Diapause in Caraphractus cinctus Walker (Hymenoptera: Mymaridae), a parasitoid of the eggs of Dytiscidae (Coleoptera)

Caraphractus cinctus Walker is a parasitoid of the eggs of Dytiscidae. The entire development of this Mymarid takes place in eggs immersed in water and attached to, or embedded in, plant tissue. The winter is passed in diapause as a full-grown resting larva (prepupa) in the host egg. Diapause of Caraphractus has been studied in the eggs of Agabus bipustulatus. The eggs of this Agabus do not undergo diapause and since the shell is transparent, the larvae of Caraphractus (usually two to three in each egg) are clearly visible during their later stages, so the time of pupation is easily determined. Experiments were made of chilling the maternal generation of Caraphractus at various stages of development, but this did not lead to diapause in the larvae of the chilled generation, or to any increase in the diapause in the offspring. When the Mymarid was bred in a north room exposed to natural daylight and darkness, some diapausing larvae occurred in mid-September and by October all were in diapause. When larvae were reared in long photoperiods (16½–24 hr. light) development was direct, but with short photoperiods (2–15 hr. light), diapause occurred. The critical period lies between 14 and 16 hr. of light. The stage sensitive to photoperiod is the full-grown larva, and when larvae are reared to this stage under short photoperiod and then transferred to continuous light, direct development is determined within 3 days. Conversely, larvae reared under continuous light and removed to short photoperiod before reaching the sensitive stage, go into diapause. The larvae are sensitive to light of very low intensity, developing direct with a ‘night’ of 0·03–0·05 f.c. Larvae becoming full grown in the end of August and beginning of September do not diapause and must respond to 2–3 hr. of twilight. Diapause induced, under artificial photoperiods at any time of the year, usually lasts about as long as the normal winter diapause. If however the prepupae are subjected to temperatures below freezing for about a fortnight (provided they have already been about 15 weeks in diapause) pupation occurs as soon as the temperature is suitable. In cold weather the pupal stage may be greatly prolonged. I am much indebted to Professor H. G. Callan for his kind permission to use the constant temperature rooms at the Natural History Department of St Andrews University. I have received most valuable help from Dr C. Muir in conducting the various photoperiod experiments, especially in regard to light measurements, and I am most grateful to him for discussing with me many of the problems that arose in the course of this research and for commenting on the typescript of this paper. To Dr H. E. Hinton I am indebted for useful advice, and to Dr Seliga of St Andrews University for his translation of Russian papers. Professor and Mrs Stibbs have been most kind in providing me with information regarding the duration of astronomical darkness in the latitude of St Andrews. I am especially grateful to Dr George Salt for his kindness in reading over my typescript and for his helpful criticisms.