The main excitatory neurotransmitter in the central nervous system in mammals is glutamate. In insects it's acetylcholine. Does this matter? Here's an answer from Gilles Laurent, in "Shall we even understand the fly's brain?" This is from the excellent book "23 problems in system neuroscience", as I have mentioned earlier.
"No, because either mechanism provides equivalent means for postsynaptic excitation over many timescales, using ionotropic and metabotropic receptors. The flow of activity within and across networks are, as far as we know, similar with either combination of neurotransmitter and receptors. On the other hand, the fact thatglutamate ended up being used in the mammalian CNS enabled the exploitation of receptor subtypes with interesting nonlinear properties (e.g. NMDA receptors) for a variety of fundamental tasks such as rythm generation or learning."