informant arrested but no another F.I.R

Section 2 of the Code of Criminal Procedure defines a cognizable offence. A cognizable offence is one in which a police officer can make an arrest without the need for a warrant or the consent or order of the magistrate in line with the First Schedule or any other statute currently in effect.

According to Section 154 of the Criminal Procedure Code, in order to initiate an investigation into a cognizable offence or case, a police officer must first receive the First Information Report (FIR) pertaining to the cognizable offence, which can be acquired without the Magistrate’s authorization, and enter it in the general diary.

The informant is stated to hves been arrested on the basis of investigation of the FIR, where the police has powers to arrest the informant, accordingly action of IO/ SHO cannot be challanged.

No another F I.R.is needed for arrest of infirmant

You have nowhere indicated whether he has been arrested rightly or wrongly

Yes.  I agree with the advice of the above expert, Sri Dr. J C Vashista Sir