Incidence and causes of deformities in recently hatched larvae of Chironomus riparius Meigen, 1804 (Diptera, Chironomidae)

The occurrence of head capsule deformities in first instar larvae of Chirononomus riparius MEIGEN, 1804 was studied under field and experimental conditions. These larvae were obtained by collecting egg masses from two polluted sites, as well as from cultures of fourth instar individuals from these two sites. Fourth instar larvae from the two populations were screened for deformities, and frequencies ranged from about 10-14 %. Although the two sites were affected by different types of pollutants (raw sewage in one case and pesticides in the other) we did not detect significant between-site differences in either frequencies or types of deformities. Newly hatched larvae from egg masses collected from the two sites displayed deformities very similar to those present in fourth instar larvae from the same sites, although frequencies were lower (often 0-5 %). These deformities included severe deformities such as mentum gaps, and in some cases a particular deformity type was repeated in several larvae from the same egg mass. Clean-water Fl first instar larvae (from egg masses laid in clean water by adults emerged from field-collected fourth instar larvae in the laboratory) likewise showed deformities. Four hypotheses for the occurrence of larval deformities are considered and discussed: a) teratogenic effects of contaminants in the water, b) teratogenic effects of contaminants accumulated in the mother's body, c) genetic inheritance of contamination-induced mutations and d) spontaneus dysgenesis (i.e. developmental failures occurring independently of genetic or teratogenic contamination effects). All the hypotheses considered are not mutually exclusive, but a combination of two or more causes could increase the deformity frequencies in first instar larvae.