The mosquito proboscis is composed of seven micro parts. Among them, labrum and maxillae play important roles in insertion. The labrum is a central needle which sucks blood; two maxillae are left/right needles aside the labrum, respectively. They cooperatively move, i.e., one maxilla goes forward, labrum goes forward, another maxilla goes forward, successively. We noticed that the maxilla has a jagged harpoon-like shape on its tip area, which has an anchoring effect. We proposed the easy insertion mechanism as follows: the equivalent stiffness, which is the ratio of the resistance force relative to the moving distance, is considered. The total stiffness in pushing out the central needle can be made smaller than zero due to the anchoring effect of the side needles. We named it as 'negative stiffness mechanism.' Equivalent stiffness during pulling/pushing a fabricated needle was experimentally estimated, which indicated the validity of proposed mechanism.