Some aspects of the host/parasite relations of Newfoundland blackflies and their mermithid parasites

Two species complexes of larval blackflies, Prosimulium mixtum/fuscum Syme and Davies and Simulium venustum Complex, susceptible to mermithid parasitism, were collected from two Newfoundland streams. The anatomical relations of the neuroendocrine centers of these simuliid larvae were histologically examined. The neurosecretory system showed certain morphological similarities to other Nematocera (viz. three pairs of neurosecretory cell clusters in the brain, one pair of nervi corporis cardiaci entering the corpora cardiaca). The retrocerebral endocrine complex is comprised of cellular corpora cardiaca glands, a single corpus allatum and surrounding peritracheal gland. The morphology of the blackfly neuroendocrine system is discussed in relation to other Nematocera and higher Diptera. Two experimental groups were chosen consisting of uninfected larvae with dark histoblasts (controls) and nematode infected larvae of similar body dimensions. Histochemical studies showed no significant differences between the endocrine glands of mermithid infected and control P. mixtum/fuscum larvae. However, the mermithid parasite did significantly increase the nuclear DNA/RNA activity of the corpus allatum gland and the corpus cardiacum gland volume and stored neurosecretory material in S. venustum Complex larvae. Such endocrine effects of mermithid parasitism are discussed in relation to their significance within the host/parasite relationship. A marked decline in the amount of fat body tissue and their glycogen concentrations in both the infected simuliid species was recorded. However, fat body nucleic acid activity was unaffected by mermithid parasitism. Mermithid-parasitized P. mixtum/fuscum larvae had a lower overall dry weight than uninfected controls, but the biomass of S. venustum Complex did not appear to be affected by mermithid parasitism