Hypostoma

Hypostoma refers to a structure found in certain animals, primarily invertebrates, that is associated with the mouthparts. In insects, it is often used synonymously with the labium, which is the lower or hindmost part of the insect mouth. The labium functions as a lower lip and is composed of several fused segments. It plays a role in manipulating food, conveying it to the mandibles for processing, and in some species, it can be adapted for piercing or sucking. The morphology of the hypostoma can vary greatly depending on the insect's diet and feeding habits, reflecting a wide range of evolutionary adaptations. For example, in predatory insects, the hypostoma might be more robust, while in fluid-feeding insects, it can be elongated and modified into a proboscis.

In some other arthropods, such as certain acarines (mites and ticks), the term hypostome refers to a specific ventral projection at the anterior end of the body, below the chelicerae. This structure is often serrated or toothed and is used by these organisms to anchor themselves to their host during feeding, particularly for blood-feeding species. The hypostome is a critical component for attachment and piercing the host's skin, facilitating the ingestion of fluids. Its appearance and degree of development can be a key characteristic used in the taxonomic identification of different mite and tick species.