They can. Two-plus months.
Or, more specifically, 79 days, says pseudonymous entomologist–blogger Bug Girl.
It’s generally thought that the male hangs around in order to have repeated matings, but also to drive off other males that want to get lucky. I found several reports of stick insect menage a trois (or sept) in the literature, including this etching of kinky stick insect activity.
Quoting a study on the topic, Bug Girl writes,
The Indian stick insect Necroscia sparaxes may remain coupled for up to 79 days (a record for insects)… In captivity, Diapheromera veliei and D. covilleae pair for 3 to 136 hours and the penis may be inserted and removed up to 9 times. The genitalia are not in contact for ca. 40% of this period, and attachment is maintained by a male clasping organ.
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