1. The mean flight-duration, in tethered flight, of the ‘large milkweed bug’, Oncopeltus fasciatus (Dallas), was measured as a function of age in days after the final moult.

2. In both sexes a peak of flight occurs at 8-10 days. In males there is a second flight peak at 30-35 days, whereas there is no second peak in females.

3. Virgin males tested at 20 days, a time when mated males were short-flying, flew longer than bugs at any other age. Thirty-day-old virgin females were not significantly different from mated females of the same age.

4. When long flights occurred in males they were in general preceded by one or more flights of less than 1 min. This was not true for females, and the difference between the sexes was statistically significant.

5. The results in 2, 3 and 4, above indicate two, and possibly three, differences between the sexes. Males and females should thus be studied separately with respect to flight activity.

6. Some individuals never flew. These were as long-lived and as reproductively active as those which did, indicating the possibility of a behavioural polymorphism.

7. The initial peak of mean flight-duration for both sexes occurs when the daily deposition of paired cuticular growth rings ceases. Maximum flight thus seems to occur at the end of the teneral period.

8. The threshold for flight declines with age. There are thus three types of flight: long-duration, high-threshold; short-duration, low-threshold; and (in males only) long-duration, low-threshold.

9. The initial flight peak is pre-reproductive and occurs at a time when reproductive value, a measure of the expected contribution of an individual to future population growth and hence of sensitivity to selection, is high. Long-flying bugs are evidently colonizers, and a migration is an evolved adaptation, not a response to current adversity.

10. It is concluded that the long-duration, high-threshold flights represent migration and the short-duration, low-threshold flights represent non-migratory ‘flits’.

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