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From top to bottom, Venera 9, 10, 13A, 13B, 14A and 14B.

enter image description here

Venera 13 and 14 have some kind of teeth on the edge of the lander. What purpose do these serve? Why didn't Venera 9 and 10 have them?

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The teeth served an aerodynamic function.

...metal teeth were added to the periphery of the impact ring in an effort to reduce the spin and oscillation during the descent and prevent the rough landings experienced by the 1978 missions.

This is also why the earlier missions didn't have them, they were added in an attempt to mitigate problems experienced on the earlier missions.

Huntress & Marov, Soviet Robots in the Solar System, p. 322 (selections, including this page, are available at Google Books here)

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    $\begingroup$ Is it known if the efforts were successful? Did those teeth actually do what they were supposed to? $\endgroup$
    – Polygnome
    Jul 9, 2020 at 13:42
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    $\begingroup$ @Polygnome the book didn't say (in fact it did not describe the issues encountered on the previous probes). I assume the teeth act as turbulators, but it didn't go into detail on that either. $\endgroup$
    – Organic Marble
    Jul 9, 2020 at 13:54
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I think the teeth should be used for camera calibration. The very dense atmosphere of Venus outside the camera window of the lander had a magnifing effect like a lens.

This effect is known from scuba diving masks, everything looks closer and bigger because of the water outside is more dense than the air inside the mask.

The exact magnification was unknown before because the optical properties of the venusian atmosphere were not known precisely.

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    $\begingroup$ More precisely, its index of refraction wasn't known. physics.stackexchange.com/questions/83480/… $\endgroup$
    – Camille Goudeseune
    Jul 8, 2020 at 18:43
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    $\begingroup$ If that were the reason, they would have needed it the first time, but possibly not the second or third times. Perhaps they didn't care about the small distortions for the kind of images they planned to take? Perhaps they used the known "observable" radius of the craft itself to calibrate the camera? (I.e. they could have taken a picture from the same mechanical position on Earth, then measured the difference in sizes to see how to calibrate the images/camera. $\endgroup$
    – jpaugh
    Jul 8, 2020 at 21:31

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