Diapause fixation and development were studied in females of a Czech population of Aquarius paludum by monitoring changes in oxygen uptake, weight and reproduction potential in field adults between August and February. The fall in oxygen uptake related to fresh weight (from > 1000 µl O2 per g per h to around 500 µl O2 per g per h) in field adults during early diapause was similar in two age cohorts, although the time of adult ecdysis differed by 50 days and thus occurred at markedly different temperatures and photoperiods of late August vs. early October. The different conditions affected the weight of females and thus also the absolute value of oxygen consumption: both parameters were much lower in the October females. The seasonal time of diapause termination in A. paludum did not differ from findings in other cold temperate insects: diapause and the photoperiodic response ended in the winter, as was shown by the possibility of insects' reactivation by 26°C in spite of a diapause promoting daylength of 12L : 12D. In most females (78.6%) ovaries matured after transfer to these conditions in mid-February, while ovarian maturation occurred in only 16.7% of females transferred in early January and no ovarian maturation was observed in females transferred in early December. Ovarian maturation was preceded by a transient increase in oxygen consumption from 600 µl O2 per g per h to 1400 µl O2 per g per h.
Great progress has recently been made in cryobiology. One field, however, has been neglected: the temporal sequence of the effects of photoperiod and temperature, and their relative importance in cold hardening. This is relevant to the question of importance of diapause in cold-hardiness. Denlinger (1991) outlined the categories of such relations and stressed a great need for further detailed research. A survey of studies done over the past decade revealed many gaps in the evidence and the ambiguous nature of the data on the photoperiodic regulation of cold-hardiness. We hope that this review will stimulate further research in this field. Among several directions where research is most needed we have stressed (1) simultaneous recording of changes in survival and dynamics of suspected cryoprotectants (stressed also by Danks, 1996), (2) checking the regulation of different phases of cold hardening, and (3) discrimination between direct and indirect (mediated via neuroendocrine system) effects of environmental cues on cold hardening.