Endosymbiosis between eukaryotic hosts and microorganisms is a common phenomenon in insects, whose typically unbalanced diets are usually complemented by their obligate endosymbionts. While much interest and focus has been directed towards phloem-feeders like aphids and mealybugs, blood-feeders such as the Lone star tick (Amblyomma americanum), Glossina flies, and the human body louse (Pediculus humanus corporis) also depend on obligate endosymbionts to complement their B-vitamin-deficient diets, and thus are required for growth and survival. Strict blood-feeding glossiphoniid leeches, contrary to the predatory species, have also been found to harbour distinct endosymbionts belonging to the Gamma and Alphaproteobacteria housed in specialised morphologically-diverse organs. The Mexican leech, Haementeria officinalis is associated to the obligate endosymbiont Candidatus Providencia siddallii (Gammaproteobacteria) . This symbiont resides intracellularly in spherical bacteriomes attached to the oesophagus, and possesses a highly-reduced genome with high A+T content and a reduced set of metabolic capabilities, all of which are common characteristics of ancient obligate endosymbionts of arthropods. Its genome has retained many pathways related to the biosynthesis of B-vitamins, pointing towards a role in supplementing the blood-restricted diet of its host. Through genomic comparison against the endosymbionts of the different blood-feeding arthropods, we were able to detect a high degree of metabolic convergence among these very distantly related endosymbiotic bacteria. These findings strongly support the widespread and conserved metabolic dependence of the strict blood-feeders in their bacterial endosymbionts and the similar constraints these have undergone in their evolution as obligate nutritional partners.