Ultrastructure and function of digitiform sensilla on the labial palp of a larval elaterid (Coleoptera)

There are typically six elongate pegs positioned in longitudinal grooves in the outer walls of the distal segment of the labial palps of larvae of Ctenicera destructor (Brown). They are termed digitiform sensilla on the basis of their form and function. Each has a subapical pore typical of a contact chemoreceptor in surface scan, and a terminally branched dendrite but not the cuticular pores typical of a chemoreceptive porous hair. The dendritic terminations are encased by the dendritic sheath, the subapical pore is plugged in the wall of the peg, and there is only one innervating neuron, all of which are typical of a tactile mechanoreceptor. These pegs respond electrophysiologically to contact and vibratory stimuli, but not to the amino acids, sugars, salts, and water tested, nor to a changed pressure in the cephalic hemocoel.

The attachment of cercariae of an ectoparasitic digenean,Transversotrema patialensis, to the fish host: behavioural and ultrastructural aspects

The behaviour which enables the cercariae of the ectoparasitic digenean,Transversotrema patialensis, to establish themselves on the surface of their fish hosts has been described experimentally using anaesthetized fish. Specialized regions of the cercarial tail, the arm processes, seem to be adapted for both specific recognition of, and rapid attachment to, the outer surface of a fish. Each arm process bears an array of nine ciliary sensory structures, the mammiform receptors, which have been tentatively identified as contact chemoreceptor organs involved in host recognition. Also present on each arm is an adhesive pad that mediates the initial attachment to the host. It is a differentiated region of the distal cytoplasm of the epidermal syncytium which invests the whole arm process. The cytoplasm of the pad region contains membrane-bounded adhesive granules, the contents of which are released during activation of the pad.