Diapause induction in the third and fourth instar of Chironomus riparius (Diptera) from Belgian lowland brooks

Diapause in IVth instar larvae linked to overwintering conditions is a well -known phenomenon in Chironomus species. Here, diapause induction in the IV th instar of Chironomus riparius MEIGEN, 1804, is described for the first t ime from a combination of field and laboratory research. These observations are relevant for understanding the ecology of Ch. riparius, a widespread s pecies in Europe, and for its use as a bioindicator in water and sediment q uality assessment. This diapause is linked to overwintering and is characte rised by an arrest in phase IV4-5a of the development of the imaginal discs in the last larval instar. The diapause could be induced experimentally by short-day conditions, i.e, a photoperiod of 8 hours light per day at 15 de greesC. However, there was a great variation in the diapause response. Unde r short-day conditions, half of the larvae went into a long-lasting diapaus e while the other half continued their development after a much shorter dia pause. Moreover, diapause effects on the size of IVth instar larvae have be en examined. The IIIrd instar also appears to be affected by this overwinte ring diapause, causing a retardation of the moult to the IVth instar. Diapa use phenomena in the IIIrd instar of Chironomus species have never been des cribed before. In long-day conditions of 16 h light per day at 15 degreesC, development is continuous: no diapause was observed. Furthermore, a desynchronisation bet ween moulting and development of the imaginal discs was observed. In all ex periments, as well as in the field, the development of the imaginal discs d efinitely began in the IIIrd instar: phases described in the literature fur the early IVth instar have been observed in the IIIrd instar.