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Mating behavior in Phasmatodea is impressive because of the extraordinarily long duration of pairings. A record among insects, the stick insect Necroscia sparaxes, found in India, is sometimes coupled for 79 days at a time. It is not uncommon for this species to assume the mating posture for days or weeks on end, and among some species (Diapheromera veliei Walsh and D. Covilleae), pairing has been observed to last 3-136 hours in captivity. Explanations for this behavior range from males guarding their mates against reproductive competitors to the view that the pairings are a defensive alliance.
Instances of overt displays of aggression between males over mates would suggest that the extended pairing behavior may have evolved to guard females against sperm competition. Fighting between competing males has been observed in the species D. veiliei and D. covilleae. During these encounters, the approach of a challenger causes the existing mate to manipulate the female's abdomen, which he has clasped by means of the clasping organ, or vomer, down upon itself to block the site of attachment. Occasionally the consort will strike out at the competitor with the mid femora, which are equipped with an enlarged and hooked spine in both sexes that has been observed to draw the blood of the opponent when they are flexed against the body to puncture the integument.
Usually a strong hold on the female's abdomen and blows to the intruder are enough to deter the unwanted competition, but occasionally the competitor has been observed to employ a sneaky tactic to inseminate the female. While the first mate is engaged in feeding and is forced to vacate the dorsal position, the intruder can clasp the female's abdomen and insert his genitalia. If he is discovered, the males will enter into combat wherein they lean backward, both clasped to the female's abdomen, and freely suspended, engage in rapid, sweeping blows with their forelegs in a manner similar to boxing. Usually when the intruder gains attachment to the female's abdomen, these conflicts resolve in the displacement of the original mate.
Lengthy pairings have also been described in terms of a defensive alliance. When cleaved together, the pair is more unwieldy for predators to handle. Also, the chemical defenses (secretions, reflex bleeding, regurgitation) of the individual stick insect are enhanced when two are paired together. Female survivorship of attacks by predators is significantly enhanced by pairing, largely because the dorsal position of the male functions well as a shield. This could indicate that manipulation by females is present: if females accept ejaculate at a slow rate, for instance, the males are forced to remain in copulo for longer and the female's chances of survival are enhanced. Also, evolution could have simply favored males that remained attached to their females longer, since females are often less abundant than males and represent a valuable prize, so that for the lucky male even the sacrifice of his own life to preserve his offspring with the female may be worth it.
Sexual dimorphism in the species, where females are usually significantly larger than the males, may have evolved due to the fitness advantage accrued to males that can remain attached to the female, thereby blocking competitors, without severely impeding her movement.