Egg sack tears have x2 range/ x2 tear height (which also increases effective range).
This item belongs to the Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.
This item belongs to the Spider Baby set. Collecting three items from this set will transform Isaac into a humanoid spider.
Notes
The chance to shoot egg sacs depends on the luck stat and is based on this formula:
At base luck (0), the chance is 14.29%, maxing out at 50% at 5 luck.
Egg sacks will also spawn blue flies or spiders if they hit Fire Places or invincible enemies like Death's Head and Stoney.
It is possible to create a large number of Spiders and Flies by repeatedly shooting a closed Host.
Synergies
C Section: Parasitoid fetuses spawn spiders/flies and slowing creep with every hit on an enemy.
Compound Fracture / Cricket's Body / The Parasite: When egg sacks are fired, they will split into smaller egg sacks. Split egg sacks from The Parasite will not look like egg sacks.
Cupid's Arrow: Egg sacks will apply their effect to every enemy hit.
Fire Mind / Explosivo : If the explosion effect triggers on an egg sack, it will produce a much larger pool of slowing creep.
Hive Mind: Flies and spiders are bigger and deal double damage.
Technology 2: Rapid damage ticks can cause a large amount of flies and spiders to spawn.
Dr. Fetus: Bomb explosions have a chance of leaving the slowing creep in a larger pool than egg sack tears, however the egg sack bombs can't spawn Blue Flies / Blue Spiders.
Fish Tail: The duplicated flies will spawn on top of the enemy and deal damage immediately.
Haemolacria: Tears retain Haemolacria appearance and only the main tear can spawn flies or spiders when damaging enemies / smaller tears can now leave slowing creep and spawn flies and spiders.
Ipecac: Only extra range applies on egg sack tears and they will not spawn flies or spiders.
Spirit Sword: The sword projectiles can still spawn flies and spiders, but the swings themselves won't / the swings can now leave slowing creep and spawn flies and spiders.
x2 range/
x2 tear height (which also increases effective range).
Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.
Spider Baby set. Collecting three items from this set will transform Isaac into a humanoid spider.
Fire Places or invincible enemies like 
Death's Head and 
Stoney.
Host.
C Section: Parasitoid fetuses spawn spiders/flies and slowing creep with every hit on an enemy.
Compound Fracture / 
Cricket's Body / 
The Parasite: When egg sacks are fired, they will split into smaller egg sacks. Split egg sacks from The Parasite will not look like egg sacks.
Cupid's Arrow: Egg sacks will apply their effect to every enemy hit.
Fire Mind / 
Explosivo : If the explosion effect triggers on an egg sack, it will produce a much larger pool of slowing creep.
Hive Mind: Flies and spiders are bigger and deal double damage.
Technology 2: Rapid damage ticks can cause a large amount of flies and spiders to spawn.
Dr. Fetus: Bomb explosions have a chance of leaving the slowing creep in a larger pool than egg sack tears, however the egg sack bombs can't spawn Blue Flies / Blue Spiders.
Epic Fetus: Bombs can now spawn Blue Flies / Blue Spiders.
Fish Tail: The duplicated flies will spawn on top of the enemy and deal damage immediately.
Haemolacria: Tears retain Haemolacria appearance and only the main tear can spawn flies or spiders when damaging enemies / 
Ipecac: Only extra range applies on egg sack tears and they will not spawn flies or spiders.
Montezuma's Revenge/
Revelation: No effect.
Spirit Sword: The sword projectiles can still spawn flies and spiders, but the swings themselves won't / 
Tech.5/
Skatole, 
The Bible, 
Magic Mushroom.
The Binding of Isaac: Rebirth
