Does dew blind grasshoppers?

Summary: Considering a 'proof' of Chazal's advanced scientific knowledge.

Post: Prompted by an exchange over at Rationalist Judaism.

The gemara in Shabbat 106b reads:

ת"ר הצד צבי סומא וישן חייב חיגר וזקן וחולה פטור א"ל אביי לרב יוסף מ"ש הני ומ"ש הני הני עבידי לרבויי הני לא עבידי לרבויי והתניא חולה חייב אמר רב ששת לא קשיא הא בחולה מחמת אישתא הא בחולה מחמת אובצנא ת"ר הצד חגבין גזין צרעין ויתושין בשבת חייב דברי רבי מאיר וחכמים אומרים כל שבמינו ניצוד חייב וכל שאין במינו ניצוד פטור תניא אידך הצד חגבים בשעת הטל פטור בשעת השרב חייב אלעזר בן מהבאי אומר אם היו מקלחות ובאות פטור איבעיא להו אלעזר בן מהבאי ארישא קאי או אסיפא קאי ת"ש . הצד חגבין בשעת הטל פטור בשעת השרב חייב אלעזר בן מהבאי אומר אפילו בשעת השרב אם היו מקלחות ובאות פטור:

Our Rabbis taught: If one catches a deer that is blind or asleep, he is culpable; a deer that is lame, aged or sick, he is exempt. Abaye asked R. Joseph: What is the difference between them? — The former try to escape;¹³  the latter do not try to escape. But it was taught: [If one catches] a sick [deer] he is culpable? — Said R. Shesheth, There is no difficulty: one refers to [an animal] sick with fever;¹⁴  the other to [an animal] sick through exhaustion.

Our Rabbis taught: He who catches locusts, gazin,¹⁵  hornets, or gnats on the Sabbath is culpable: that is the view of R. Meir. But the Sages rule: If that species is hunted, one is liable; if that species is not hunted, one is not liable.¹⁶  Another [Baraitha] taught: He who catches locusts at the time of dew is not liable;¹⁷  at the time of dry heat [midday], is liable. Eleazar b. Mahabai said: If they advance in thick swarms, he is not culpable.¹⁸  The scholars asked: Does Eleazar b. Mahabai refer to the first clause or to the last? — Come and hear: He who catches locusts at the time of dew is not liable; at the time of dry heat, is liable. Eleazar b. Mahabai said: Even at the time of dry heat, if they advance in thick swarms he is not culpable.

Why is one not culpable when one catches locusts in the time of dew? Rashi explains:

חגבים בשעת הטל - עיניהם מתעוורות והרי הן ניצודין ועומדין:

Namely, that their eyes are blinded, and behold they can be caught as they stand in place.

Now, how did Rashi obtain such an insight into the sight of locusts and grasshoppers? There seem to be at least three possibilities:

1. There was such a contemporary (scientific) belief, either in the time of Rashi, or in the time of Chazal which was passed down to Rashi.
2. Rashi, and/or Chazal, said this via ruach hakodesh.
3. Rashi made an educated guess, based on cues in context.

I would point out a little bit of evidence for (3), in that this brayta about locusts is somewhat parallel to the earlier brayta about gazelle. And that earlier brayta read: ת"ר הצד צבי סומא וישן חייב. This could then be a guess, and extrapolation. What could the dew do to these locusts, that would pass? Temporary blindness.

Are there any any other cues in the gemara that locusts, in particular, are susceptible to blindness? Well, if we look earlier in the same masechta, in Shabbat 77b, we read:

רבי זירא אשכח לרב יהודה דהוה קאי אפיתחא דבי חמוה וחזייה דהוה בדיחא דעתיה ואי בעי מיניה כל חללי עלמא הוה אמר ליה א"ל מ"ט עיזי מסגן ברישא והדר אימרי א"ל כברייתו של עולם דברישא חשוכא והדר נהורא מ"ט הני מכסיין והני מגליין הני דמכסינן מינייהו מכסיין והני דלא מכסינן מינייהו מגליין מ"ט גמלא זוטר גנובתיה משום דאכל כיסי מ"ט תורא אריכא גנובתיה משום דדייר באגמי ובעי לכרכושי בקי מ"ט קרנא דקמצא רכיכא משום דדיירא בחילפי ואי קשיא גדיא ומתעוורא דאמר שמואל האי מאן דבעי דליסמיה לקמצא לשלופינהו לקרני' מ"ט האי תימרא דתרנגולתא מדלי לעילא דדיירי אדפי ואי עייל קטרא מתעוורא 

R. Zera met Rab Judah standing by the door of his father-in-law's house and saw that he was in a cheerful mood, and if he would ask him all the secrets of the universe he would disclose [them] to him. He [accordingly] asked him: Why do goats march at the head [of the flock], and then sheep? — Said he to him: It is as the world's creation, darkness preceding and then light.²¹  Why are the latter covered, while the former are uncovered?²²  — Those with whose [material] we cover ourselves are themselves covered, whilst those wherewith we do not cover ourselves are uncovered. Why is a camel's tail short? — Because it eats thorns.²³  Why is an ox's tail long? — Because it grazes in meadows and must beat off the gnats [with its tail]. Why is the proboscis of a locust soft [flexible]? Because it dwells among willows, and if it were hard [non-flexible] it [the proboscis] would be dislocated and it [the locust] would go blind. For Samuel said: If one wishes to blind a locust, let him extract its proboscis. Why is a fowl's [lower] eyelid bent upwards?²⁴  — Because it dwells among the rafters, and if dust entered [its eyes] it would go blind.²⁵

Thus, if the proboscis (or rather, antennae) of a locust is dislocated or removed, it goes blind.

