Lecture 11 Sample term test Dipteran flight Chabrier, click mechanism Myogenic muscles Non-click mechanism

Order Diptera, An Introduction to the Study of Insects 1954.Borror & Delong Most of the Diptera can be readily distinguished from other insects to which the term ‘fly’ is applied (sawflies, stoneflies, caddisflies, dragonflies…) by the fact that they possess only one pair of wings; these are the front wings, and the hind wings are reduced to small knobbed structures called halteres, which function as balancing organs.” Halteres are diagnostic of true flies. They are evolved from the metathoracic (hind) wings of dipteran ancestors. During flight they move up and down out of phase with the mesothoracic wings. The mesothorax of Diptera is hugely developed beyond the pro and metathorax, reflecting the specialization of this segment for flight.

Tagmata Among animals that are metameric [Arthropoda, Annelida], that have a body plan of serial segments, there is a phenomenon termed tagmatization. Subsets of these segments become grouped to serve a common function. The ancestors of insects were serially segmented [each segment primitively with a pair of jointed appendages]. A number of these grouped during evolution to become: head, thorax, abdomen. The head with sensory organs (antennae, eyes, ocelli, tongue etc.); the thorax specialized for locomotion both ambulatory and flight; the abdomen specialized to change volume. Adaptiveness of thorax tagma and the abdominal tagma of insects.

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Chabrier model

Radial Stop

Non-click mechanism of dipteran flight A shows the mesothorax of the fly seen in lateral view. The longitudinal flight muscles within the thorax are shown (green) running between anterior and posterior phragma. Phragmata (plural) are transverse thin sheets of inwardly projecting cuticle to which the flight muscles are attached. When these longitudinal flight muscles contract with the start of the downstroke they make the scutum more arched, i.e. the phragma on which these muscles insert fore and aft, are pulled closer to each other and the scutum above bows upward in the longitudinal median plane. But because of the 'hemispherical' shape of the scutum (and a 'scutal inflection') this same muscle contraction creates lateral forces in the transverse plane. That is, the scutum actually becomes less arched in the transverse plane, or to say it another way, the right and left parascutal shelves on either side tend to move away from each other, following an outward force which acts through the first and second axillary sclerites onto the pleural wing process (see B).