There was practically no better species at detecting the Psionic Energy aside from the Mind Flayers than the Arachnids. Zehal retrofitted the Psychic Disruptor Device into the Device that would be releasing a particular type of Psionic Energy that could be registered only by the Arachnids.
At the same time, this signal would drive them into a frenzy, and they would desperately search for the source of the signal because they would perceive it as an ultimate threat to their very own existence.
One of the primary forms of communication between the bugs was done through the Psionic Energy; the Arachnids were species that could expertly and masterfully read the Psionic Energy Signatures and then determine if something was a threat or not.
They were able to tell if someone was going to attack them just based on the residual remnants of the Psionic Energy in the environment that was released by a person, which was a very common tactic for the bugs.
