May 26, 1923] 

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NATURE 
795 
‘ 
’ Vision and Light Sensitiveness. 
In my former letter (NATURE, April 14, p. 498) I 
endeavoured td avoid dogmatism on this very 
obscure subject, and aimed rather at stimulating 
further research in what seems to be a phenomenon 
of great importance. Though I go all the way with 
Mr. Locket (NarurE, April 28, p. 570) in cautioning 
the utmost reserve in accepting my hypothesis, 
_ especially as, in my letter, it is supported, and 
designedly so, by no more definite evidence than the 
flicking of a fly; still, in the interests of research, 
I feel called upon to add some comments to his 
letter. 
I have collected a large mass of evidence which, 
however, cannot be published yet, as it is far from 
complete. Moreover, as much of it appears to 
contradict the conclusions of great authorities, some 
of whom Mr. Locket mentions, it obviously cannot 
be urged until I have repeated each essential portion 
of it more than once, and explored in each experiment 
every possibility. 
First of all, then, I submit that it is of the utmost 
importance to differentiate clearly between sensitive- 
ness to light and vision, both in experiment and in 
deduction; for vision is by means of eyes, light 
sensitiveness not necessarily so. Plateau has proved 
(Journal de I’ Anat. et de la Physiol., 1886, p. 431) that 
certain myriapods distinguish between light and 
darkness by the general surface of the skin ; though 
Forel and Lord Avebury have proved this not to 
be the case with ants. It would be interesting to 
know if it isso in Typhlopone, where there are no eyes. 
Wherever there are eyes I think we may, for the 
present, assume that there is sensitiveness to light, but 
_ this is far different from presuming that there is vision. 
I have been unable to discover any evidence for vision 
in the Epeire studied, or in Tegenaria domestica or 
Agelena labyrintheca, when proper precautions have 
been taken. In these instances a fly, with the wings cut 
off, will not disturb them ; but, if the fly have wings or 
stumps of wings with which it can buzz in the forceps 
then the case is very different. The necessity for 
taking every precaution may be shown by the extreme 
sensitiveness of T. domestica to the presence of 
carbon dioxide, so that even slight breathing on 
the specimen causes movement. Though this spider 
will revive after a collapse of five minutes in a 
“vacuum at 6 mm. of mercury, though it will live for 
some minutes apparently with comfort in coal gas, 
an atmosphere of carbon dioxide kills it instantly. 
Another precaution, most essential to success, is 
to avoid casting shadows on the animal under 
observation ; in the case of the black-bellied taran- 
tula (Lycosa narbonnensis), and in very many others, 
there is a manifest seeking for light. This is most 
apparent when the young are on the parent’s back. 
Phe, possibly, lies the solution of this enigma. 
This spider carries her young on her back with, so 
far as I can see, no food for about six months. These 
fasting youngsters grow strong, expend energy, and 
certainly do not become emaciated. Do they get 
their energy from the air alone? Do they possibly 
get it from the sun, as vegetable life does? JT 
cannot yet answer this. Here the investigation would 
extend from the so del de in Euglena viridis to the 
facts of modern heliotherapy. 
Turning to Mr. Locket’s remarks on the ants, the 
species employed for the most part were Formica 
fusca and I’. sanguinea. In these insects Forel (Rec. 
Zoolog. Suisse, 1887) has proved that there is normally 
sensitiveness to light which is destroyed by varnishing 
the eyes, and Lord Avebury mentions (‘* Ants, Bees, 
and Wasps,’’ 13th edition, p. 405) their sensitiveness 
NO. 2795, VOL. 111] 
to “ultra-violet rays much beyond our limits of 
vision.”” This same authority says quite plainly 
(op. cit. pp. 273, 272, 266, 256, 251) of Lasius niger 
that, ‘‘ though it seems clear that they are helped 
by sight,”’ they do not trust much to their eyes, 
and are “‘ little guided by . . . surrounding objects.”” 
In another experiment, ‘‘ if she {the ant] were much 
aided by sight, then she would have had little ' 
difficulty in finding her way back.’’ On the other 
hand, he concludes from further work (pp. 268, 270) 
that by altering the position of the lights ‘‘ the ant 
went wrong,” and that, “in determining their 
course the ants are greatly influenced by the direction 
of the light.’ Here the difference between mere 
light sensitiveness and vision is strongly supported. 
Forel’s experiment (‘‘ Senses of Insects,”’ pp. 
124-128), noted by Mr. Locket, had seemed to me 
conclusive until I repeated it, taking care to supply 
several control ants the eyes of which were varnished 
with a transparent fluid. “From the results I concluded 
that the difficulty in finding home was as much 
due to the annoyance of being handled and varnished 
as to being hoodwinked. 
When working on the flies, including both species 
mentioned by Mr. Locket, I did not varnish the 
eyes; I used a second sheet of glass between the 
fly’s back and the moving object. Still this experi- 
ment with the second glass was not Tepeated often 
enough to allow me to state my results with assurance, 
and I agree with Mr. Locket that varnish should 
have been employed. With regard to the motion 
of air,—and this is the kernel of Commander Hilton 
Young’s hypothesis,—vibration due to sound waves 
and simple air currents must be treated separately. 
Many insects and spiders are extremely sensitive to 
the former, and there can be no doubt that the fly 
is sensitive to the latter, though I doubt whether to 
the extent suggested by Commander Hilton Young. 
I cannot agree with Mr. Locket in his use of the 
ocelli as an explanation. It was, I think, Johannes 
Miiller’s opinion that they were especially useful for 
close vision. But we have the authority of Plateau, 
Forel, Réaumur, Marcel de Serres, Dugés, Lord 
Avebury, and-others, that varnishing of these organs 
made no difference. Though many, with Forel 
(Rec. Zoolog. Suisse, 1887), Lebert (“ Die Spinnen 
der Schweiz ’’), and Pavesi (Ann. Mus. Civ. di Genova, 
1873, p. 344), Suggest that the ocelli serve for vision 
in semi-darkness, and the eyes for vision in full light, 
the experiments I have made in this field, necessarily 
obscure, have been fruitless, so far as I have been 
able to devise them. 
The instance of the male Attid, given by Mr. 
Locket, I have not worked on yet; as well as the 
many examples of what appears to be lethal fascina- 
tion as in Mantis religiosa. There is clearly an 
enormous amount of work to be done,—so far I 
have not touched the scorpions,—and I speak, as I 
did in my former letter, only of those species which 
I have studied, necessarily few relative to the vast 
kingdom under discussion. So I can end no better 
than again, with Mr. Locket, cautioning reserve. 
J. P. O’HEa. 
St. Beuno’s College, St. Asaph, 
April 29. 

Phosphorescence caused by Active Nitrogen. 
In a letter under this title in Nature of May 5, 
Pp. 599, Prof. E. P. Lewis announces his recent 
discovery that active nitrogen excites phosphorescence 
in a number of solid compounds. [I should like to 
mention that during the summer of last year I 
observed the same phenomenon in the case of an 
x2 
