The morphology and position of putative neurohemal areas in the peripheral nervous system (ventral nerve cord and retrocerebral complex) of the cricket Gryllus bimaculatus are described. By using
antisera to the
amines dopamine,
histamine,
octopamine, and
serotonin, and the
neuropeptides crustacean cardioactive peptide,
FMRFamide,
leucokinin 1, and
proctolin, an extensive system of varicose fibers has been detected throughout the nerves of all neuromeres, except for nerve 2 of the prothoracic
ganglion. Immunoreactive varicose fibers occur mainly in a superficial position at the neurilemma, indicating neurosecretory storage and release of neuroactive compounds. The varicose fibers are projections from central or peripheral neurons that may extend over more than one segment. The peripheral fiber varicosities show segment-specific arrangements for each of the substances investigated. Immunoreactivity to
histamine and
octopamine is mainly found in the nerves of abdominal segments, whereas
serotonin immunoreactivity is concentrated in subesophageal and terminal
ganglion nerves. Immunoreactivity to
FMRFamide and
crustacean cardioactive peptide is widespread throughout all segments. Structures immunoreactive to
leucokinin 1 are present in abdominal nerves, and
proctolin immunostaining is found in the terminal
ganglion and thoracic nerves. Codistribution of peripheral varicose fiber plexuses is regularly seen for
amines and
peptides, whereas the colocalization of substances in neurons has not been detected for any of the neuroactive compounds investigated. The varicose fiber system is regarded as complementary to the classical neurohemal organs.