Diptera, the true flies, are among the most diverse and ecologically important insects. Adults are easily recognised by their single pair of wings (the forewings) and club-shaped halteres (derived from the hindwings). Two major subgroups occur: the lower Diptera and the more robust higher Diptera. The lower Diptera (below), traditionally treated as the suborder Nematocera, form an assemblage of primitive groups of generally delicate (‘mosquito-like’) flies with multisegmented, often long, adult antennae. Larval lower Diptera have a complete head capsule and horizontally working mandibles. Higher Diptera, suborder Brachycera (meaning ‘short-horned’) (p.220), are generally more robust flies with relatively short antennae, and their larvae have reduced head capsules and mouthparts that usually work vertically (like a snake’s fangs), rather than horizontally. All measurements given are body lengths.