Arrenurid water-mite larvae, ectoparasitic on zygopteran imagines (Odonata), attach themselves to the host s cuticle and pierce it with the cheliceral blades to obtain the host s tissue fluids. Promptly after anchoring in feeding position, secretions of the larval mite are forced into the host beneath the attachment site, where a subcuticular vesicle, bounded by a delicate gelatinous membrane, appears in the epidermis layer. The vesicle constitutes a local space of thin fluid into which the larva ejects a liquid that rapidly gels and forms a slender resilient blind sac, the stylostome. The vesicle provides a buffer zone for the nascent stylostome, protecting it against vehement defence reaction on the part of the host. The early stylostome undergoes a phase of extreme inflation of the distal thin-walled portion, which is associated with expansion of the primary vesicle. Epidermal ceiis adjoining the primary vesicle undergo lysis and fuse with the vesicle, transforming it into an epidermal abscess, whose ceii juice acts as a pool of nutrient to the larval mite. Initially short, wrinkled and thin-walled, the stylostome increases in size, elongating and remodelling as larval engorgement proceeds. Eventuaily the stylostome breaks the rim of the abscess, which becomes modified to a melanized sleeve around the origin of the stylostome. When fully formed, the stylostome is mostly lodged in a cleft in the epidermis separated from the haemocoele and deeper tissues by the sheet of epidermal basal lamina. The delicate meshwork of the basal lamina, closely applied to the outside of the stylostome, seems to prevent haemocytic reactions to the stylostome in later developmental stages.