OBSERVATIONS ON THE FLIGHT MUSCLES OF SITONA WEEVILS

SUMMARY. In macropterous weevils of S. hispidula F. marked variation occurs in regard to the development of the muscles of flight, some insects being able to fly while others are permanently flightless. Both forms are widely distributed and may occur in the same locality. In the flightless insects the histology of the fibrous muscles is entirely different from that of the normal muscle; the discs of the fibrous muscles are reduced in size and the phragmata of the metatergum are suppressed. Forms intermediate betweeen the two extremes occur. A study has been made of the post‐metamorphic development of the flight muscles in S. lineata , a species which is an excellent flier, and it has been found that in the newly emerged weevils the condition of the flight muscles and their chitinous supports closely resembles that of the abnormal form of S. hispidvla. In S. lineata , however, the normal condition is usually attained a short time after emergence. The flightless macropterous weevils of S. hispidula may therefore be regarded as individuals in which the normal post‐metamorphic development of the flight muscles and their chitinous supports has been arrested. In the normal form of S. hispidula it has been found that the muscles of flight usually degenerate during the winter, but in S. lineata they are functional for a much longer period and weevils of this species fly readily in spring. Evidence obtained by breeding S. hispidula suggests that unfavourable environmental conditions during ontogenetic development may be responsible for the production of the flightless weevils.