Laudator Temporis Acti

Avianus, Fables 3 (tr. J. Wight Duff and Arnold M. Duff, followed by their Latin text):

While a crab was walking backwards and tracing its crooked way,
it banged its scaly back in the rocky pools.
Its mother, eager to go forward with step unhindered,
is said to have delivered a warning to it in such words as these:
"Don't go zigzag and choose these crooked ways, my child,
and don't seek to move backwards and slantwise on your feet.
Step out vigorously with straightforward effort and
plant your footsteps safely in the onward path."
"I will do so," the young crab replied, "if you go ahead of me;
and, if you show me the correct road, I will follow the more surely.
For it is exceedingly foolish of you, when you are attempting the most crooked of courses yourself,
to set up as censor and criticise the faults of another."

Curva retro cedens dum fert vestigia cancer,
    hispida saxosis terga relisit aquis.
hunc genetrix facili cupiens procedere gressu
    talibus alloquiis emonuisse datur:
ne tibi transverso placeant haec devia, nate,        5
    rursus in obliquos neu velis ire pedes,
sed nisu contenta ferens vestigia recto
    innocuos proso tramite siste gradus."
cui natus "faciam, si me praecesseris" inquit,
    rectaque monstrantem certior ipse sequar.        10
nam stultum nimis est, cum tu pravissima temptes,
    alterius censor si vitiosa notes."

4 emonuisse Ellis: praemonuisse codd.
11-12 del. Cannegieter
12 si Ellis: ut codd.

Lines 11-12 look to me like the moral of the fable, addressed to the reader, despite Robinson Ellis ad loc.:

Wopkens can hardly be right in explaining these verses as a general reflection; they would then have been introduced by Nam, nor would stultum have been enforced by nimis which gives the remark a sort of personal character.

They are introduced by nam!

Babrius, Fables 109 (tr. Ben Edwin Perry):

"Don't walk aslant!" said a mother crab to her young one.
"Don't drag yourself crosswise over the wet rock."
"Mother and teacher," replied the young crab, "first
walk straight yourself, then I'll do so by watching you."

"Μὴ λοξὰ βαίνειν" ἔλεγε καρκίνῳ μήτηρ,
"ὑγρῇ τε πέτρῃ πλάγια κῶλα μὴ σύρειν."
ὁ δ᾿ εἶπε "μῆτερ ἡ διδάσκαλος, πρώτη
ὀρθὴν ἄπελθε, καὶ βλέπων σε ποιήσω."