapped inside the old exoskeletons of the mature larvae.