PHOTOPERIODISM IN INSECTS AND MITES 591 



Stimulation. Unlike plants, all such reactions require only low intensi- 

 ties: 1-2 ft-c of white light in Melatctranychus; less than 1.0 ft-c in 

 several Lepidoptera; and approximately 0.01 ft-c in the embryos of 

 Bombyx. The important question of the wavelength dependence of the 

 reaction requires further attention, but it is known that the violet, blue, 

 and blue-green regions of the spectrum are especially effective in 

 Bombyx, GraphoUtha, and Metatetranychus, all these genera being 

 nearly or totally insensitive to red. There appears nevertheless to be 

 some variation between species. In a recent investigation by Geyspitz 

 (1957) the following descending order of sensitivity was estabUshed 

 for two genera of Lepidoptera: violet, green in DendroUmus; green, 

 violet, red in Pieris. A third genus, A crony eta, appeared to be equally 

 sensitive to violet, green, and red, although this may have been a 

 consequence of the high intensities used. Unfortunately, no response 

 curves so far obtained have sufficient accuracy for any firm conclusions 

 to be drawn as the nature of the photosensitive pigment. It is worth 

 noting in this context that there is as yet no evidence that the photo- 

 response can be reversed by other wavelengths in the manner of the 

 red, far-red reaction in plants. 



Although it has been repeatedly demonstrated that photoperiod acts 

 directly on arthropods, and not indirectly through the food plant, in- 

 formation is still lacking as to the precise location of the photoreceptor. 

 The receptors in the larvae of DendroUmus are certainly in the anterior 

 region of the body, for it is possible to place the head in a light-tight 

 hood for 12 hr a day while exposing the thorax and abdomen to 

 permanent light — a treatment that invariably results in diapause initia- 

 tion (Geyspitz, 1957). DendroUmus, Pieris, and Acronycta also show 

 a well-defined locomotor response to light which is mediated by the 

 eyes. Since a comparison of the wavelength dependence of the be- 

 havioral and photoperiodic reactions revealed certain similarities, Gey- 

 spitz concluded that the eyes were also responsive to photoperiod. This 

 argument must, however, be treated with some reserve in view of the 

 earlier experiments by Tanaka (1950) on the larvae of the silkworm 

 Antheraea pernyi. In this species photoperiodic induction remains 

 unimpaired after the lateral ocelli have been destroyed by cauteriza- 

 tion. 



