I think both of the answers here are good answers, but since you seem unsatisfied let me try to contribute.
When playing as Protoss or Terran I often find its useful to use my Pylon/Supply Depot to help give me vision around my base. This isn't, by any means, a replacement for scouting, but it does give me a little advanced notice on drops. Since most maps have your main on a cliff, spreading out these (otherwise useless) structures helps monitor not only the edges but extends the range of your viewing.
Now this technique won't stop a drop, and it won't even really give you too much advance warning, but when every second counts, these are invaluable. You also have the benefit of power for Warpgates. If your opponent drops near the edge, these aren't advanced enough warning, but they do provide a nice distraction while you move your army. If he bypasses them and moves to your mineral line, they're a more advanced warning.
By comparison, Overlords almost trivialize this problem as you can spread them about the map, around your base and even around your opponent's base.
I know its not a solution, but it should help.
The truth is, there is no cheap, effective, sure fire way to guarantee you catch drops. Certainly map control helps, as does scouting, but if there was an easy fix for this terran's would probably never drop.
Forgive me for the quality of this answer I'm on the road.
Unlike a lot of people I really feel that using drones for defense as Zerg is a completely legitimate strategy. Zerg is very much a momentum race, so many top players strategy revolves around: Power drones and build units at the last possible second. Unlike Terran or Protoss, Zerg can produce a very large army very quickly so I feel using your Drones to delay is a fairly legitimate strategy.
However, I dislike opinions, even my own, so let's see this strategy in practice:
This first video is from a pair of Plat level players. You'll notice that by pulling drones off the mineral line he's able to get a very easy surround (which whoever this caster is doesn't seem to understand) and really take them apart with few losses (believe he loses 3 Drones). This delays the 10 pool long enough for his Spine Crawler to get up and really defend him well. The key point I want you to take away here is:
Drones can do Damage
This next video shows a Zerg defending against a 2 Gate back in beta (when Zealots were 5 seconds faster). The Zerg player here completely misses on his scouting. Notice how the Zealots are forced back every time he pulls all his Dones, but when he only sends in a few (or the Queen alone) he gets eaten apart. Drones are only really strong in numbers and when they can get a surround (much like Zerglings). The thing he does do well is sending his drones back to mine every time the Zealots back off. This is the second key point:
If you're not fighting, you need to be mining. Pulling drones is fine, but don't leave them lying around
This third video is a little more high level. I actually saw this match live and Blizzcon and the experience was exhilarating. Cool here is defending an early Bunker rush by Boxer. He immediately pulls 3 Drones (1 from scouting) to take out the Bunker. Boxer shows some impressive micro and manages to get the Bunker up and a Reaper inside... just in time to get destroyed by Zerglings. This is the third main point:
Drones are delaying tactic, you are using them to stall until real units can get out.
This last video is your money video because its exactly what you're dealing with: 7 Pool into Spine Crawler Rush. We see here that the defending player really shows knowledge of all the principles we talked about. When he sees the Spine Crawlers he pulls a large amount of Drones off the mineral line, forcing repeated cancels (every cancel costs 25 minerals). Next when Zerglings come he uses the mineral trick and then
backs out into the open to get a good surround on them and really take them down quickly. Finally, all of his actions are a delaying tactic so he can get his Zerglings out.
So what have we learned:
- Drones can do Damage. Try to get a good surround, preferably out in the open where he can't use buildings to block you
- When you're not fighting, you need to be mining. When your Zerglings finally pop and you start winning you need to back off and secure your economy
- Drones are a Delaying action. Zerg is a momentum race and you're just using your Drones to delay until you have enough momentum to win.
FAQ
But Tzenes, what if he just backs off and powers Drones and has a larger economy than me?
Omnes non possunt omnis
. To pull off this rush he is making a large sacrifice, this means you have a lot of room to breath. Worry less about him backing off and more about repelling the rush. By the same token, as soon as the rush is countered, stop producing Zerglings. Chances are he can't afford a second rush, so building a lot of units here is just letting him catch up.
Best Answer
Background info
This is what time their spawning pool will finish based on their build order (in-game time):
Additionally, the build time for a pair of
zerglings
is 20 seconds, so theirzerglings
will be on the field at:Whenever you spot the incoming
zerglings
(with your scoutingoverlord
), you can make an educated guess on what build order the opponent went for (based on what time it is in the game, and where you've spotted thezerglings
on the map).Scouting
All of that being said I think you should
drone
scout. Unless you are Master or Grand Master, the potential scouting information you will receive outweighs the lost mining time of your scoutingdrone
. Plus if the other player did go for an early pool, you will have the better economy anyways.It is good that you are positioning your second
overlord
along the attack path, and you should be doing this with everyoverlord
in the early part of the game (until they have some sort of anti-air).Build order & Defending
In
ZvZ
, a pretty safe build is14p/14h
. Yourzergling
will spawn later than his, but you usually only have to stall for ~30 seconds. You don't necessarily have to throw down aspine crawler
. Aspine crawler
takes 50 seconds to build, whilezerglings
only take 20 seconds. When you're defending, don't be afraid to attack with your drones (and if your micro is good, pull back the weak ones), to stall a little until yourzerglings
are out.There are a lot of videos on youtube about defending against a 6 pools.
One last thing: while your defending against the
zerglings
in your base, keep an eye on the minimap! Thoseoverlords
you placed along the attack paths earlier are giving you vital information: if you see morezerglings
reinforcements coming, keep building more of your own. A lot of players lose when they fail to scout the reinforcements, and try to squeeze in some drones when they should been building morezerglings
.