Foregut anatomy and ultrastructure of the soil-spraying larvae of Bittacus cirratus Tjeder (Mecoptera: Bittacidae).

The larvae of Bittacidae in Mecoptera are remarkable for their distinctive soil-spraying behavior. To adapt the soil-spraying behavior, the larvae bear compact longitudinal muscles on the midgut, and well-developed circular muscles on the rectum. Whether the foregut evolved specific morphological adaptations associated with this behavior, however, remains unknown to date. In this study, the foregut of fourth-instar larvae of the hangingfly Bittacus cirratus Tjeder was investigated using light and transmission electron microscopy. The foregut comprises a short pharynx and a long esophagus. The esophagus is composed of a cuticular intima, a single layer of thin epithelial cells, and longitudinal and circular muscles. The cuticular intima thickens and differentiates into three layers: the endocuticle, exocuticle, and epicuticle. The epithelial cells are rich in mitochondria, microtubules, glycogen, and rough endoplasmic reticulum. The muscle cells are characterized by a particularly high density of myofibrils, along with distinct Z-lines and abundant mitochondria. The longitudinal and circular muscles are well-developed to support soil-spraying behavior, facilitating rapid soil transport through the esophagus, and the trilaminar intima protects the epithelium against abrasion by soil particles.