Quiz 21: Hexapods
The characters that distinguish hexapods from all other arthropods are- • Presence of six legs • All legs are uniramous • Presence of three tagmata-head, thorax and abdomen with appendages on the head and thorax.
Insects are different from other hexapods in the following ways: • Presence of ectognathous mouth parts • Have usually two pairs of wings
Indirect flight muscles are not attached to the wing and cause movement of the wing by altering the shape of the thorax while direct muscles are attached directly to the wing. The wing is attached at the Tergum part of the thorax. When the indirect muscles contract, it pulls the Tergum down towards the sternum which causes the wing to move upward. The wing moves downward with the relaxation of Sternotergal muscles and the contraction of the longitudinal muscles of the thorax. Indirect muscles can beat much more rapidly than direct muscles as they have the asynchronous type of neural control while direct muscles have the synchronous type. In synchronous type a single nerve impulse stimulates one muscle contraction and one wing movement. In asynchronous type, the muscle contractions are not synchronous with the nerve stimuli. Hence, if a single nerve impulse is given, the muscle contracts and relaxes for more than one time. Only occasional nerve impulses are needed to keep the indirect muscles relaxing and contracting. This helps the wing to move faster than the nerve impulses given. The wing moves up and down according to the contraction and relaxation of these muscles. This is the reason why indirect muscles can beat much more rapidly than direct muscles.