As close as I can understand these Scriptures they apply to the fallen Angels:
And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, 8 but they did not prevail, nor was a place found for them in heaven any longer. 9 So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world;he was cast to the earth, and his angels were cast out with him.
Revelation 12:7-9 NKJV
The passage does not say that those Angels are demons. However; other Scriptures do lead us to understand they are:
If Satan also is divided against himself, how will his kingdom stand? Because you say I cast out demons by Beelzebub.
Luke 11:18 NKJV
The Scriptures refer to Satan’s helpers as both angels and demons:
“Then He will also say to those on the left hand, ‘Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels'” (Matthew 25:41 NKJV), compared to:
And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet. For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
Revelation 16:13,14 NKJV
Even though I cannot give you any verses which say that Demons are fallen Angels these Scriptures taken together along with the description of the Angel opening the gate to the bottomless pit leaves little doubt in my mind that they are the same (Rev. 8:3-5).