122 A. D. LEES 



have a long day response, since diapause is prevented in long 

 days and induced in short days. The moth Acronycta and the red 

 spider mite Pcmonychus uhni belong to this group^' ^. The re- 

 sponse curve shows a characteristic high rate of change at the 

 critical photoperiod, which ranges from about 13 h of light 

 daily in insects from southerly latitudes to 20 h in insects from 

 the extreme North. In at least a few insects, however, this 

 general pattern is reversed. In such 'short day' species a short 

 photoperiod prevents diapause. The commercial silkworm 

 Bombyx mori is the best known example. Others include the 

 Desert Locust Schistocerca^ and the leafhopper Stefwcranus'^. 

 It is noteworthy that the diapausing and non-diapausing forms 

 of the latter species are strikingly heteromorphic. 



Since these and many other light-responsive species are 

 phytophagous, the question arises as to whether the plant 

 serves as the insect's photoperiodic receptor. By transferring 

 them serially between different host plants, the insects or mites 

 can be exposed to one type of photoperiod, their hosts to 

 another. Experiments of this type have shown that in nearly all 

 cases the insects respond in their own right. However, the soil- 

 dwelling larvae of the cabbage root fly Erioistichia appear to be 

 an exception, for although their diapause behaviour is unaffected 

 by placing an opaque cover on the soil round the root of the 

 plant, the light treatment of the latter is quite influential^. But 

 it is not yet known whether photoperiodic or photosynthetic 

 events in the plant provide the 'cue'. 



Some insight into the mode of action of the cycle of illumi- 

 nation has been obtained by varying the lengths of the light and 

 dark periods independently. In the mite Panonychus, for 

 example, there is no doubt that both light and darkness plays 

 an integral part in the photoperiodic response, the length of the 

 dark period being particularly critical^. So far as the dual 

 significance of the light and dark phases is concerned, other 

 arthropods seem to resemble Panonychus: but the details of 

 the response differ considerably. A feature which seems to be 

 typical of the group (and untypical of plants) is that long 



