Adult males look like insects and measure 0.04 to 0.3 inches (1 to 7 millimeters) in length. The surfaces of their compound eyes are rough and resemble blackberries because each lens is slightly separate and clearly distinct from the surrounding lenses. Their antennae (an-TEH-nee), or sense organs, are branched and resemble tiny antlers. The mandibles, or biting mouthparts, if they have them at all, are cone-shaped and pointed downward. The first segment of the three-segmented thorax, or midsection, is short and saddle-shaped, while the last segment is much larger and houses most of the flight muscles. The front wings are twisted and knob-like. In flight they are used as balancing organs. The hind wings are clear and fan-shaped, with few supporting veins. Some of the leg segments are fused together, and the feet sometimes lack claws. The abdomen is distinctly segmented.
Most adult females strongly resemble larvae (LAR-vee), or young animals, and measure 0.08 to 1.18 inches (2 to 30 millimeters) in length. They lack wings and legs and have greatly reduced mouthparts, antennae, and eyes. The head and thorax are covered by a thick exoskeleton, or hard outer covering, and are joined together in a single body region. This part of the body sticks out from between the abdominal segments of the host's body. The abdomen is large and barely shows any traces of segmentation. The abdominal exoskeleton is very thin. The abdomen swells up like a balloon when it is filled with eggs.
The larvae have two distinct forms. Those hatching from eggs move freely in the environment and lack antennae, but have phylum class subclass • order monotypic order suborder family simple eyes with one lens each, mouthparts, and legs. The abdomen is tipped with two long, threadlike projections. Once they find a host they transform into a grub without legs. The pupa forms inside the hollow exoskeleton of the mature larvae. In most species male and female characteristics are clearly visible in the pupa.
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