Rashi, on the daf, explains:

ואי קשיא נדיא ומתעוורא - ואם היתה קשה תנוד ותעקר כשתכה בעצים:

ומתעוורא - יסתמו עיניה שמראות עיניה תלויה בה כדשמואל:

Thus, the sight of a locust's eyes is dependent upon its antennae, as Shmuel said. Note that he states יסתמו עיניה, that is that its eyes are blinded. I am no expert in the modern-day metzius, but as I understand it, the feelers are supplemental to the eyesight, but the eyes would not cease their input were the feelers dislocated or removed.

But this statement that יסתמו עיניה seems parallel to the Rashi on Shabbat 106b that  עיניהם מתעוורות. It seems logical that Rashi would be working with a theory which finds explicit support elsewhere in the gemara. So, if we were to ask how the dew temporarily blinds the grasshoppers, the answer, within Rashi's worldview, might well be that it weighs down the feelers so that the cannot function properly.

However, there is no absolute need to interpret the gemara in Shabbat 106b as Rashi does. Another interpretation is possible. The gemara, again:

תניא אידך הצד חגבים בשעת הטל פטור בשעת השרב חייב

Another [Baraitha] taught: He who catches locusts at the time of dew is not liable;¹⁷  at the time of dry heat [midday], is liable. 

Note that the gemara does not mention blindness, nor even that it is even the dew that necessarily causes the locusts to stop moving. All we know is that at the time of dew, in the early morning, they don't move about much, such that it is not tzeida, while in midday, the time of dry heat, they do move about, such that it is tzeida. The difference in the environment at these two times is not just one of moisture, but one of temperature.

Indeed, it appears that modern accounting for grasshoppers not moving about much in the early morning is the cold temperature. Thus, for one example:

Grasshoppers are fabulous trout bait. They are easiest to catch in the early morning before the sun warms them and they become more active. 

With all this as background (and even without), it was surprising to see the following comment by "pianoman" at Rationalist Judaism

Pianoman said...

We can't ignore the statements by Chazal and Rishonim that do in fact reveal marvelous scientific wisdom. For instance:

...

3. Rashi on the Gemara in Shabbos 106b had already noted the phenomenon of the grasshopper’s blindness 400 years before the field of optometry began.

This is surprising because it is strange that someone would need to know optometry to determine blindness. Wasn't Yitzchak blind, and don't we know this well before 400 years ago, when the field of optometry began? If a grasshopper was blind, couldn't one determine this without optometry? For instance, see if it bumps into a wall!

Furthermore, see Rabbi Natan Slifkin's response:

Grasshoppers: The reason why they can be caught in the early morning is that they are cold, not because of dew blinding their eyes.

So the blindness does not seem to accord with the actual metzius. What is so amazing about that?

It seems that there a better writeup of the idea by Rabbi Yehuda (Leo) Levi, in Science in Torah. He writes:

There are a number of problems, in my view, with this inspirational "proof" of the foreknowledge of the Sages, and of Rashi.

Basically, the author understands Rashi's comment in a specific way -- that dew enters their eyes and blinds them. Meanwhile, one could understand Rashi's comment in a completely different, plausible way. But let us grant his reading of Rashi. Because of modern understanding of dew and eyes, he believes there to be a difficulty with Rashi, as he understands it. The answer to this might lie in a medieval understanding of the way dew works, or of the way the eye works. For example, though Rabbi Levi (above) asserts that dew "is essentially just pure water", according to Aristotle, honey comes from dew. It is possible, for instance, for Rashi to have believed that the dew similarly transforms on the surface of the grasshopper's eyes. Thus, his question makes some modern assumptions.

The author answers his own question by appealing to modern science. Since nowadays we know of the complex eyes of locusts, we can explain Rashi (as per his understanding) is a way which is 'reasonable'.

Note that the word 'reasonable' is Rabbi Levi's word choice. "Because the body temperature of insects is equal to that of their surroundings, it is reasonable to assume that the dew will also condense on the eye of a grasshopper, temporarily blinding it." Not that he is citing scientific studies which have determined this to be the case, but that it is reasonable, or plausible, to offer this explanation.

Meanwhile, as far as I have seen, modern science does not state that this is what happens. Modern science does not state that grasshoppers go temporarily blind from dew condensing in their eyes. Rather, it appears that the modern belief is that they are not blind at all, but are simply not as active, due to the cold. (Which, as I noted above, works well with the wording of the brayta.)

Is this inspirational? I suppose it could still be inspirational. After all, whether Rashi is wrong or right, he clearly based his explanation on knowledge of insect temperatures, dew, and optometry, a science which would have its beginnings only in the sixteenth century!

Except there is no reason to make that assumption except to save Rashi. It need not be read into the gemara (which could be based on the simple effect of temperature on their activity); it need not be read into Rashi, for he might well be talking about the impact of dew on antennae. That someone asks a qeuestion on shaky grounds and resolves it by retrojecting modern ideas backwards, in a way that does not even accord with reality -- I don't find that inspirational in the least.

Should one go out of his way to debunk such silly proofs? I could see good reasons to leave people with their faith in Chazal and then by extension, Torah MiSinai, etc. On the other hand, there is value in Truth, and in finding the true meaning of a gemara and a Rashi. More than that, there are unintended consequences of such inpirational proofs. To cite Rabbi Natan Slifkin on The Dangers of False Inspiration:

Third, we live in an era of great hostility towards those who follow the rationalist approach of the Rishonim in these matters. Spreading the idea that Chazal, in their pronouncements about the natural world, were millennia ahead of their time, assists the campaign of those who condemn the rationalist approach.

Update: Rabbi Slifkin pointed out to me this picture, which would appear to indicate that dew does not in fact  cause problems with the eyes in the manner suggested above by Rabbi Levi: