Ultrastructure of the integument of first-instar Hypoderma lineatum and H. bovis (Diptera: Oestridae)

The ultrastructure of the integument of newly hatched, 60-d-old (in vitro-cultured) and 6-7-mo-old (recovered from the host) first-instar cattle grubs, Hypoderma lineatum (Villers) and H. bovis (L.), was studied. Changes in the cuticle during development of the first instar included increased thickness of the procuticle (0.8-1.4 to 7.5-7.9 microns) with an accompanying increase in number of lamellae (4-7 to 35-40), increased thickness of epicuticle (0.11-0.24 to 0.92 microns), and development of new epicuticle layers. The epithelial cells in newly hatched larvae also changed with age when compared with those in culture and those recovered from hosts. These changes included the appearance of dense plaques on the apical cell membrane and an increase in the folding of the basal cell membranes, with an accompanying increase in the amount of extracellular space. The basement membrane, separating epithelial cells from the hemocoel, also increased in thickness during the first instar. These changes reflect developments required to accommodate increased size but, in the case of the epithelial cells, suggest altered function. Ruthenium red staining on the cuticular surface of larvae cultured in vitro was indicative of the presence of negatively charged molecules, which may influence the host recognition and response capabilities.