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It has been argued that the copulatory structures of the weevil genus Sitona Germar have their equivalents in the opposite sex. Hence, the male intromittent organ represents a species-specific key to unlock the corresponding genital tract of its mate, and the female internal genitalia displays several positions that match the functional armature and surface prominences of the endophallic internal sac from the conspecific male. However, this claim can lead to a major challenge due to the perceived lack of variation in the females copulatory organs across the species of a large genus, Sitona.  Therefore, a special case can be very helpful for modeling an elaborate correspondence of male and female genital traits in this group. One of the most intricating and unusual forms of the genitalia in the tribe Sitonini that is assumed to be the representative of an advanced type of the “lock-and-key” relationship belongs to S. fairmairei. A pair of peculiar “bursal plugs” along with the two slender accessory pouches were revealed inside the female bursal lumen of this species that were considered to operate in concert with the unusual ejaculatory pump and anchoring device of the male transfer apparatus respectively, in the process of insemination. The Modification of the pumping device and the way that the endophallic armature threads along the female spermathecal duct in S. fairmairei were compared to another species, S. cylindricollis. Our results provide evidence on how the biomechanics of copulation can drive the evolution of a “cooperative lock-and-key” isolating mechanism in the genital traits of the weevil genus Sitona, as well as the significance of a clear understanding about the reciprocal morphologies of male and female copulatory organs.

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According to a perspective, male and female copulatory organs have not more extreme morphologies. However, a skewed bias in males, -as well as the scanty taxonomic data on female genitalia, may explain why most evolutionary biologists failed to find acceptable evidence for the reproductive co-evolution in insects. A recent finding shows that the mechanical footing has a cooperative essence in the weevil genus Sitona. In this viewpoint, successful mating is achieved by the dynamic correspondence between the male endophallus and female genital lumen. The present work addresses some missing evidence in the genital traits of this taxon that may keep the hampered comprehensions under lock and key. The female bursal lumen in the studied species is provisioned with a pair of forklift-shaped structures (named the “codelocks”) that may provide for stabilizing the characteristic position of the male bio-syringe during copulation. This structure varies between the species based on the shape and direction of its paired levers, so that an elaborate algorithm possibly serves as the fuel of this device. However, in one of these, Sitona fairmairei (Allard, 1869), the levers perform a dual function in accordance to the lockback mechanism of the male copulatory organ. This species is also featured for having several campaniform mechano-receptors on the flanges of the male bio-syringe which are involved in the in-vivo navigation of this armature towards female spermathecal duct.