Alimentary canal of adult Acalymma vittata (Coleoptera: Chrysomelidae): Morphology and potential role in survival of Erwinia tracheiphila (Enterobacteriaceae)

We describe the morphology of the alimentary canal in adult Acalymma vittata (F.), the vector of Erwinia tracheiphila (Smith) Bergey et al. emend. Hauben et al. (Enterobacteriaceae), the causal agent of bacterial wilt in cucurbits (Cucurbitaceae). The foregut includes a pre-oral cavity, pharynx, oesophagus, and crop but lacks a well-developed proventriculus. The midgut occupies approximately 65% of the length of the gut, has distinctive ventricular crypts throughout its length, and is lined with a peritrophic membrane, but lacks caeca for harboring symbionts. The hindgut comprises the colon and rectum and four Malpighian tubules. The cuticular intima of both foregut and hindgut bears rows of spines and is thrown into numerous folds. Transmission electron microscopy showed bacteria resembling E. tracheiphila within the hindgut 1 and 30 d after the beetles fed on E. tracheiphila spread between cotyledons of cucumber, Cucumis sativus L. (Cucurbitaceae). Our observations suggest that the midgut is not appropriate for long-term retention of E. tracheiphila because of the absence of caeca and the presence of a peritrophic membrane. Temporary and long-term pathogen retention may be associated with rows of spines and numerous folds within the foregut and hindgut.