3D reconstruction of a model phytophagous mite at nano-resolution reveals a peculiar ingestion and digestion system and structural adaptation for a tiny animal.

The fine structure of the globally distributed phytophagous mite, Tetranychus urticae, has been elucidated using serial block-face scanning electron microscopy (SBF-SEM) combined with 3D reconstruction. The reconstructed models reveal previously unknown internal adaptations tailored to its unique lifestyle. These include a specialized food ingestion structure to facilitate effective cell-sucking, a semi-closed-loop tracheal system optimized for efficient oxygen diffusion in a minute organism, and a pair of well-developed silk glands utilized for water repellency, defense and dispersion. Notably, the midgut occupies nearly the entire body cavity, housing 189 (male) and 636 (female) suspended digestive cells, except for the reproductive system. No observable connections or channels were found linking two midgut regions (ventriculus and posterior midgut), nor were valve structures or controlling muscles detected in this region in most cases, suggesting the presence of an intermittent, dynamically regulated midgut. This adaptation may address the functional demands of unique digestive cells suspended within the ventriculus. However, a funnel-shaped midgut capable of forming temporary channels for the disabled transport of digestive cells from the ventriculus to the posterior midgut was identified. These findings profoundly advance our understanding of how minute animals adapt their internal systems to specialized lifestyles.