Diaclones & Microman were produced by Takara in the late 70's and early 80's. Diaclones & Microman were a great hit but they were only released in Japan. They were probably one of the first toys that had a robot emerge from a modern vehicle or jets. Diaclone & Microman are mostly remembered as being "Pre-Transformers". This is because almost all the Diaclones & Microman were later re-issued as Transformers.
Please beware of Knock offs. The companies that did them, did a extremely good job! So look for the markings of Takara on them. Stickers stating Diaclone doesn't mean it is Diaclone. I have been fooled a couple times already. Also there is one Knock-off Company called" Four-Star", ever seen a Praying mantis Kickback?
Before Transformers were ever produced, Takara did produce Diaclones in North America under name Kronoform and Diakron. It is unknown which came first, but Kronoform has better artwork and boxes. These toys are Diaclones just sent to North America and were repackaged in different boxes. They may be packaged with different accessories.
In the transition of the Diaclones & Microman's becoming Transformers, Hasbro & Takara made some decisions to not produce some of them for the Transformers series. These toys can range from the entire toy itself (Triple Changer Helicopter) or a difference in color ("Red" Suntreaker). Another color difference you've probably heard of is the very famous Diaclone "Blue" Bluestreak. Though it still is roughly myth if it was ever produced under the Transformers name.
Hasbro also purchased rights from other companies to be produced them as Transformers. Takatoku produced the "Deluxe" Insecticons in Japan before Transformers, as Beetra. They had their own toy line and cartoon show(?). There was also a Lady Bug bot, but was never produced even for the Beetra line because Takatoku was bought by Bandai.
