Giant Ants: OD&D situates these creatures on the 3rd level table. This monster doesn't have any statistics specified in Vol-2, or even a suggestion for damage in Sup-I (like most giant animals do), so we necessarily turn to the Monster Manual. The basic type of giant ant is AC 3, HD 2, Atk 1 for the canonical 1d6 damage; the text indicates that this is a worker ant ("90% likely") -- and the simulator measures this simple creature at only EHD 1, which would suggest that we move it one table lower in OD&D. But the text also indicates another option (presumably 10% of ants): the warrior ant with HD 3 and a poison sting (save or die, of course) -- and this type the simulator assesses at EHD 5, which would put it one table higher than shown in OD&D. So here we have a dilemma: the worker ant is too weak, and the warrior ant too strong, for the given location in the OD&D random monster tables. I've left it as the warrior type in my charts, just because the poison sting makes it a little more interesting; perhaps that's a mistake thematically.
Giant Beetles: The type is shown on OD&D random table level 4. Sup-I (p. 18) suggests that Giant Beetles be given 3-30 (3d10) damage per bite; that seems highly excessive in our context of non-revised OD&D combat, because even Giants are originally given only 2 dice of damage (d6's). Looking at the Monster Manual with its half-dozen options for Giant Beetles, the one that seems the most appropriate is the giant Boring Beetle, with AC 3, HD 5, and Atk 1, now reduced on average to 5-20 points (5d4). If I lower this to simply 2d6 damage, to match it with Giants (and the only type on my list below Giants to get more than 1 die of damage), then the assessed EHD is 5, the same as its Hit Dice -- so no bonus to XP, but nicely situating it in the middle of the Level 4 table where OD&D says it belongs.
Giant Snakes: OD&D places these creatures on the Level 3 table; and there are so many issues to juggle with this one (again, we can only guess at it assumed Hit Dice, Attacks, etc.). Sup-I suggests that the Giant Snake should have both a poison bite and a constriction attack (if I read it correctly); a combination that doesn't actually occur on Earth for any type of snake. The Monster Manual thereafter takes a more naturalist stance, splitting up the types of snakes into either a giant Constrictor HD 6+1 or Poisonous HD 4+2 types (among others: Amphisbaena, Sea, and Spitting snakes, not considered here). So we have a number of different options to consider for our OD&D serpent.
Let's take a step back and consider the pulp literature for the type. Conan is known to deal with monstrous snakes in many of his stories (including famous cover artworks), so let's look there. Fortunately, I found a post compiling those giant-snake encounters on the Official Robert E. Howard Forum precisely one day before it's scheduled to be shut down (currently at www.conan.com; I'll be copying the content in comments below when I get a chance). Poster blanor notes that Conan fought giant snakes in 4 stories (Hour of the Dragon, Queen of the Black Coast, Beyond the Black River, and The Devil in Iron). As I look up the scenes in the various stories myself, I note that in each case they are described as a tremendous python, a constrictor type that tries to crush Conan or someone else in its coiling mass (the word "python" is directly used in all of the stories except for Devil in Iron; although tellingly the description of a "wedge-shaped head" is used uniformly in all of those stories). The serpents are each described as being variously 20 or 40 feet long (and The Scarlet Citadel has a whopping 80-foot snake that chases Conan for a while, but he never actually comes to blows with it). But the creature in Beyond the Black River is given an additional terrifying aspect:
That was the reptile that the ancients called Ghost Snake, the pale, abominable terror that of old glided into huts by night to devour whole families. Like the python it crushed its victim, but unlike other constrictors its fangs bore venom that carried madness and death. It too had long been considered extinct. But Valannus had spoken truly. No white man knew what shapes haunted the great forests beyond Black River.
Taking the Conan stories as a cue for the basic type, my list features the MM Giant Constrictor as the basic Giant Snake: AC 5, HD 6+1, can bite and constrict at the same time for 1d6 damage each (this requires an initial successful bite; thereafter it gets 1d6 constriction every round automatically, and can still bite normally as well; the victim can act normally meanwhile); this gets assessed as EHD 6, the same as its HD (so no XP bonus), but placing it one level higher than in the OD&D tables (and on the tail end of that table, in fact, almost into Level 5).
So perhaps we should consider some other options to rectify that. Take the MM Giant Poisonous type (HD 4+2). Unfortunately, the save-or-die poison makes this even more dangerous, at EHD 8 -- clearly into the Level 5 table, then. Or we could simulate the infamous "Ghost Snake" at HD 6 with both constriction and poison -- of course, that's even more deadly, at EHD 11 (pretty deep into the Level 5 monster table at this point). We could consider a mini-Ghost snake, 3 HD with both constrict & poison, as per Sup-I -- that's EHD 5, still one level too high.
If we decide to dial things down to a simple snake of 3 HD and constriction as its only ability, then in this case, we get EHD 3, properly placing it on the Level 3 table as OD&D is printed. (The constriction ability by itself always generates EHD = HD.) Of course, as DM you can create a snake of whatever hit dice and abilities your heart desires. For example, the Holmes Basic D&D Sample Dungeon has a 2 HD snake in room "S" (no special abilities noted; are we meant to assume this is a constrictor, poisonous, both, or neither?).
What do you use for baseline giant ants, beetles, and snakes in your OD&D game? Bigger, badder, and more fantastical stuff considered in the next post.