Fig. 4. Chemicals that decreased on solitarization in final instar nymphs were tyramine, citrulline and acetylcholine. Tyramine decreased to 30% of its gregarious value within 24 h of isolation and remained low during all stages of solitarization, but increased rapidly within 24 h of crowding. Citrulline decreased to less than 10% of gregarious amounts after 24 h isolation and remained low during all stages of isolation and gregarization monitored. Acetylcholine declined slowly over a period of 24 h to 1 generation of isolation and remained at approximately 80% of the gregarious amount during further generations of isolation. It did not recover following 1 stadium of crowding. Values (mean ± S.E.M.) are sum amounts in the three sampled regions of the central nervous system, standardized to give the mean long-term gregarious value as 1. Stages as in Fig. 1.